Build a USB Powered AA NiMH and NiCd Battery Charger

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Build a USB Powered AA NiMH and NiCd Battery Charger

Im always complaining about all the chargers and wall warts I need to carry with me when going on a trip. This project, which can charge a pair of AA NickelMetal Hydride (NiMH) or Nickel Cadmium (NiCd) cells using a laptops USB port for power, arose to address part of that problem. (By the way, if you want tolighten your laptop load, take a look at the MoGo Mouse.)

Any USB port can supply 5V at up to 500mA. The USB standard specifies that a device may not use more than 100mA until it has negotiated the right to use500mA, but apparently no USB ports enforce that requirement. This makes the USB port a convenient source of power for devices such as this charger.

Build a USB Powered AA NiMH and NiCd Battery Charger http://www.stefanv.com/electronics/usb_charger.html

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There are commercially available USB AA charging solutions available, but they each have some drawbacks:

The USBCell is a 1300mAh AA NiMH cell with a removable top that allows it to be plugged directly into a USB port. No separate charger is needed.

Unfortunately, the cell capacity is very low (most NiMH AA cells are 2500mAh these days), and each cell requires its own port.

There is a two cell USB powered AA charger available, sold under a variety of names, but it charges at a very low 100mA rate. The distributor calls it anovernight charger, but at 100mA, a 2500mA cell would take about 40 hours to charge (40 instead of 25 due to the inefficiencies of charging at lowcurrents).

I found a 2/4 cell charger that can be powered by a USB port, auto adapter, or wall wart, but it is as large as the wall charger Im trying to replace. Differentones can be found here and here, but these take 10 to 12 hours to charge 2500mAh cells.

[December 2007 Update:Sanyo has introduced a USB powered charger for their Eneloop batteries. This charger has none of the drawbacks listed above, andwill charge a pair of 2000mAh cells in about 5 hours, or a single cell in half that time. Although designed for Eneloops (see my review), it will work with

regular NiMH cells as well. Watch for a review on this site soon. ]

The charger in this project is designed to charge two AA NiMH or NiCd cells of any capacity (as long as they are the same) at about 470mA. It will charge700mAh NiCds in about 1.5 hours, 1500mAh NiMHs in about 3.5 hours, and 2500mAh NiMHs in about 5.5 hours. The charger incorporates an automatic chargecut-off circuit based on cell temperature, and the cells can be left in the charger indefinitely after cut-off.

Specifications

This charger has the following specifications:

Size: 3.8L x 1.2W x 0.7H (9.7cm x 3.0cm x 1.5cm).Cells: Two AA, NiMH or NiCdCharging Current: 470mACharge Termination Method: Battery Temperature (33C)Trickle Current: 10mAPower Source: Desktop, Laptop, or Hub USB portOperating Conditions: 15C to 25C (59F to 77F)

The CircuitThe heart of this charger is Z1a, one half of an LM393 dual voltage comparator. The output (pin 1) can be in one of two states, floating or low. While charging,


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the output is pulled low by an internal transistor, drawing about 5.2mA of current through Q1 and R5. Q1 has a beta of about 90, so about 470mA will flowthrough into the two AA cells being charged. This will fully charge a pair of 2500mAh cells in just over 5 hours.

USB powered AA charger schematic.

During charging, R1, R2, and R4 form a three-way voltage divider which yields about 1.26V at the non-inverting input of Z1a (pin 3, Vref).

TR1 is a thermistor that is in direct contact with the cells being charged. It has a resistance of 10k at 25C (77F), which varies inversely with temperature byabout 3.7% for every 1C (1.8F). R3 and TR1 form a voltage divider whose value is applied to the inverting input (pin 2, Vtmp). At a temperature of 20C(68F), TR1 is about 12k, which makes Vtmp about 1.76V.

Once the cells are fully charged, the charge current will literally go to waste, in the form of heat. As the cell temperature rises, TR1 s resistance drops. At 33C(91F), the resistance will be about 7.4k, which makes Vtmp about 1.26V, which equals the Vrefvoltage.


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B ild USB P d AA NiMH d NiCd B Ch h // f / l i / b h h l


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Battery voltage versus time. The cellsare full when the voltage peaks, andthe charger shuts off shortlythereafter.

As the temperature rises above 33C, Vtmp will become less than Vref, and the open-collector output of Z1a will float high. Therefore, the current flowingthrough R5 is greatly reduced, as it is now limited by R1, R2, and R4. As a result, the current flowing through Q1 and the cells is reduced to a 10mA tricklecharge rate.

Also, because R4 is now connected to +5V through R5 and Q1 instead of being held at 0.26V by Z1a, the Vrefvoltage changes to about 2.37V. This guaranteesthat as the cell temperature drops, the charger wont turn back on. In order for Vtmp to reach 2.37V, TR1 would have to reach about 20k, corresponding to atemperature of about 6C (43F), which should never happen in a room temperature environment.

Z1b is the other comparator on the LM393 chip, and a close look at the schematic reveals that its performing the same comparison as Z1a. Instead of driving thecharging transistor however, it drives an LED that indicates that charging is in progress. R6 limits current to the LED to about 10mA. By running the LED fromits own comparator (which is on the chip whether we use it or not), the LED current has no effect on Vref.

Finally, C1 is there to ensure that charging starts when a pair of cells is inserted. With no cells in place and the charger off, C1 has about 1.9V across it (5V 0.7V Vref). As soon as the second of two cells is inserted, the positive side of C1 is suddenly forced down to the battery voltage (about 2.4V). This immediatelyforces the negative side 1.9V lower than this, to about 0.5V. Since this is connected to Vref, Z1as output goes low, causing charging to start. After a fewmilliseconds, C1 adjusts to the new voltage difference imposed by R1, R2, and R4 on one side and the cells on the other, and no longer affects the circuit.

Construction

The circuit is best built on a printed circuit board. Refer to my article on the subject, Making Excellent Printed Circuit Boards. Here is the printed circuit layout:


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B ild USB P d AA NiMH d NiCd B tt Ch htt // t f / l t i / b h ht l


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Copper side. Actual size is 3.8" x 1.2" (9.7cm x 3.0cm).Click to enlarge.

Begin by installing all the resistors and the capacitor. The resistors should be installed lying flat. Install LED1, being sure to orient it so that the negative terminalis the one connected to pin 7 of Z1b.

Component placement diagram. Click to enlarge.

Install Z1 next, ensuring that pin 1 (indicated by a small dot or identation on one corner of the IC) is oriented as shown in the placement diagram. If you wish, usea socket for Z1.

Transistor Q1 is mounted on a small heatsink. First bend the leads back 90 just where they start to narrow. Dont bend them too sharply or they might break.Insert Q1 into its lead holes, and slide the heatsink underneath. Hold everything in place with a clamp while soldering the leads. With the clamp still in place, drillthe hole for the heatsink bolt.


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Charger with all electronic components installed. Note that there isspace under Q1 for the heatsink. The board area where the batteryholder will go has been scuffed up to aid adhesion.

Installing the battery holder is the next step. I used a 2-cell holder made by cutting the two outer cell positions off of a side-by-side 4-cell holder. You can ofcourse just buy a 2-cell holder, but none was available when I went to the parts store. My approach has the additional advantage that the cells are easier to insertand remove, because the sides of the holder dont curve inwards over the cells.

Before installing the holder, remove a long section of the centre divider to make room for the thermistor. Also solder some leads to the cell holder terminals.Glue the holder in place on the circuit board, flush with the sides and ends of the board. When the glue has dried, drill through the TR1 holes in the board to makematching holes in the battery holder. If you did everything carefully, these two holes should be right on the centre line, where you removed the section of divider.

Insert the thermistor through the holes, and then put a pair of AA cells in the holder. From the copper side, push up on the thermistor so it is in firm contact withthe cells, and then solder it in place. Then remove the cells, and connect the battery holder leads to the holes marked B+ and B- on the placement diagram.


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Build a USB Powered AA NiMH and NiCd Battery Charger http://www stefanv com/electronics/usb charger html


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The completed charger with one cell in place. The 2-cell holder wasmade by cutting the outer positions off of a 4-cell holder. Notice howthe thermistor is installed so as to make physical contact with the cells

being charged. A small heatsink keeps Q1 cool.

The last step is to connect a USB power cable. Either buy a cable, or cut one off of a discarded USB device such as a broken mouse. Cut the cable to the desiredlength, and strip about 1 of the outer covering off the end. Roll back the shielding, and find the +5V and GND wires. These will generally be red and blackrespectively. Strip and tin the ends of them, and solder them to the USB+5V and USBGND terminals of the charger.

Testing

Before connecting the charger to a power source, inspect your work carefully. Be sure all the components are oriented correctly (specifically Q1, LED1, Z1, andthe battery holder).

For initial tests, I used a USB hub for

power. A pair of #11 hobby knifeblades between the cells and thecontacts let me hook up a voltagemonitor.

For initial tests, I suggest you use a powered USB hub. By using a hub, you ensure that the charger is not drawing power from your computer, since a defect in thecharger could damage the power source. Note however that most powered hubs wont output any power unless the hub is connected to a computer. Alternatively,you could use a regulated 5V power supply, temporarily connected to the +5V and GND traces on the circuit board.

With power applied, check that the LED is off. If it is on, use a 330 resistor to short out TR1 for an instant (this makes the circuit think the cells have gottenextremely hot). If the LED doesnt extinguish, theres something wrong.


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Cell Type Charge Time

700mAh NiCd 1.5h

1100mAh NiCd 2.5h

1600mAh NiMH 3.5h

2000mAh NiMH 4.5h

2500mAh NiMH 5.5h

It is important that the two cells being charged are of the same type and at the same level of discharge. If the cells are mismatched, one will become fully chargedbefore the other. When it reaches 33C, the charger will shut off. If the second cell needs more than about 200mAh more than the first cell, it will not havereached a full charge.

This charger, with a suitableenclosure, is ideal for use on trips,using a laptop to power the charger.The laptop should be plugged in toavoid running down its battery.

In general, if two cells are used together in a single device (digital camera, GPS, etc.), then they will remain in sync, and can be charged together.

When charging is completed, the charger will switch to a 10mA trickle charge. This is sufficient to overcome the cells natural self-discharge rate, but low enoughthat the cells can be left in the charger indefinitely. However, do not leave the cells in the charger unless the charger is plugged into a powered-up USB port.Otherwise, the cells will supply power to the circuit and be drained in the process.


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When using this charger with any computer, make sure that the computer is not set to go into a power saving mode that turns off power to the USB ports. If thishappens, charging will stop, and the cells being charged will discharge. When using a laptop as a power source, its best to plug in the laptops power supply, sincethe charger uses a significant amount of power, and will probably take longer to complete than the laptop battery will last.

If powering this charger from a USB hub, be sure to use a powered hub. A non-powered hub will not be able to deliver enough current to the charger, since itmust share the 500mA coming from the computer with the ports in the hub (typically four). The extra cable length also tends to reduce the voltage reaching thecharger.

Charging AAA Cells

If the springs in the battery holder are long enough, the charger can also be used to charge a pair of AAA cells. However, it is then necessary to insert shimsbetween the cells and the sides of the battery holder to ensure that the cells remain in contact with the thermistor. Only charge modern AAA cells, having acapacity of 700mAh or more.

Parts List

Some parts can be obtained at Radio Shack, but larger electronic supply houses like Digi-Key are more likely to stock all the parts needed.

Part Description

R1 56k W, 5% resistor



R4 47k W, 5% resistorR5 750 W, 5% resistor

R6 220 W, resistor

TR110k @ 25C thermistor, approx. 3.7%/C NTC

Radio Shack #271-110 (discontinued)

C1 0.1F 10V capacitorQ1 TIP32C PNP transistor, TO-220 case

y g p _ g

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Z1 LM393 dual voltage comparator IC, DIP

LED1 Red, green, or yellow LED, 10mA

Other2-cell AA battery holderUSB cableSmall heatsink

Note that the Radio Shack thermistor has been discontinued. Although I have not tried any of them, there are other similar thermistors available, such as theVishay #2381 640 54103 (Digi-Key #BC2298-ND). The temperature coefficient is slightly different (about 4.6%/C), but over the range were interested in, isclose enough. Using this thermistor, the cut-off and turn-on temperatures would be about 32C (89F) and 10C (50F) respectively.

Alternatively, you can use the resistor values below with the Vishay thermistor to raise the cut-off temperature back to 33C, while lowering the turn-ontemperature to 3C (37F).

PartAlternative Resistor Values to use with

Vishay #2381 640 54103 Thermistor





I have not tested this combination, but the values were computed using the same program that I used to compute the values that were used with the Radio Shack

thermistor. Do not mix and match values from this table with those listed above. If you change any of the values to those in this table, change all of them.

If anyone finds an alternate source for the Radio Shack thermistor, please let me know.

Like 133 people like this.

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Leave a Comment

194 Comments

Amy DonovanDecember 04, 2007

Please include a sentence or two about the need to recycle rechargeable batteries. These types of batteries contain heavy metals and should never bethrown away; they should always be recycled. We need to get the word out to the public! See http://www.rbrc.org/call2recycle/.

Amy Donovan, Program DirectorFranklin County Solid Waste Management DistrictGreenfield, MAhttp://www.franklincountywastedistrict.org

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AnonymousDecember 09, 2007

NiMH batteries should be recycled to reduce waste but they do not contain heavy metals.

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Lead acid and NiCd are the rechargeable battery chemistries that use heavy metals and must always be recycled properly.

T.S. LibertanJanuary 09, 2008

The use of C1 is quite clever. Didnt understand it at first, its role is quite subtle. Probably still dont understand it

Using temperature to halt the charging seems quite dependent on a number of things e.g how well the thermister is thermally coupled to the cells.

Also do cells only become hot when theyre nearly charged, or do they heat up significantly at any point in the charging process?

I was thinking of a 4xAA charger using a 12v bicycle dynamo. I was just going to use a LM317 set to four times whatever the open voltage of a NiMH cellis (4 x 1.36v?). I thought that once charged the cells wouldnt draw any current. I think the dynamo is rated at ~500mA maximum, so maybe a tokencurrent-limiting resistor could be used, or not. Any thoughts?

By some miracle I bought 25 6ft USB extension cables on ebay today for 2 dollars, and Ive always wanted an excuse to play with a thermistor. I think Imight try your charger.

Anyway, all I wanted to say really was congratulations a well explained circuit is a rare thing.

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Stefan VorkoetterJanuary 09, 2008

T.S., youre right about thermal coupling being important, which is why I stress in the instructions to install the thermistor to ensure it is in firm contact with

the cells being charged.

NiMH cells do warm up a bit during charging, but they start to heat up quickly once they are full. NiCds on the other hand cool during charging, and start

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BenMay 16, 2008

Would I be able to use a slightly lower charge rate to allow the power from the usb port to drive the load as well? In other words, what is the lowest chargecurrent that can be used within reason?

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Stefan VorkoetterMay 16, 2008

Ben, by "load", I assume you mean "the load connected to the battery"? If so, you dont need to modify the circuit to do this. The load will just take asmuch of the current as it needs, and the rest will charge the batteries.

However, this may not be the circuit you want, since it will charge the batteries until they are full, and then stop. It will not restart until the batteries areremoved and reinserted.

You probably want a much simpler circuit that simply continuously charges the batteries at a very low current (just a bit more than is used by the load).

Stefan

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ChrisMay 29, 2008

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Solving that for Vref gives 2.36V (when I did the original calculations, I rounded off as I went, and thus arrived at 2.37V).

Vicente FittipaldiJune 05, 2008

Congratulations, Stefan. This circuit was the best that I found in the Internet. Simple, efficient and economic to mount.

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Rick Scott

June 13, 2008Hi Stefan,

The reason the circuit draws curent when not powered by the USB port comes from diodes junctions in the circuit you have not considered in your analysis.The first diode is the collector-base junction of Q1 which will be forward biased because the battery voltage is higher than the base. This allows thebatteries to apply power to the circuit through R5.

The next part of this problem is not visible in your schematic and is something most people arent aware of. Virtually all microcircuits except for some RFdevices have ESD protection diodes/circuits between each interface pin and the power and ground pins of the device. For analysis purposes, they look likereversed biased diodes when the device is used within its normal operating conditions.

With no power supplied the power pin of Z1 (pin 8), the voltage applied to the output pin (pin 1 in your circuit) through R5 will forward bias the ESD diodebetween pin 1 and the internal voltage bus connected directly to pin 8. This will then provide enough power to make the comparators operate which drawscurrent and also turns on the LED. All of the circuits are now being powered by the batteries, but the voltages are much different than the normal operatingcondition.

The internal ESD protection diodes need to be considered for any circuit that can have a voltage applied to its input/output pins when its not powered by

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its power supply. This problem is very common in interface circuits between different "boxes".

Rick

Stefan VorkoetterJune 13, 2008

Rick, thanks very much for your detailed analysis and explanation. The behaviour Ive observed makes perfect sense now. I guess the simplest way toprevent all this would be to put a diode into the circuit between the collector of Q1 and the positive battery terminal. Theres enough of a voltage differenceeven at full charge to allow for another 0.7V drop.

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Rick ScottJune 15, 2008

Hi Stefan,

Youre welcome. Have you considered using a schottky power diode like the 1N5806. With a diode like this, the forward voltage drop is only around 0.4V,which would provide more overhead if the USB power supply voltage is low.

Rick

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OvidiuJuly 05, 2008

Hello Mr. Stefan! I was surfing the internet for a project for mu electric circuits class and I found yours to be the most interesting and I have started doingit for my final examination for this class. I have a little problem! I use de Orcad pspice schematics and there I have to put a equivalent circuit for thebateries and the thermistor and I am in reall problem could you lend me a hand? I have to draw that battery voltage versus time and i dont know howPlease help

serkanJuly 11, 2008

I try to simulate it with Multisim workbench. I need to know that what can i use instead of rechargeable batteries in simulation?

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AshwinAugust 11, 2008

That is a wonderful design. I simulated it on Multisim and it is superb. Just started work on the PCB

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Gigi

September 27, 2008

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Hello Mr. Stefan

May I suggest you to use a second thermistor (or a combination resistor+thermistor) for R3 ? In this case the circuit will sense the difference betweenambient temperature and cells temperature, allowing it to operate at ambient temperatures over 33 degrees C and also ensuring a faster chargingtermination (after the cells are charged) when ambient temperature is lower. The second thermistor should be placed away from heat sink and cells, outsideof an enclosure.

Gigi

stuntmasterOctober 14, 2008

hi Mr stefan i have question should I always charge 2 cells at the same time or could i charge only just one cell at time?

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ArgenOctober 26, 2008

Hi!!

I was wondering, here I cant fin thermistors of 10K. The Only thing I can find in my country are thermistors of 5ohms, any ideas about it?? Can I replace itin any other way?

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Stefan VorkoetterNovember 25, 2008

Haley, what the USB spec says and what is actually implemented are not the same thing. The spec allows a USB port to restrict current to 100mA tonon-enumerated devices. However, no one actually puts in the circuitry to enforce this limitation. Most computers and USB hubs just supply 5V to all theports, with little or no current limitation.

AndrasJanuary 14, 2009

I fully understand your point in using the USB as the power source. However, if I want to use something else as a power source, I can use anythinggenerating 5V and at least 500mA, right?

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Stefan VorkoetterJanuary 14, 2009

Yes, you can use any power source that provides a reasonably regulated 5V at 500mA or more.

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honda4life

January 28, 2009

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thx

Stefan VorkoetterFebruary 09, 2009

Irwan, if you want to charge a 1000mAh NiCd in one hour you need to change two parts: Your power supply, and the 750 resistor. Your power supply will

need to be able to supply at least 1200mA. The resistor should be changed to about 300. Youll also need a bigger heatsink on the transistor.

32.

Ersin AyraMarch 13, 2009

Hi Stefan,

I built the circuit, its really fantastic. Cheap, reliable and performs very well. The only question is how to protect it against damages and short circuit(casing). I didnt want it to put it in a box, because its now very small and smart, I didnt want to enlarge it with a box. Maybe with silicon or protolin youcan make a self-box Did you have an idea or a solution for it? Again congratulations for the circuit and thanks for sharing it with us.

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Stefan VorkoetterMarch 13, 2009

Ersin, I still havent made a box for mine either, primarily because Ive only used it on my desk at home so far. However, one way to package it would be to

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use translucent heat-shrinkable tubing of the appropriate size. Just slide the tubing over the circuit, shrink it with a heat gun, and then cut away the tubingwhere it covers the battery holder. This method is frequently used for electric model speed control circuits.

HasheemApril 09, 2009

Stefan, ive constructed your circuit but there is a heat problem. When i bind the circuit to my computer, LM393 starts to heaten. The voltage i take frommy usb port collapses to 2,5 volts when i bind it. can you please help me, what can be the problem? im waiting for your answer urgently, thank you.

35.

Stefan VorkoetterApril 10, 2009

Hasheem, without seeing the circuit, it is difficult to diagnose a problem like that, but the three most likely causes are: 1) youve inserted the IC backwards,2) youve connected the USB power leads backwards, or 3) youve made an error making the PCB.

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HenriqueApril 11, 2009

Thanks Stefan!

Your project are doing very sucecss here on Brazil.

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Rene'April 19, 2009

Hey Stefan., love your project, I have a few Questions, hope you can be of help, "While charging, the output is pulled low by an internal transistor,drawing about 5.2mA of current through Q1 and R5". ***** How do you know the current through R5 is 5.2mA, is it from a data sheet? ***** You using aheatsink for Q1 because it heats up, could you provide more information, tell me what equations you used to lead you to select the heatsink you using(such

as junction case temperature etc), in a nutshell could you provide more heatsink design information in you project. ***** I know, im very inquisitive:)

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Ren, the current through R5 is calculated using Ohms law: V=IR. In this case, there will be a 0.7V drop through Q1 (because there always is across theB-E junction of a transistor), and a 0.4V drop through the internal output transistor of the comparator. That leaves 3.9V across R5.

For the heatsink, I used the TLAR (That Looks About Right) method. Worst case, the transistor is only dissipating about 1.5W, so it doesnt need much of aheatsink.

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Rene'April 23, 2009

Thank u very much for clarifying that, i undastand now, but i do poze more questions."As the cell temperature rises, TR1s resistance drops. At 33C, the

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Rene'April 26, 2009

Wow, thanks very much, u very helpful, that helps me undastand alot, yet again i come with questions

"current flowing through Q1 and the cells is reduced to a 10mA trickle charge rate"

Q1: How did you get 10mA?

"Also, because R4 is now connected to +5V through R5 and Q1 instead of being held at 0.26V by Z1a"

Q2: What does the 0.26V represent?

"the positive side of C1 is suddenly forced down to the battery voltage (about 2.4V). This immediately forces the negative side 1.9V lower than this, toabout 0.5V"

Q3: How does this forcing down of the capacitor take place? is there a term or process that causes this? i would like to read up more on it.

Q4: How do u know it will be forced down to 0.5V?

"After a few milliseconds, C1 adjusts to the new voltage difference imposed by R1, R2, and R4" Q5: is this difference = the voltage division result of R1,R2, and R4?

Rene'April 26, 2009

Wow, thanks very much, u very helpful, that helps me undastand alot, yet again i come with questions

"current flowing through Q1 and the cells is reduced to a 10mA trickle charge rate"

Q1: How did you get 10mA?

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"Also, because R4 is now connected to +5V through R5 and Q1 instead of being held at 0.26V by Z1a"

Q2: What does the 0.26V represent?

"the positive side of C1 is suddenly forced down to the battery voltage (about 2.4V). This immediately forces the negative side 1.9V lower than this, toabout 0.5V"

Q3: How does this forcing down of the capacitor take place? is there a term or process that causes this? i would like to read up more on it.

Q4: How do u know it will be forced down to 0.5V?

"After a few milliseconds, C1 adjusts to the new voltage difference imposed by R1, R2, and R4" Q5: is this difference = the voltage division result of R1,R2, and R4?

Rene'April 27, 2009

"With power applied, check that the LED is off"

Q: Y must the led be off? does it indicate that charging process has not begun?Whats the purpose of the LED??

"voltage at battery terminals should be increasing. After a while, the rate of increase should slow down"

Q: the battery voltage rate at terminals depend on temperature?? coz of the thermistor. So does the rate slow down because the batteries are warming up asit gets charged? temperature increaseresistance decrease-voltage decrease????V=IR

"As the cells reach about 75% charge, the rate of increase will speed up again. Finally, when the cells reach 100% charge, the voltage will start decreasing,and the cells will start to get warm. 15 to 20 minutes later, the charger should turn off." Q: if the battery voltage rate depends on temperature why does itspeed up when its 75% charged coz the batteries should be pretty warm at that stage and the charging process should be coming to an end?????temperatureincreaseresistance decrease-voltage decrease????V=IR

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More answers for Ren (however, I suggest you get yourself a good book on basic electronics and try to figure some of these out for yourself; this is not acomplicated circuit):

A1: Use Kirchoffs law to calculate the current through R5 when Z1a is floating, and multiply the result by the beta of Q1.

A2: The low-level output voltage of Z1a with a 5.2mA input current (see the datasheet).

A3: The process is called "connection". When two points in a circuit are connected, they have to be at the same voltage.

A4: 2.4V 1.9V = 0.5V.

A5: Yes.


Final answers for Ren:

Answer to first question: The purpose of the LED is described in the article.

Answer to second question: The voltage at the battery is increasing because the battery is being charged. The thermistor doesnt control the voltage, onlywhether or not the battery is being charged.

Answer to third question: See answer to second question. The battery voltage increases more rapidly as it gets to about 75% charge because that is whatNiMH (and NiCd) batteries do when you charge them at a constant current.

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RolandoMay 26, 2009

This circuit can recharge two AA batteries in serial?

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PLJune 29, 2009

hi Stefan,

built your circuit and its working! however, could ask for some help regarding the circuit. can i have a brief explaination of the whole circuit? what eachcomponents do and all ? Because i need to understand it before i can explain it as well. it would be great if you would be able to provide me theinformation. cheers!

rgs, PL

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Stefan VorkoetterJune 29, 2009

PL, if you read the article, youll notice that its all explained in great detail in the section entitled The Circuit.

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Richard SweetappleJuly 07, 2009

Hi Stefan,Hello from Queensland Australia. Thank you for sharing this project with us. In this part of the world our ambient temperatures vary from 0 to 38degrees Celcius. To overcome the narrow temperature range of the circuit my thoughts are to replace R3 (22k) with another TR1 and a 20k trimpot inseries. This should allow me to recharge the batteries when the ambient temperature is above 33 degrees and also terminate the charge earlier when theambient temperature is very low. Another modification I have made is the addition of a urethane foam cover over the batteries to get a quicker switching

response.

nurulJuly 13, 2009

hello.thanks for sharing. actually ive try to simulate it by using proteus. but error occur because of the lm393.ive check the connection and im afraid thereis no wrong.can u suggest any other component that i can replace?

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nurulJuly 24, 2009

help me please stefan.ive construct the circuit on project board. the current flows to charge the battery is too small, about 10mA.where should itroubleshoot?

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October 13, 2009

Ive built the circuit and it works perfectly with 2AA NiCd/NiMh Batteries; I set it up to charge at 1Amp changing R5 for one of 330Ohms aprox. Im usinga wall adapter 5VDC 2Amp

I have a trouble.

I need to charge 3AA NiCd/NiMh bateries in serie, instead of 2AA. When I put the 3AA in serie, the charge current drops from 1A to 200mA. What doyou think its happening??

Thanks for your answer

Stefan VorkoetterOctober 13, 2009

Yuber, 5V is not enough to charge 3 AA batteries in series at high currents. At 1A, a single AA NiMH battery will reach about 1.6V, so 3 will be 4.8V. Thereis a minimum of 0.7V lost in the transistor, so with 5V, youll only have 4.3V available. The simple solution is to use a 6V power supply instead of a 5V one.

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Stuart HallidayOctober 17, 2009

Most USB 2 Ports do now need to be negotiated today so no way will I be able to get more than 100mA out of a single port.

More so when USB 3 comes out with its slightly larger current later this year.

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Stuart, I have yet to encounter a USB port that wont deliver 500mA even without negotiation. All my machines have USB 2 ports, and they have all beenable to operate this charger. There are also commercial USB-powered chargers (such as Sanyos Eneloop charger) that also do not negotiate for current, andthey work fine. The thing is that to actually limit the current and only allow the higher current once it has been negotiated requires more circuitry in theUSB port, which most manufacturers will leave out in order to save costs.

MarcosOctober 28, 2009

Hi Stefan, i only found thermistor of 1K and 20, i will have problem if i use them?

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LukeDecember 10, 2009

Hi Stefan, hello from Poland. Thanks for your project, it is great and easy to make. However, I have a question. Is it possible to use TIP42 instead ofTIP32? I did everything with accordance to this article, connections between elements on PCB are good, elements are also OK, but my charger doesntwork. Admittedly, I use power supply instead of USB, but it is able to supply 1000mA so its enough. I think, that the problem is in TIP42 which I usedinstead of TIP32. What do you think about it? Is the transistor a problem or maybe something else?

Thank you in advance for your answer.

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KyriakosDecember 19, 2009

Luke, as far as I can recall, TIP42 is used to handle negative voltage. TIP34 and TIP36 will work fine if you use a power supply.

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tirochingDecember 30, 2009

can i know how the resistor r5 effect the circuit

for ur example when input is 470mA, so resistor r5 R5 = 1.6 x I = 750ohm

but i try using 400ohm y i cant see got any different ,,, the current input to battery still the same 470mA

and can u tell me what circuit simulator software u r using .

because i using multisim software cant find out the transistor tip32c and the ic LM393

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Stefan Vorkoetter

December 30, 2009

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Tiroching, R5s value is computed based on the available voltage (5V), desired output current, beta of the transistor (about 90) and the voltage drop throughit (0.7V) and the output transistor of the op-amp (0.4V). The formula is:

R5 = (5 0.7 0.4) * 90 / OutputCurrent

For an output current of 0.47A, R5 works out to 747 Ohms. If you replace R5 with 400 Ohms, you should get an output current of about 0.88A. In practiceyou probably wont, because the USB output voltage will drop with that much load.

Marloe UyFebruary 05, 2010

Hello Stefan,

Im planning to make your charger. However, i want to charge 5 AA battery. Can you help me on what should i change on the circuit?

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This charger cant charge more than two at at time, because there isnt enough voltage available from a USB port.

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Marloe Uy

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February 18, 2010

Im planning to use a power supply for the source what will be the proper voltage to use for charging 5 AA battery


9V supply should work just fine. Youll have to change R5 to about 1.5k

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tirochingMarch 19, 2010

i trying ur circuit using the input current from other power supply 5V and current just arround 200mA to 250mA

R5 = (5 0.7 0.4) * 90 / OutputCurrent

then i calculate out .for using 250mA.should i need to use 1.4k ohm????

but at the up site u give 1 of the fomula

R5 = 1.6 x I Use the nearest standard value. For example, if you measure a current of 510mA, replace R5 with an 820 resistor. If the measured currentwas 420mA, use a 680 resistor.

how can i calculate out for using 250mA current to

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That formula was to correct the value of R5 if the resulting current was too high or too low from the designed current. The formula you have used is correctfor obtaining a lower current.

calMarch 26, 2010

I have gone through the web, and I guess yours is the only project available that explains in detail as to how to charge Ni-Cd batteries through USB.

However if I want to charge 4 batteries 1.2v each (4.8V), within USB 5v limit, what changes should I make. I guess if you can include this in your project itwill be really complete and so useful for all of us.

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The best way to charge 4 batteries is to build 2 chargers. You cant charge 4 batteries in series through the USB port. Four batteries will get up to about6.4V during charging, which would require 7.1V from the power source. The article is already complete and useful.

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calMarch 29, 2010

Thank you Stefan for your reply about charging 4 batteries via USB. First of all I apologise, yes, the article is full and complete! I am just wondering howdo some companies like this one http://www.meritline.com/usb-battery-charger-268p-28333.aspx

accomplish this. Is there a way to do this. Pls help me reg this cals


Notice that it can charge either 2 or 4 AA cells. When charging 4, its basically charging the two sets separately, like if you built two separate chargers.

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calMarch 29, 2010

OK!! got it, thnx a lot !!

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Aditya ParanjapeApril 03, 2010

I have completed the circuit work but I am not able to figure out what type of USB cable to use. Which wires in a usb cable carry current? There are 4

wires of the colour White, Green, Blue and orange. Can you please tell me which two wires carry current.


All USB cables that I have seen have white, green, red, and black wires. The red wire is +5V and the black wire is ground. I dont know which wires in yourcable are +5V and ground.

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celineApril 03, 2010

how can i calculate out r1, r2, r4 from the three way voltage divider which yields about 1.26v at non-inverting input of Z1

can u provide me the concept and the fomula of calculation

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mayApril 03, 2010

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how can i know the Z1 pin 1 during charging the voltage is 0 3volt



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how can i know the Z1 pin 1 during charging the voltage is 0.3volt

how to calculate out?

Lord CupcakeApril 20, 2010

I notice that in order for the charger to turn back on, TR1 would have to reach about 43F. How would the charger turn back on in a room tempatureenviroment once I removed the batteries to plug in my next pair.

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The charger turns back on when new cells are inserted. See the last paragraph of The Circuit section for the explanation. C1 is the key.

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In looking through your comments, I noticed that people had trouble with the LED lighting when the power source was removed and the batteries were still

inserted. The proposed solution was to put a diode between the positive battery terminal and the collector of Q1. The way I understand it, the cathodeshould be at the positive terminal of the battery, but I cant be certain. Can you please clarify? Also, one side of C1 is connected to both the positive battery

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terminal and the collector of Q1 Which side of the diode should the capacitor end up connecting to or does it not matter



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terminal and the collector of Q1. Which side of the diode should the capacitor end up connecting to, or does it not matter.


You are correct about the orientation of the diode. Put the diode between the battery and junction of C1 and the collector of Q1.

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Also, the TIP32C on Digikey has a hFE of 10. Is this the right part, or can I substitute another, like the 2SB0953AQ, Digikey part no.http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=2SB0953AQ-ND

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The listed hFE is at 3A. If you look at the graph on the data sheet, youll see its around 105 at 500mA. That is of course the maximum. The examples Itested were all around 90.

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of what I would look like is below. (I dont know if it will display right, or if you will understand it, but its worth a try for clarifications sake)



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of what I would look like is below. (I don t know if it will display right, or if you will understand it, but it s worth a try for clarifications sake)

||


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Im not really familiar with using those parts as temperature sensors, so Im not sure what would be involved, but its probably possible.

philMay 01, 2010

Thanks for your help with that. I have modified the circuit a little in order for it to sense ambient temperature. I have changed R1 to a 16000 ohm resistor,

R2 to a 33000 ohm resistor, R3 to the Vishay thermistor you specified as a replacement http://search.digikey.com/scripts/DkSearch/dksus.dll?WT.z_header=search_go&lang=en&site=us&keywords=BC2298-ND&x=2&y=15, R4 to a 11000 ohm resistor, R5 stays the same R6 stays thesame, and TR1 to the Vishay alternative. This yields ~1.85V at Vref during charging (shut off voltage for Vtemp) and ~3.813V after charging has ended(turn back on voltage for Vtemp).

In order for the charger to shut itself off, the thermistor connected to the batteries would have to become ~13C (23.4F) hotter than the ambienttemperature. This shut off temperature goes up to ~16C (28.8F) above ambient @ an ambient temperature of 60C (140F), and down to ~9C (16.2F)above ambient @ an ambient temperature of-40C (-40F).

In order for the charger to go back to charging the batteries after being shut off, the batteries would have to reach ~12C (21.6F) below ambient. This shutoff temperature goes up to ~19C (34.2F) below ambient @ an ambient temperature 60C (140F), and cant turn back on @ an ambient temperature of-40C (-40F) because -40C is the lowest operating temperature of the components. This should never happen, as there would have to be a really freakyset of circumstances for the batteries to get colder than the ambient temperature after charging.

I hope that this is helpful, and if you find any mistakes, please let me know. I have not actually tested this setup, but it should work.

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SamMay 31, 2010

Dear Stefan

Firstly, thank you for sharing this brilliant circuit with us. I have a few questions Id be grateful if you could help me with:

1) As I intend using this device in tropical countries with higher ambient temperatures, and I dont want to mess around with additional thermistors to adjustfor ambient temperatures, can I simply adjust R1 R2 and R3 (and TR1 even) to activate the circuit at higher temperatures, say 35-40C in other words canI assume the temp of the batteries will just continue to rise as they continue to be charged and dissipate energy or do battery cells peak at a certaintemperature? The aim of this would be to avoid the circuit being triggered by high ambient temperatures whilst still providing a temperature-activatedcharging cut-off.

2) Can the circuit be modified to ensure it just stops charging once Vtmp > Vref and does not trickle charge with 10mA? If so how is this possible?

3) I intend building a version of this circuit to charge 3 NiMH AA cells in series (i.e. 3.6-4V total) not from USB but from a solar PV panel rated at 5.5Vand 65mA. I have been told I can just trickle charge these cells using a diode but I want to use a more sophisticated charger like yours. Do you think this isfeasible using your charger circuit or is the solar cell just too small for the job? i.e. Would your circuit just drain too much current for this solar panel? Orwould the solar panel voltage (5.5V) just not be high enough for 3xAA?

4) I cannot find a TIP32C here easily in the UK can I just use any general purpose PNP transistor as a substitute, or even a lower power one such as2N3906 given the low current rating of solar cell I intend using? Or are there certain properties of the TIP32C I need to look out for?

5) Can the LED from output pin 7 simply be connected (via a resistor) to the battery (+) terminal instead of the +5V rail? In a solar cell charger wherepower source is limited could it save power this way by drawing on the battery rather than the solar cell to light the LED?

many thanks Sam

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May 31, 2010

Yes, it would be possible to adjust the resistors for a higher temperature shut-off. Id have to calculate new values that would work.

It would be difficult to totally eliminate the trickle charge, but by using much higher values for R1, R2, and R4, the trickle charge current would bereduced proportionally.65mA is actually about 1/3 to 1/4 of the recommended slow charge rate. In effect, youre already trickle charging at that rate, so just using a diode isprobably the best way to go.You need to choose a transistor with a collector current limit higher than your charging current. Youll also need to adjust R5 to get the desiredcharging current, depending on the beta of the transistor.Drawing power for the LED from the battery wont help, since the batterys power is ultimately coming from the solar panel anyway.

Simply using a diode is probably your best choice. That will reduce the voltage a bit, and naturally taper the charge rate as the cells become fullycharged.

SamJune 01, 2010

Hi Stefan

Thanks so much for getting back to me, very helpful and much appreciated.

In terms of creating a tropical climate version how about just an additional resistor between TR1 and ground to provide the additional resistance (in effectbiasing Vtmp), and might even be used with a SPDT switch to toggle between a temperate and tropical version? But otherwise I look forward to hearingyour new calculated R values.

In terms of charging rate, using both the hFE / beta / gain of the transistor and your equation for R5 = 1.6 x I, suppose it could be modified with a SPDTswitch (between pin 1 and base of Q1) to toggle between 2 values for R5 to select slow and fast charge? I dont know much (if anything!) about batteriesbut might there be a certain charge current which is more efficient or more beneficial to max capacity or long-term health of the battery rather than the maxUSB current (i.e. 500mA)?

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Stanley MontanoAugust 29, 2010

me parece muy buena aportacion

Arpit NemaSeptember 04, 2010

A very nice and well explained circuit.I have a question though. Can a differentiator circuit ( http://www.allaboutcircuits.com/vol_3/chpt_8/11.html ) be

used in place of the thermistor? Because it should be better for places with large variation in ambient temperature through the year (like where I amcurrently it varies from >34 C to


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Arpit NemaSeptember 05, 2010

No no.What I meant to ask was using the dV/dt to stop the charging,across the batteries.No measuring of temperature at all. So will it work?

Stefan VorkoetterSeptember 05, 2010

Ah, I see what you mean. In theory yes, but would probably be very difficult to make work, for the same reason (the times involved are very long).

Typically, a dV/dt charger will use a sample-and-hold circuit to sample the voltage at some time, and then compare the voltage against the sample a shorttime (a few seconds) later. See my reply to Ahsan in the older non-Facebook comments below.

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Ben NeweySeptember 12, 2010

i need help getting these parts and there any special tools needed to do this??

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p ,

Sorry Ben, but I cant give you much help getting the parts. I suggest you order them from DigiKey. Youll probably have to order the alternate thermistor

and corresponding values for R1-R4. As far as tools go, normal electronic assembly tools are all youll need.

ToNeSeptember 28, 2010

Can this work with one battery, i am working on a mod.

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ToNeSeptember 28, 2010

Can this charge one battery only with out over charging?

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ToNe. It will work fine with a single cell, but wont be as efficient. Just make sure that the thermistor is in physical contact with it.

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To measure the charging rate, insert an ammeter in series with the battery (i.e. between the positive terminal of the battery holder and the collector of Q1).

Benjamin JusufovicNovember 16, 2010

I have completed a full numerical analysis of your circuit. It is a clever design but I believe that you are missing a pull down resistor at your collectorterminal. If you make this resistor large, it will not dissipate power and will serve as a voltage reference for your coupling capacitor, allowing for the circuitto begin charging as you explained in your report. Without it, you might run into some problems when trying to charge your batteries.

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Benjamin, youve clearly left something out of your numerical analysis, because the circuit does work as described, both theoretically and actually.

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Lnw ????November 22, 2010

good

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Sam LumajangNovember 25, 2010

Its very good circuit.

Pranjal SinghDecember 21, 2010

nice!

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Brady WenJanuary 17, 2011

is it necessary to have the thermistor? can it be replaced by another component? say,a normal resistor? thanks

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Stefan Vorkoetter

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January 17, 2011

Brady the thermistor is the key to the operation of the entire circuit I suggest you read the description of how the circuit works



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Brady, the thermistor is the key to the operation of the entire circuit. I suggest you read the description of how the circuit works.


ah yes,ive just read it.my bad,didnt read fully before i posted

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and thanks for the idea, i will try it out soon

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Jah FrancisFebruary 11, 2011

can u precise for duds like me? the A-Z So i can do it and praise u

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Not sure how much more precise I can be. Everything is in the article.

Constantine RaphaelFebruary 13, 2011

wow

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PaddyFebruary 15, 2011

Thanks for all your hard work on the battery tests. I was about to order batteries for some occasionally-used remote controls and came across your review;just what I needed as long as it doesnt turn out that the Sanyo VIPs handed you a big wad of cash to talk up their batteries!

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Stefan Vorkoetter

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February 15, 2011

I guess youre referring to my Eneloop review article (http://www stefanv com/electronics/sanyo eneloop html ) No Sanyo didnt pay me anything They



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I guess you re referring to my Eneloop review article (http://www.stefanv.com/electronics/sanyo_eneloop.html ). No, Sanyo didn t pay me anything. Theydid send me a few other things to test _after_ they discovered my review, but Id had no contact with them at the time I published it. Note that there are

other brands of low self-discharge batteries too. See my comparison review: http://www.stefanv.com/electronics/low_self_discharge.html

Marcelino Espinosa

March 23, 2011

Can i charge 4 batteries with the usb charger?

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Marcelino, the only way to charge 4 cells is to charge them 2 at a time.

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Robert Van CleefApril 04, 2011

Very interesting concept. I need something for 9v batteries but this is a promising start. Thanks

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Greg WilliamsApril 11, 2011

Will this work for other batteries? I am trying to charge a 4 cell 4.8v 200 mAh NiMH battery pack for a small robot? Since this circuit is based ontemperature, I am thinking it might work?


Greg, see my comment to Marcelino below.

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Scott Schroeder

April 18, 2011

Well put together; from schematic, to partlist VERY detailed with even a built by pic. Every detail you can imagine you need you get with this rundown. Well done sir.

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Im building this for my eneloop batteries. Cost much cheaper with a good feature and quality. Good advantage for us electronics enthusiasts. Thanks for

sharing this Stefan.

Ludwig Van Jauhari

April 29, 2011

I was wondering if i can charge single cell with this charger without replacing any components? is there any negative impact on the battery if i do that?

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Ludwig, charging a single cell will work fine. The charge current will be the same as for two cells. The only concern is that Q1 will get hotter, since itsdissipating more power (about 1.5 to 2W), so a bigger heatsink would be a good idea.

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Fadi SarkisMay 16, 2011

I think it will function correctly but it prefer to put a 0.2 fuse before the USB ports and use a bigger heatsink for the transistor.

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Fadi, a 0.2A fuse wont be enough, since the circuit draws almost 0.5A from the USB port.

Rapshaddie KhanMay 26, 2011

thanx for the article i will try it and let u know what happened

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Andrs Ignacio Fuentes CartesJune 29, 2011

Thanks!!!!

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Aaron Gomez

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Ismail KaziSeptember 23, 2011

this is helpful. could you also help with solar panel 2 AA battery charger? Solar panel is 6V/100mah


Ismail, at 100mA, it will take about 40 hours of direct sunlight (i.e. panel pointing directly at the sun) in order to charge two AA 2500mAh cells. In anycase, the charge current is low enough that you can simply connect the panel directly to the battery through a diode. You wont need any circuitry other

than that.

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Wiwied Soeparto

October 03, 2011

if i need to charge the 3.7v battery pack, which component need to be replaced and what are the values?

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Bintang SedayuOctober 04, 2011

biki hhh



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bikin ahhh


Wiwied, to charge a 3.7v battery, you need a completely different circuit. This charger is for NiMH batteries only, but a 3.7v battery is Lithium Ion. Youwill destroy the battery if you try to charge it with this charger.

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Jake DidionOctober 29, 2011

Stefan, I just finished building your circuit successfully. Very very cool thanks a lot for your help with this website!

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Mervinlee TanNovember 10, 2011

First of all, thank you very much for this wonderful circuit. I was curious on how to make this then I stumbled upon your site. great work sir! For my

question, I am curious on how you computed the values of r1, r2 and r4 and how r4 works. thank you!

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Clemens ArthDecember 29, 2011

This is really nice work and although already some time passed since you did it you tackled a still active problem I guess. Stefan, I highly appreciate itand I wonder if you can comment on changing the hardware for charging a 3.6V NiMH battery pack. In principle it would involve a third cell only, but Iguess some resistors have to change for the higher charge voltage. Do you think this could work out?

Abdullah Bin Mat IsaFebruary 01, 2012

Nice. and complete information thanks.

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Robert PhilipsFebruary 06, 2012

Is there no way to modify the circuit to charge four 1.2V AA batteries?

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John Albert PinedaFebruary 24, 2012

Can I use a 1 micro fard 50v capacitor than in 10v capacitor?



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Can I use a.1 micro fard 50v capacitor than in 10v capacitor?

Anil Sethi JnrMarch 09, 2012

DUDE !You are truly Awesome ! Extremely Legendary Work, i have only started learning electronics this year and this tutorial is perfectly & simply worded evenfor the likes of me !

Many thanks for sharing!

I also have a small solar panel but need it to charge a 9-volt battery for my Knight-Rider Circuit!

(5 volt Solar Panel @ 70mA & a 9 volt battery is 175mah, Ni-mH)

Could i also just connect the solar panel straight to the battery with a diode (also what model number diode 1N4148, 1N4007, also which way does thediode go & on what terminal of the battery/solar panel ?!!? ? ?) ?

Many thanks in advance if you could find a second or two to reply !

And apologies for bothering you with what seems very trivial work for you !

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Also i am having EXTREME difficulties trying to find C1, a 0.1F 10V capacitor in the UK, NOT even ebay sells one !

From what ive been learning about capacitors, as a rule of thumb, ish, whatever the input voltage of the circuit is, the capacitor should be double thisamount



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amount.

So for the 5 volts your awesome circuit uses, a 10 volt capacitor makes perfect sense (even to a learner like me!)

But the UK is in the damn-stoneage when it compares to countries like USA for sourcing components.

The closest i have found to getting the mentioned capacitor in the ingredients list (also from USA from ebay!) is an SMT/SMD capacitor which i dontmind using as i have very steady hands, but would this capacitor below work with through-hole components ???

http://www.ebay.co.uk/itm/0402-0-1uF-X5R-10V-Taiyo-Yuden-RMLMK105BJ104KV-F-100pcs-/160318833833?pt=LH_DefaultDomain_0&hash=item2553bf44a9

Again, any help in this direction would be most-appreciated !


Anil, a 5V solar panel wont charge a 9V battery unless you incorporate some sort of voltage boosting circuit.

The voltage rating of a capacitor should be at least double the operating voltage. So any 0.1uF capacitor of 10V rating or MORE will do. These days, even

the small caps have very high voltage ratings. A 0.1uF cap is probably the most common component in existence, so Im sure youll find one. If the UK_were_ in the stone age, then maybe youd be able to find a 10V one.

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Anil Sethi Jnr

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March 10, 2012

Many many thanks for the extremely speedy response Stefan, i am truly grateful!



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I am trying to mirror your exact ingredients list as i want my attempt of making your most-excellent & very efficiently put-together USB AA batterycharger as close as possible but have tried looking for a 0.1uF 10v capacitor from 4 of the UKs MAJOR component sellers and NONE of them are sellingwhat i need, except the SMT capacitor (Surface-Mount-Technology, as you know, lol) !

Please would you be able to post any tech specs for the exact capacitor you used in your design or would it be ok to use the below capacitor (very cheap,100 for 0.50 pence!): {even though the below capacitor is ONLY 0.5mm (H) x 1mm (L) x 0.5mm (W) in size !!!!!!! ?}

http://cpc.farnell.com/multicomp/mcca000505/mlcc-0402-x5r-10v-0-1uf/dp/CA06746?Ntt=0.1uF+10V

And lastly could you please point me in the right direction for voltage doubling circuits/reading guides as i have been searching for the past two weeks for away to double voltages for my projects but still havent even found as much as a tutorial regarding these circuits of they are far too complicated for mylearner-level ?!!?

Once again i humbly thank you for your help as i stand back in awe of you great tutorial !!!


I cant find ANY 0.1uF 10v capacitor !!!

Not even one !

Not even one priced at a ridiculously high price!

Only the surface-mount capacitors that match your ingredients list exactly, but would this mean that all the components have-to be surface mounttechnology or can i get-away with using the SMT capacitor with the through-hole components despite the huge difference in size ?

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{also all LEDs sold here in UK are at a very minimum of 20mA, cant find a single LED rated for 10mA, again, unless its an SMT component!}

Is it ok to use a 20mA LED instead ?!!?



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So sorry about harassing you again with these trivial questions as im sure a man of your calibre would have more important things to do other than tobaby-sit a learner in electronics, but the help is extremely appreciated & very gratefully received !


Anil, as I implied in my previous response, the voltage rating of the capacitor doesnt matter so long as it is AT LEAST 10V. In short, you can use prettymuch ANY 0.1uF capacitor. The only place you wont be able to find one of these is in Antarctica.

With LEDs, the current rating is also a maximum. Any red LED with AT LEAST a 10mA rating will do.

Voltage doubling circuits that can produce sufficiently high current to be useful for charging a battery are non-trivial. On the other hand, for a 9V battery,youll only need about 100mA or so. Im afraid youre on your own finding what you need though. Google is your friend.

170.


No worries, youve clarified the capacitor issue (again, lol) which was more my pressing dilema!

Again many thanks !!

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Kent, you cant use this circuit to charge a Lithium-Ion battery. The charging method for these is completely different. Its not just a matter of using the



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right voltage.

Simeon

April 23, 2012

I dont know if somebody ever try to make this charger because I did it.Very good in theory.But very unstable in practice. What I mean?I dont know maybe the problem is with my thermistor It seems its more than 10% / degree: room temp charger wont start few minutes in refrigerator here we go a half an hour in refrigerator here we go even without a batteries.So for now I put a potentiometer instead R1 and make it somehow but it is not the end. Maybe I should try different thermistor.

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Stefan Vorkoetter

April 23, 2012

Simeon, the charger works fine if you use the thermistor specified in the article. You cant just substitute a part like that for a totally different one andexpect it to work.

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SimeonApril 24, 2012

Hi Stefan.



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You are right I cant make substitutes and expect to work the same way like yours. Also R1 from my previous post is actual R3 and the potentiometer is47k. It works for now. The problem is in the thermistor I live in Bulgaria and its imposible to find exactly the same thermistor here. So I have to start withits parameters at first. If I give you the parameters of my thermistor can you calculate the other resistors for me? I would be very grateful.I find the idea and the sheme very good and thats the reason I start making it. Maybe should be used two thermistors just to measure temperaturedifference not the batteries 33deg. because its not the right temp for summer in many countries. The second one instead R3.Thanks.


Hi Simeon: Yes, a design with two thermistors would be a bit more flexible, but its best to charge at a reasonable room temperature anyway. But I realizethat that isnt always possible. Anyway, if you tell me the specs for your thermistor, I can figure out the resistor values.

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TonyApril 25, 2012

Can I replace the 2-cell to 4-cell battery holders? any changes need to be made?

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I am from India.

I have read the whole article & all the comments till date.(since 2007)

Your circuit is so good and I will definitely make it and post comments



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Your circuit is so good and I will definitely make it and post comments.

I will keep a watch on your site for similar circuit ideas.

Thank you for sharing!

EricMay 15, 2012

Hi Stefan, you are a very patient man. Are you sure it wont charge three cells??? Just kidding- thanks for the info!

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GamenJune 12, 2012

Hi Stefan,

Thanks for your great circuit.When correct thermistor cant be found, think a npn transsistor like BC547 could be used instead of thermistor changing resistor R3 and adding a resistoron this network acording to triger second amop. Using only base and emitter pins (np union), this voltage np union decreases when temp increases.transistor is more sensitive to temp than diode (like 1N400x).Thanks again for be so patient.

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August 25, 2012

Hi there Stefan. First, great work! Congratulations.I got a question about the project: can i use it to charge a 9v single battery with no changes in the circuit? I want to use it in a circuit for leds and need somemore tension 1 2v will not do the work



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more tension. 1,2v will not do the workThanks!


No Guilherme, it cannot charge a 9V battery, since the power source is only 5V. To charge a 9V battery, you would need a step-up converter.

187.

GraemeSeptember 08, 2012

Hi StefanFirst off Champ! Thanks for all your time and effort for not only developing and show it to all of us this project but to also answer all the question still

coming years later as well.Like Guiherme, I too would like to charge a 9v battery as well.As for step-up converters I have been looking at the TPS6734IFIXED 12-V 120-mA BOOST-CONVERTER SUPPLY.Since the rechargeable 9v battery I have is only 350mAh the 120mA max should be fine (yes/No?)If so then my main question is, should the step-up converter circuit be in-front of your circuit or after it?Ive looked at the data sheet for the LM393 and it can handle the 12 volts.

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If there is anything I should add, change, adjust or remove.Any advice would be much appreciated.Thanks Champ!

189



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ColinSeptember 14, 2012

Hi Stefan,I want to use your circuit for battery backup. I am not electronics eng. Please would you express a view on my intended addition to your circuit if it isstupid yes or no.

P Channel MOSFET

BAT IN - Vout| | | |

| v | | | || || |USB IN-|>| |Schottkey

Diode

189.

Colin

September 14, 2012

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Hmmmmm, me picture got screwed

P Channel MOSFETBat In >> DUSB In >> G



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USB In >> GDiode >> between G & SVout to >> S

ColinSeptember 14, 2012

Sorry, me again I forgot to mention the MOSFET is a P-channel enhancement-mode device.

191.

AmyOctober 03, 2012

Hi Stefan:

Great circuit! btw how do you simulate the Battery voltage versus time? I tried to simulate it on Altium, should I replace the battery with something else inorder to get the waveform (as it stays constantly at 2.4 V with a battery source at the moment)

Thanks for your help!

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You need a more sophisticated battery model (i.e. one that models state of charge, internal resistance, etc.) in order to simulate any sort of battery chargercircuit. However, I didnt use a simulator at all. I designed the circuit on paper, drew up the schematic in an old DOS schematic drawing program, and builtit. Its too simple to need to be simulated. But if I were to simulate it, I would use MapleSim these days.

keeping fitOctober 06, 2012

Hmm is anyone else encountering problems with the images on this blog loading?Im trying to find out if its a problem on my end or if its the blog.Any feed-back would be greatly appreciated.

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