INGENUITYUNLIMITEDOur regular round-up of readers' own circuits. We pay between£10 and £50 for all material published, depending on lengthand technical merit. We're looking for novel applications andcircuit designs, not simply mechanical, electrical or softwareideas. Ideas must be the reader's own work and must nothave been submitted for publication elsewhere. Thecircuits shown have NOT been proven by us. IngenuityUnlimited is open to ALL abilities, but items for consideration inthis column should be typed or word-processed, with a briefcircuit description (between 100 and 500 words maximum) andfull circuit diagram showing all relevant component values.Please draw all circuit schematics as clearly as possible.Send your circuit ideas to: Alan Winstanley, IngenuityUnlimited, Wimborne Publishing Ltd., Allen House, EastBorough, Wimborne, Dorset BH21 1PF. (We do not acceptsubmissions for IU via E-mail.)Your ideas could earn you some cash and a prize!

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50MSPS Dual Channel Storage Oscilloscope 25MHz Spectrum Analyser Multimeter Frequency MeterSignal GeneratorIf you have a novel circuit idea which would beof use to other readers then a Pico TechnologyPC based oscilloscope could be yours.Every six months, Pico Technology will beawarding an ADC200-50 digital storageoscilloscope for the best IU submission. Inaddition, two single channel ADC-40s will bepresented to the runners-up.

290 Everyday Practical Electronics, April 2001

12V Sealed Lead/Acid Charger – CCyycclliicc BBaatttteerryy UUssee

CONSTRUCTORS have often been advisedthat it is unwise to charge a sealed 12V

lead/acid battery directly from a simple “cartype” charger which usually consists of atransformer, bridge rectifier and a meter thatgives some indication of the charging current.There are good reasons for this, including thefact that a simple car battery charger is notsuitably current limited and can quicklysizzle a badly discharged sealed lead/acidbattery.

It is recommended that the charging cur-rent for a sealed lead/acid battery is limited to25 per cent of the battery’s Ah (Ampere-Hour) rating. For example, an 8Ah batterycan supply one amp over an eight hourperiod, four amps over two hours and eightamps over one hour and so on.

It is unlikely that the output voltage from astandard charger is suitable for charging a12V sealed lead/acid battery. A constant andstable voltage of between 2·4V and 2·5V percell is required for cyclic charging whichequates to 14·4V to 15·0V for a 12V battery.However, the circuit diagram of Fig.1 showsa method of charging sealed lead/acid batter-ies using a basic car battery charger with theaid of an L200 voltage/current regulator chip.

The off-load output voltage from a typicalbasic car battery charger is 13·0V as mea-sured, which is taken to a 2,200µF 50V elec-trolytic capacitor C1. This smoothes thecharger output and increases its available d.c.voltage to just over 20V, providing enough“headroom” to overcome the voltage dropacross the L200 regulator and diode D1.

On The LimitThe value of the current limiting resistor

(R1 to R6) is determined by measuring theopen circuit voltage across pins 2 and 5 of theL200 (with power applied to its input). This isthe reference voltage and should be in theregion of 450mV which is divided by therequired output current (2·0A maximum). Forexample, Vref/required current = 0·45/0·2(200mA) = 2·25 ohms.

The output of the charger has anadjustable current limit, consisting of six

low value resistors wired to a 1-pole 6-waywafer switch. This enables the current tobe reduced, enabling a good range ofsealed lead/acid batteries to be charged.The resistor/switch combination is con-nected between pin 2 and pin 5 using shortleads.

The diode D1 prevents any current flowingfrom the battery being charged, through thepotential divider (R7 and VR1) should thecharging source be removed with the batteryconnected. With the selected current limitresistor in circuit and power applied to theinput of the regulator IC1, adjust VR1 for avoltage of between 14·4V to 15·0V as mea-sured between the cathode (k) of diode D1and 0V line.

When the above adjustments are completethe battery may be connected and the charger

switched on and left as the battery will auto-matically draw less current as it reaches itscharged state. A full charge should take about10 to 14 hours.

This can be monitored by the ammeter ofthe battery charger, but a more accuratemethod is to monitor the voltage across thecurrent limiting resistor using an externalvoltmeter. The actual charging current canthen be determined by the application ofOhm’s Law, i.e. the voltage across theswitched resistor network / the value of resis-tance in ohms.

(Readers wanting to know more about theL200 should check Andy Flind’s feature arti-cle “Using The L200CV’’ in EPE July 1998 –ARW).

David Allen,Cheltenham, Glocs.

Ω Ω Ω Ω Ω Ω

Ω

µ

FIg.1. Circuit diagram for the 12V Sealed Lead/Acid Charger. (The low-ohmresistors (R1 to R6) can be from the W21 series.)