LiPo Battery Safetycompetitionx.com /rc-articles/lipo-battery-safety/

Learn from our mistakes before you make yours!

I consider myself a pretty knowledgeable RC enthusiast/racer. I’ve raced at every level of competition,driven everything from 1/18-scale to 1/5-scale motorcycles and travelled all over just to play with toy cars.I’ve worked with NiCd, NiMH and LiPo batteries and been around every form of brushed and brushlessmotors. Even with this kind of resume, things still happen that can be considered a ‘newb’ move.

A couple months ago, I had a LiPo battery catch fire while working on one of my cars. I did everything right;used a high-end charger, had it set on the correct mode, positive lead to positive plug on the battery andhad a balance board correctly connected. Even with all this, it still went up in flames and burned my race-ready Tamiya F104X1 into a pile of molten plastic. Thankfully I was able to get it out the back door before itburst (which it actually did as I was throwing it out…arm hair doesn’t smell very good), but this made methink about getting a real experts opinion and sharing with you my experience.

I contacted my good friend Ling. He worked in the product development department of a company thatdeals with medical devices. During his time there, he was in charge of developing a wallet sized devicethat was designed to vaporize liquid insulin. For those of you that don’t know, this type of device requires aton of power in a tiny package. His research led him to the understanding of battery construction, failureanalysis and audits of many battery manufacturers. To say that he has some knowledge of LiPo batteries isan understatement. After he heard about my LiPo woes, he contacted me and we had a chat. We decidedthat this was probably a good time to clear the air about LiPo batteries and this little interview is the resultof that chat.

TONY: Ok Ling, what exactly is inside a LiPo battery? How similar is it to the older NiMH cells?

LING: A LiPo cell is a classic battery construction. It’s composed of an anode, a cathode and a separator.These components are inside the aluminum pouch and it’s filled with a liquid electrolyte. Oddly enough, the“Polymer” in modern Lithium Polymer cells refers to the electrolyte – which isn’t a solid polymer at all. Theanodes are composed from a copper foil current collector coated with a fine particle graphite slurry. Mostcathodes are composed of a LiCoO2 (lithium cobalt dioxide) coated onto an aluminum foil current collector.There are some other mixtures that they throw in there, but for the most part – it’s LiCoO2 based. The

Standard LiPo Battery

LiPo Compatible Charger

polymeric separator does just that, it physically and electrically separates the anodes and cathodes.

Surprisingly, RC LiPos are the exact same chemistry usedin your cell phone or laptop. There is a tradeoff betweenLi-Ion cells: high energy vs. high power. Laptops and cellphones fall into the high energy category and RC LiPosfall under high power. The difference is really just thearchitecture of the cell.

The high energy type of cell generally uses a singleanode/cathode pair with a separator between the two. Thissandwich is generally rolled up in a “jellyroll” format tocreate either a cylindrical cell or a prismatic (rectangular)cell.

The high power type of cells are made of a multitude of anode cathode pairs. This is a stacked cellconfiguration. A typical LiPo is composed of 20-30 pairs of anode/cathodes. The separator is either a “Z”shaped accordion or a spiral wrapped between the anode/cathode. Think of an RC LiPo as a whole bunchof smaller cells where all the anodes are all welded together and all the cathodes are welded together. Theactual thickness of a one Lithium Polymer cell (typically under 10mm thick) is actually limited by thenumber of anode/cathode pairs that are ultrasonically welded together. If there are too many parts ofanode/cathodes, the ultrasonic weld will fail.

TONY: Good to know about the high energy vs. high power. Guess I won’t be hooking up a 6500mAh 65Crated battery to my iPhone. Speaking of that, what does the C rating mean?

LING: Technically, it means the relative chargeand discharge capability. For simplicity, take thecapacity in amp hours and multiply it by the Crating. So for a 5 amp hour battery, 1C = 5 amps,2C = 10 amps etc. The discharge C rating is themaximum continuous discharge rating that a cellcan handle. There are no industry standardswhich define when a cell fails a C rating, whetherit’s the number of cycles, ending temperature,capacity, etc.

In the RC world, the C rating means next tonothing. I can pretty much guarantee you that justabout every RC LiPo would self destruct whendischarged at the C rating on the label unless it’s ashort burst. For a 5000mAh battery, a 40C discharge is 200 amps. A complete discharge at 40C would lastexactly 1.5 minutes There is absolutely no RC application that dumps the battery in just 1.5 minutes. Agenuine Deans connector starts melting at 80 amps continuous – a fake Dean fails at even less.

TONY: Well, I always figured some of those numbers were a bit exaggerated. I’m assuming sticking to aquality, brand name battery is probably the best way to get an accurate C-rated pack. Other than that, isthere any way to determine a quality pack vs. a cheap pack besides price?

LING: For the average consumer, it’s very hard if not impossible. High quality packs are distinguished byhigher safety and abuse limits, higher cycle life, better cell to cell consistency, longer shelf life etc. Higherquality cells have higher standards for incoming material inspection, more in process inspection (high pot

testing, X-ray detection of stack etc), and generally a much cleaner production line that limits the amountof impurities that can enter the cell. Once the cell is pouched, pretty much all cells look alike (high qualityvs low quality) on the outside. The raw cost of a cell from a high quality manufacturer can cost about 4Xmore than a cell with the same specs from a low end manufacturer. Any high quality pack WILL cost moreto produce, however, all expensive packs are not guaranteed to use high quality cells.

TONY: So, as far as LiPos go, it can be a ‘get-what-you-pay-for’ situation. Are there any benefits to hardcases vs. soft cases?

LING: Absolutely! Remember the copper anode and aluminum cathode layers I mentioned before? Theyare separated by a very thin polymer. A dent (especially along the edges) will cause the sharp edges of thecopper and aluminum to damage and start eroding away the thin separator. This is exasperated by thenaturally expansion/contraction of the battery during charge and discharge. Eventually, the eroded hole willlead to an internal short circuit. Best case, the cells short out internally and no longer hold a charge for anextended amount of time. Worst case, the short builds up temperature and leads to thermal runaway – ie.fire. So yes, absolutely, hard cases are beneficial.

TONY: Good to know…I’ll be placing all of my soft-case packs in a case! Hard-case packs from here onout! Let’s talk about charging them. Are there any basic charging guidelines?

LING: There is not much to it. Almost all chargers follow the standard CC/CV charge profile pretty well.With higher charge rates (like 2C and above), there is almost always a big drop in cycle life associatedwith it. The exception is Lithium Phosphate (LiFe), which does very well at higher charge rates, but thosearen’t too common yet…

TONY: So stick with the standard charging guidelines as far as charging amps?

LING: For LiPos, I’d recommend charging 1.5C or under with a charger designed specifically for LiPos.

TONY: A question I’m sure everyone wants an answer to…is it necessary to balance a LiPo pack everytime you charge it?

LING: This is a loaded question. For a high quality 2S pack (7.4V), it’s highly unlikely that the batteries willever need balancing. For higher cell counts, 3S and above, it’s a good idea to balance charge. For cheappacks, it’s always a good idea to balance charge.

TONY: Well, for the sake of safety, is it a problem to balance LiPo packs regardless of their ‘supposed’quality?

LING: Balancing is an insurance policy. It prevents a cell from getting overcharged. Overcharging is asurefire way to get a LiPo cell to erupt into flames. Unlike NiMH or Ni-Cd, Lithium Polymer batteries cannottolerate or absorb any overcharge.

TONY: What does it mean when the battery pack looks puffy or bloated?

LING: It just means that the electrolyte has started to break down. A lot of things can cause this: over-charge, over-discharge, over-heating, cell contaminants, internal corrosion, poor material purity, etc. Ittakes very little liquid to form a whole lot a gas. Unfortunately, puffing is very bad. The anode/cathodelayers are vacuum sealed in an aluminum pouch. This vacuum seal helps prevent the anodes/cathodesfrom shifting around and shorting out on each other. Once puffy, the vacuum is gone and the cell is muchmore likely to internally short out and, as mentioned before, an internal short may lead to fires.

TONY: So, let’s say I’m going to take a short RC break. Are there any good storage tips?

LING: Shelf life is largely determined by temperature. A rule of thumb is with every 10 degrees, the C isdouble the degradation rate, so keeping the cells in a cool spot is the good idea. Also the cells like to bepartially charged – usually between 20-80% of full. A fully charged cell stresses the internal chemistry anddegrades the cell much faster. Think of it as compressing an overstressed spring – if you hold the springmomentarily, it pops back in shape. If you hold it down for a long time – it tends to take a set.

TONY: So a springy LiPo cell is a bad thing. Check. Well, I actually have a few puffy cells here that I’vebeen hanging on to for, ummm, whatever reason. What is the correct procedure for destroying a LiPopack?

LING: I’ve read about the salt water type disposals. The salt conducts and drains the battery; the water isthere in case there is a fire. That will work just fine, but I think it’s cumbersome and overkill. Personally, Ijust discharge the cell at a low amperage rate, usually as low as I can get it. You can run it in your RC car,but I would turn off the low voltage cutoff. You definitely want to go well below the LiPo cutoff. A drainedbattery has no stored energy. A cell with no stored energy cannot ignite.

TONY: I’m assuming you’ll want to run it at a slow speed so as not to stress it out? I’d hate to haveANOTHER car catch on fire as I do this.

LING: If you have a discharger, I’d use that and a LiPo sack. This way youdon’t have to risk a RC vehicle. A discharger can easily take down the energyin a LiPo. If you don’t have a discharger, you can run it at low speeds to bleedoff the stored energy, though there is still a little bit of risk.

TONY: Ok, well, that should just about do it. Thanks Ling, I appreciate yourtime in helping with this article.

LING: No problem buddy, I’m here to help. Just let me win the next time werace – OK?

TONY: You got it!

WRAP-UPAs you can see, there’s quite a bit going on with LiPos but, if used correctly, they are just like any othercell. I’m sure there are a TON of other questions that could be asked so, in the name of safety, I will beanswering any additional questions you might have about LiPo battery safety on this web site.