5. INTERNAL SHORT CIRCUITS – this is mostly caused by contaminants getting into the cell. You would be amazed how many manufacturers operate in a nearly open air environment. One of my overpaid consultants uses a very simple tool – a magnet – and waves it over the exposed electrodes (prior to cell assembly) and it’s amazing all the metallic contaminants that little magnet picks up. Contaminants can poke through the separator over time. If you are lucky, the cell will just have a high self discharge rate. If you are unlucky, the microshort will cause a localized heat buildup and create a thermal runaway condition – and thus another cozy fire. Another source of internal shorts is the punching process the manufacturer uses to stamp out the anode and cathode electrodes. Most of the low end manufacturers use a low cost steel rule die that cost maybe a couple hundred dollars. The better manufacturers use a high end die that cost a couple orders of magnitude more. The problem with cheap steel rule die punch is that it tends to leave burrs on the electrodes. These burrs have a tendency to puncture the separator and create microshorts. This microshort will create an area of localized heat. In most cases, this will cause the cell to puff up. In bad cases, this localized heat may be enough to ignite the cell. Every time you charge a cell, the cell will expand about 5% in the thickness dimension. This expansion/contraction may cause the burr to eventually rub through the separator. The vibrations and shock from an RC car also rubs the burr against the separator. Most of the lower end manufacturers don’t perform a simple high pot test (apply a high voltage and check for current leakage) to check for small internal shorts during the manufacturing process. The big Sony recall was largely attributed to burr type contaminants as well as Dell’s non-standard pulsed fast charge technique during the CV part of the charge cycle.

What can YOU do to minimize the risk of this type of failure? - Not much actually. It’s not like we are able to audit every cell manufacturer and even if we did, most of the RC cell distributors hide the manufacturer information. Of all the factories I visited, Saehan Enertech had the highest level of quality control and cleanliness followed by Kokam. Enerland is passable though they lag behind Saehan and Kokam. I don’t think I am allowed to list some of the factories I visited that are downright dangerous.

As a distributor – if a lipo cell manufacturer claims that they can produce a custom size cell tooling for you under $2K, then it’s an indication that their electrode punching process is dangerous.