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Using LI Po Batteries Safely

In order to present as clear a picture as possible and to guide in the safe use of Li pos, no matter the manufactuerer, you will find at www.fmadirect.com under the Support section the Kokam Battery Systems Ap Note, Ap Note # 2 in pdf to download.

The following is a brief supplement to the Charging and safety sections of the Ap Note. The principal things to remember: Li Ion and Li Poly cells have Lithium in them and that is why they have five times the energy density of other chemistries. Powdered Lithium, if heated sufficiently , can ignite and burn. Understand: we can test, report, educate, add in any kind of safety device but, as long as Li is present there may be some way that it might be ignited. In the ultimate, suppose a lighning strike hits your model! The only thing to do in all this is to charge the packs in such a way that , if they do ignite, no harm is done.

Please do take time to read the following and download the information in the Ap Note.

All high energy density batteries including Ni Cd, Ni MH, Li Po, and LI Ions and the chargers used require common sense and caution. If any are overcharged or shorted, great heat and pressure result. Ni Cd and Ni Mh cells have a mechanism to vent excess gas pressure as do Li Ion cells. These cells all have in common, a thin metal can enclosure. I have experienced explosion of Ni Cd cells when the vent did not function properly. One such occurred at 1 AM in a deathly quiet shop as I worked on an Army radio system in 1984. That was behind me and about 15 ft away. You probably never saw a 55-year-old, 225 lb guy clear a 4 ft workbench flatfooted! I didn’t even bother sending the 4AH cells back to Sanyo since it was a charger malfunction that caused the event.

Li Ion cells truly can explode as they are sealed in a metal can. They too have vents. However, Lithium is a metal that, as a powered material can burn if ignited. This is true of several metals, not just Li. Magnesium burns readily even in solid form. Thermite is powdered iron that, when ignited, has been used to weld steel. Finely powered aluminum is the “fuel” for almost all solid rocket motors. Some solid rocket motors are made of extruded nitrocellulose, an organic material. Organic materials burn when ignited, just like paper. Powdered, sintered nickel takes a very high temperature to ignite.

Li Po cells also can vent if charged at too high a voltage. There is a narrow range of choice of the electrolyte for use in Lithium Ion cells. Remember that Li Po cells are a form of Li Ion; they derive their name from the fact that Li Po cells are housed in a plastic (polymer) envelope. If the envelope has a small Vee cut in the join line, that serves as a vent. The major difference with Li Po is that the envelope can swell when pressure builds to form the infamous “silver sausage”.
Any cell is ruined when pressure that causes venting is experienced.

If a Li Ion cell suffers ignition, the vent cannot act quickly enough to prevent rapid pressure build up. When this happens, the can fails instantly and catastrophically just as it can in a Ni Cd/Ni MH if the vent does not function properly. The pressure release is, therefore, explosive just like popping a balloon only with massively more force. This is why all Li Ion cells used in OEM applications such as cell phones have a protective circuit on them.

The failure mode that leads to explosion in a Li Ion leads to an event called “venting with flames” in a Li Po cell. The basic phenomenon is called thermal runaway. If, say, a Li Po cell is charged at six to seven volts, well above the nominal 4.2 V limit, the electrolyte can begin to “boil” and develop voids as temperature rises above about 180 degrees F. If this abuse continues for, say, ½ hour, the electrolyte, being organic, can eventually ignite. As we said earlier, it takes a lot of heat to igniter Lithium. In a solid rocket motor, ignition is initiated essentially by a high explosive blasting against the propellant.

If the thermal energy release of the electrolyte used is high enough, the Lithium can be ignited. In tests I have conducted, the electrolyte burns at about the intensity of burning paper when it has a heat gun blasting it. When I light the fireplace I winter, I wad up newspaper in softball size wads and put in a layer before I put wood on the grate. If I have light, dry kindling, just igniting the paper with a lighter lights the fire. Last winter was so nasty that we ran out of kindling. I found that the thermal output of the wadded paper could be increased sufficiently to ignite reasonably dry maple logs by blasting the paper with my Monokote iron. The point: Subtle but significant changes can affect ignition. Not every overcharge event causes ignition.

If the lithium ignites, it burns with an intensity and gas generation that can cause “venting with flames” that is the gasses exit the envelope with a swoosh, not a blast. If you have the pack in your airplane when this happens, your airplane is going to be damaged. If you have the pack on a highly flammable car seat, the seat is likely to catch fire.

Does this happen often? Not really; we have about a dozen such events reported in the past 18 months out of perhaps 100K cells in the field and, probably, a million or more charges. In all instances, analysis of the event has shown that the cell/pack was charged at too high a voltage and/or there was a fault in pack assembly.

Methods that are as stress-free as possible that permit one to use Li P cells in a completely safe way are outlined in Ap Note 2 located at www.fmadirect.com Open the home page, click on Support then scroll to Ap Notes to open or download the pdf file for Kokam Li Po battery Systems.

It is a simple matter to operate safely. Just as you are asked to avoid smoking while handling an open can of glow fuel, keep your hand out of the prop, don’t whittle toward yourself, and don’t fly while drinking, it is suggested that the simple warnings posted at our web site be followed. Remember, safety is a matter of discipline. Remember also, that we take care to educate the user about these things.

The safety device used by George Maiorana to change KOK 1500 4S3P and 4P packs is shown in the Ap Note. $15 spent on such a charge center is the cheapest insurance in the world. The white lining is sheet rock. I tested sheet rock by directing a propane torch flame against one side while I touched the backside with no harm. George has been handling several packs of KOK 4S4P cells with the LIPO 402 charger and his charge box for several months with no problem. Testing to date at FMA Dorect of the Kokam Safety Guard indicate that it could prevent such a charger failure from resulting in a battery fire. Very soon now, we will post a full report on Safety Guard as part of the subject Ap Note showing the various ways it can be used. A shipment of Safety Guards is being readied at this time.

George also charges the 8S4P pack used for his conversion of the 80 inch, 9 lb Great Planes Lancair, attached, that is reported to fly like a dream.

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