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Hi all,

Can I ask what charger was being used and whether it was a balanced charge?

I am only curious because I am not a massive advocate of Li-Po bags because I prefere prevention to be better than a marketing cure.

Of course, if your batteries are on fire, then a bag to take them outside would be great, but I'd preferred the fire not to have started in the first place.

I am concerned that everyone may think that using a Li-Po bag makes them safe, instead of understanding and minimising the risk imposed in using Li-Po batteries.

There are mainly two scenerios in which you are more likely to have an issue.

Bit of background (sorry if its teaching you to suck eggs)

Li-Po batteries used in RC racing typically consist of 3 cells connected in parallel, connected in series to another 3 cells connected in parallel.

The physical configuration of the batteries doesnt really matter for the point of this, can be saddle, square or stick packs, its all the same inside.

The reason for the 3 cells in parallel is because of the high discharge current required to do the fun hobby we all enjoy, very very quickly.

Typically each individual cell will happily discharge at more than 25C and charge at around 1C. This means that if you had a 5000mAh set of batteries, you can theoretically discharge them at 5000mA x 25 = 125A for a very short period on time (about 60s).

Ofcourse the connectors, battery tabs, wires and solder would fall off but theoretically this is possible. The 1C charge means that you can charge these batteries at 5000mAh x 1C = 5A.

Scenario 1

This all sounds great but if two of the cells inside the battery are no longer connected - i.e. breaks in shipment, manufacturer being terrible, or during vibration whilst using them, then the problems begin.

Back to the example, we had 5000mAh owing to the 3 cells in parallel, two disconnect, now we have only (5000mA/3) = 1666mAh. Then we go and charge them at 5000mA and bobs your uncle you are now charging at 3C.

Cells catch fire or if you are lucky, just swell.

The problem exists in the discharge scenerio, but is less likely to start a fire owing to now the maximum discharge current is 1666mAh x 25C = 41A which cars might get close to, but only for a small part of the track. This also explains why most issues are on the bench, not on the track. Typically, on the track incidents are caused by unlucky vibration, causing mechanical failure of solder joints, which by chance causes issues. Or your ESC being terrible (and from Tamiya in my experience) !!!!

The battery monitoring gadget specified previously will not correct this scenerio, if there is a single cell that remains connected inside the battery the potential or voltage reading will be the same.

The best way to reduce this risk is to always use a charger with a balanced charge whilst charging.

Scenerio 2

Using the same issues in scenario 1, once the bad (or broken) battery has less cells than the good battery thats in series and you DO NOT use a balanced charge, then the faulty battery will get to 4.2V before the good battery.

The charger, is not aware that one battery is at 4.2V whilst the other one is still charging (lets say for example, 3.9V). It only knows the total voltage i.e. 3.9V + 4.2V = 8.1V. The constant current part of the charge cycle will continue to charge the broken battery, which will go over voltage and is most likely to catch fire.

The best way to reduce the risk is to always use a balanced charge whilst charging.

So, from all this writing "or ranting", I suggest everyone uses a balanced charge !!!! It is the safest way to charge the battery packs. The charger will monitor the "middle" voltage and try to balance the cell, which will take ages if its broken, because typically chargers only have 1A balancing current.

This makes a very nice visual (or timely) aid. If it takes ages to charge from a balanced charge, then something is wrong !!!

Li-Po bags do have there place in our hobby, but unfortunately, all Li-Po bags are made from a material which cant be see through. I'd prefere to have a Li-Po bag available to put my batteries in if they did spontaneously decided to catch fire, then walk outside with them disconnected from my charger. This does however mean you also need a heat resistance pair of gloves, but at least you can see what is going on !!!

Cheers

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