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PLODING NiMH 23/43 CELLS

No doubt you will have read or heard the stories of batteries exploding and that the Norwegian Federation has temporarily suspended all electric racing. We have been monitoring these ‘incidents’ but obtaining hard facts is not easy, making it difficult to draw any meaningful conclusions.

Given that there are thousands of cells in circulation, such incidents have been exceedingly rare in the past. The only serious incident that has occurred to date in UK was when a receiver pack blew, that has permanently damaged a person’s eyesight. Although there will have been more injuries caused by scalpels or super glue in eyes than caused from exploding cells, you still need to take care!

The NiMh cells have a chemistry that allows the cell to 'self-discharge' over a relatively short period, (days as opposed to weeks). Lately, some of the 4200 cells seem to self-discharge more rapidly than other/previous versions. More importantly the amount each individual cell within a pack discharges varies (i.e. cell 1 might be .9v, cell 2 .8v, cell 3 .9v, cell 4 .7v, etc.)

If charged in this state, some chargers will not know when to stop the charge process, as some cells are demanding to continue with the charge, whilst others could be over charged. It is also important to control the delta peak (voltage drop-off after completed charge) to a low value when charging. Once the cell has achieved full charge, any further charging will produce gas. The cells have a pressure vent, but if that is blocked or not working, the gas pressure could cause an explosion.

High charge rates will also produce more gas and reduce the cell’s useful life. It is possible that internal gas pressure may cause distortion to some internal parts that could result in a short circuit within the cell. An internal short circuit combined with gas pressure is likely to be a 'big bang'.

Drivers need to recognise that the 'matchers' recommendations are intended to achieve maximum performance. It is the recommendation from the cell manufacturer that should be used.

Best Practice for use of NiMH cells.
1. Equalise cells before charging if they have been stored (with charge) for more than 2/3
days. If any of the cells in the pack show that they do not need to be equalised (i.e. the
light does not show or goes out straight away) then remove the pack from the equaliser
and charge for a short period (approx. 5 mins. should be adequate). Then carry out the
equalising process.
2. Never exceed the manufacturers recommended fast charge rate. This should be 1C max.,
even if the manufacturer states higher.
(C being rate of charge based on capacity of the cell, e.g. a 4200 mAh cell = 4.2 amps)
3. Disregard any charge rate recommendations by the 'matcher' if it is more than 1C.
4. Use a maximum Delta-Peak setting of 3mV per cell (6-cell pack = 3mV x 6 = 0.018V).
5. Never repeak cells after main charge.
6. Use a temperature cut-out as an additional safety feature set at 42 deg. C max., located on
the hottest cell (usually middle cell). If charging on a cold day then consider reducing this
to around 35 deg. C.
7. Allow cells to fully cool to ambient temperature before further charges. Be aware the
centre of the cell cools slower than the outer casing. Do not put them in water to cool down;
in an emergency cover the cells with a damp/wet cloth if you have to quickly cool them.
8. Store cells with some charge (30/50%).


BRCA Electric Board 16.11.07

Some good advise there from the BRCA Elect Board, thanks for bringing that up Paul.

I've seen NiMh cells explode, been right next to them when they went off like a gun cartridge and it is not nice, there are bits of flying metal and debris all over the place like shrapnel. If you hear the cells hissing during charging you know something is wrong, if any cell is too hot to touch or the heat shrink splits during charge, discard it.
Sadly the worst cells for this are those in stick packs, which most RTR users run and in rotor starts etc, because they cannot be balanced before recharging by virtue of their construction, they can get out of balance very easily and if charged fast they are more likely to pop than a sideby side pack that has been looked after on a balance discharge board. So don't think this only applies to racing, and it applies to receiver packs too obviously!

Great post matey i think everyone should read this and make sure there chargers are set correctly, most decent chargers have an internal setting for adjusting the delta peak THIS SHOULD BE CHECKED as it could be set to high from the factories.

TOP POST.

Marti

We had a lipo explode at the Southend indoor meet last night. No apparent reason but I don't know the background, but it went with a big bang, smoke, heck of a smell and bits of the cell pack all over the place. Luckily it was unattended so no one was hurt. Makes you wonder about the advice to never leave them unattended!

was it in a sack john.... me thinks not?

lipos sacks should become a rules we got it at stafford now

I'd put money on the charge cables shorting out....

LiPo should always be attended when charging, you'd have sen the pack swelling way way before it got to the stage of smoking let alone exploding, and if it had been in a LiPo sack it would have been invisible, so still exploded, admitedly it might have contained the burst, but being charged just on a bench and hence visible means it could have had the charge ternminated way before it got to the dangerous stage and disposed of safely!

I'm not a big fan of LiPo sack rules at all! I'd rather see what's going on and step in to prevent it happening than just clean up the mess later!

that's all well and good charging on the desk top if your going to sit there and monitor it or fiddle with your car for the whole time its charging, at least have a sack if you are going to walk away........ forget about the damaged cell what about the persons eye that gets taken out because someone walked away............!

Even if you just put something on them to stop the shrapnel flying would be a bonus like a bar towel, and i know they burn but would stop the bits.

Only my view be gental with me..........

Anyone seeing a LiPo swell would pull the charge leads surely, rather than sit there and watch it pop. In a sack you've got no chance of stopping it.

Yep agreed, i would for sure but there's some out there with the " its not my problem " kinda attitude, or maybe it just goes unnoticed.

Best to have something is all im saying if you have to walk away. better to be safe than sorry

I have a couple of good LiPo chargers I trust implicitly, not expensive, but good and safe, to the point where they monitor the cells individual internal resistence and stop charging if they think there's a problem, so I have never had a single issue in five years of LiPo usage, and nor have I seen one, even wit ha cell bent 90 degrees around a chassis, so I can only conclude that a drastic failure like that described was a user error, a serious one at that!

I even did a LiPo failure test, trying to justify LiPo bags, but when none of my chargers would do any damage to an already dead, dying and fatigued cell, I had to resort to a NiCd charger, and even then it took a 10 amp charge, and ten re-peaks for it even to get hot!

I've got the pictures and test resultys here somewhere, pretty amazing how pufffed and nasty a cell has to get before it decides to finally give up, more than enough warning. Even when abused. Typically a LiPo cell that is damaged suffers more on the discharge cycle, which is why sop many planes and helis died in the early days, they charged alright, but let go under load. Sure enough the ones I was punishing and puffing finally let go when I applied a 50Amp load to them, whilst puffed up like balloons.

I've been hit by exploding NiMh cells with no warning at all!

Only last Sunday, impressed that a colleague was able to top up his lipo pack after a heat in half the time it took me, I asked what amps he was charging at. 5 was the calm reply. He had a 3200mah pack. Like it or not, there are a vast number of perfectly normal individuals out there that still haven't grasped the simple rules. Sorry Dez, I disagree. If you can't trust people to do things right and safety is an issue, you have to resort to blanket rules, like use a bag. I know that when I'm on the driver's rostrum, the last thing I'm thinking about is how my lipo is doing hooked up to my charger. If someone is sitting next to my pit table (unlikely since he's probably on the rostrum with me) he'll probably be focused on preping his car, not watching my cell pack.

And yes, I've seen NiMh explode in the pits too, a screw driver rolled against it's exposed Corally tubes. Not nice at all.