NiMh battery pack explosions

[SilverFox]

The Norwegian RC federation banned electric powered racing earlier this month following a series of NiMh battery pack explosions. According to this news story, they now have lifted the ban.

We think of NiMh cells as being safer and while we understand that they may vent, we generally consider them safe.

Caution... If you push NiMh cells beyond their limits, they can explode, sending shrapnel flying over quite some distances. People who have witnessed both Li-Po "rapid venting with flame" incidents, and these NiMh pack explosions have offhandedly commented that while they appreciated the rapid venting with flame that the Li-Po packs demonstrated, they were more frightened by the NiMh pack explosions.

Most, but not all, of the problems have been with packs made with IB 4200 cells being charged at 6 amps on hobby chargers. This works out to charging at around 1.4C. Many of us have, and are using, hobby chargers, and I am sure that some of us are charging at rates above 1C. I believe most RC racing people regularly charge NiMh packs at 2C.

The hobby chargers seem to have a flaw. They will try to charge whatever you hook up to them. Unlike the consumer 15 minute chargers, the hobby chargers do not seem to check for questionable cells or packs.

I think there are enough safety features built into the 15 minute chargers, but it is quite sobering to hear about the RC pack explosions.

The general recommendation is to charge at rates no higher than 1C.

Stay safe and charge responsibly...

Tom

 

[Stregone]

Wow. I've never heard of that being a problem with RC cars. Planes on the other hand... The high performance stuff pulls over 100 amps and drains a pack in a minute or two. Not uncommon at all for those to pop.

 

[Alonzo]

Wow, and I thought NiMH where safe?
But you're saying that you're safe as long as you use regular commercial chargers?

 

[LuxLuthor]

LOL!

NiMH = Explosion With Shrapnel
Li-Ion = Rapid Venting With Flames
Dynamite = Minor gaseous & particulate expansion

Love the characterizations.

 

[SilverFox]

Hello Stegone,

I don't think there would have been as many concerns if the explosions happened during discharging, however the packs exploded during charging.

It is not uncommon to use matched cells in a battery pack. It seems that some matchers are going beyond matching the cells on capacity, internal resistance, and mid point voltage, and are also running tests to see what charge rate heats up the pack to a temperature that allows for the "best" performance. It seems they often recommend charging at a rate that is higher than what the battery manufacturer recommends. I believe IB recommends a maximum charge rate for the 4200 cells of 4 amps, but "matchers" have found that the cells heat up better when charged at 6 amps.

Now we get down to the nitty gritty... If the battery manufacturer recommends 4 amps, and the matcher recommends 6 amps, perhaps you can get the extra "edge" by charging at a higher rate... Who knows what goes on "in the pits."

I first thought that IB had some "problem" batteries, but I have run across stories of other brands (GP and Sanyo) also exploding. In addition, Schulze, in their latest firmware upgrade, has limited the charge rate in Auto C mode to 1C. Previously, in Auto C mode with cells of low internal resistance you could find charge rates higher than 1C for some periods of the charge. It seems Schulze is also concerned about this.

It also seems that in order to get a little extra into the IB cells, they made the cans a little thinner. There is some speculation that if the can was thicker, the cell would merely pop its top and not explode.

Getting back to the 15 minute chargers, when charging 2000 mAh cells, they are charging at around 3.7C. In thinking about this I have decided that I will continue using my 15 minute chargers. They are charging each cell individually, and monitor temperature during the charge. They also reject cells that have higher internal resistance, and seem to reject charging cells that are already charged. As long as all of these safeties are in place and working you should be able to avoid NiMh explosions.

Tom

 

[yellow]

pfff

vaporware

 

[dano]

R/C Flyers do push the cells pretty hard.

Electric R/C racing pushes them hard, but not to the limits of a plane.

Trinity has released a 4600MAH sub-c NiMH in various configurations: matched, "pushed", etc. In order to stuff that much capacity into a sub-c sized can, there has to be something removed for space limitations.

When I was racing competitively, I vented quite a few cells, but nothing to to an outright explosion. This was before the age of super-hi capacity NiMH cells (i.e. 3000MAH + in capacity).

I think these super hi-cap. cells are problematic, in all sizes. There's a huge push in electric cars to move to brushless motors and LiPo packs. The output and speed are amazing, and without the issues of a hyper-NiMH cell pushed beyond its design limits.


--dan

 

[65535]

From the article it sounds like a bunch of cells were botched, I have personally seen cells dead shorted, once in RC car pits 6 cells were put on a specialized discharger backwards and and vented rather violently at a few hundred degrees enough to melt the solder holding the cells together. The other time I had the cells in my pocket and they were shortted on a metal antenna, melted the solder, but had the batteries out of my pocket before they hurt me, both cases were 6 cell 33oomaH sub c cells.

These exploding cells probably had bad vents, which led to explosion. Regulations state that all cells must be able to vent the pressure in the cells before it can build up enough to rupture. Until now never seen a vent fail.

 

[LuxLuthor]

From the article it sounds like a bunch of cells were botched, I have personally seen cells dead shorted, once in RC car pits 6 cells were put on a specialized discharger backwards and and vented rather violently at a few hundred degrees enough to melt the solder holding the cells together. The other time I had the cells in my pocket and they were shortted on a metal antenna, melted the solder, but had the batteries out of my pocket before they hurt me, both cases were 6 cell 33oomaH sub c cells.

These exploding cells probably had bad vents, which led to explosion. Regulations state that all cells must be able to vent the pressure in the cells before it can build up enough to rupture. Until now never seen a vent fail.

When I saw they were IB SubC's, I agree the QC on them has been poor for at least a year. It is why CBP dropped their brand about a year ago, and why Sweden and Belgium reacted by just banning that brand of IB 4200 battery.

 

[65535]

I have an unused pack of IB cells 6 sub C's, I doubt I will have any problems they are older, and matched by orion IIRC. I think they are 3800mAh. Haven't raced in quite a while.

 

[paulr]

I've always been skeptical of high rate charging series packs, as opposed to separate -dV monitoring of each cell using independent charging channels like consumer 15 minute chargers. The USL worries me a bit for this reason. On the other hand it is done in cordless tool packs all the time, and I don't hear about them exploding. So I wonder what the difference is between those and the RC packs.

 

[LuxLuthor]

I don't think the concern extends to all high rate NiMH charging packs, only to the recent IB particular brand....which seems to at least have a defect with their vent, and when aggressively used in particular RC application. It may be that other high capacity cells were made to be competitive with IB, and also have compromised on specs/quality.

I don't see the USL application with more conservative cells & Triton charging as remotely related to this 6C charging RC scenario. As Tom says, charging at 1C, and following other sensible guidelines will lead to favorable experiences.

 

[SilverFox]

Hello LuxLuthor,

A point of correction...

They were charging at 6 amps, not 6C...

4200 cells charged at 6 amps works out to around 1.4C.

There is more to it than just the vents. The vents have changed from a spring loaded device to a "rubber" disk. Many favor the older spring loaded type vent thinking it is more reliable and consistent. However, it seems that in order to gain capacity, the ratio of + to - electrode has been reduced to nearly 1 to 1. There is usually an excess of - electrode to allow for oxygen recombination in the event of an overcharge. This could be contributing to the explosive nature of the problem.

Tom

 

[LuxLuthor]

Tom, thanks for clarification. I misread the charging rate, and that does raise more concern as you point out. Then the other question is how many high capacity cell brands use these "shortcuts," and how new of a manufacturing change is it. In other words, is it more of an isolated defect, or a widespread general issue for all high capacity brands. I only saw that one series of articles that singled out the IB model.

 

[65535]

I usually find myself charging my 3.3Ah cells at 6 amps. It's pretty common int he RC business, cells seem to have the best voltage curves at about 140F (60C?). Higher voltage wins more races.