Case of the Exploding NiMH Pack.

[xyster]

Most of us are very aware of the risks with lithium cobalt, mainly when accidentally overcharged. I came across this post about a similar problem when NiMH chargers don't terminate the charge properly. I remember some discussions here about nickel batteries venting and oozing, but never outright exploding as this person describes.
http://www.rcgroups.com/forums/showthread.php?t=731212

I'm an airplane guy so I'm posting this here. Recently I inheirited an electric truck. All was going fine until last week. I was using a MRC Super Brain 959 delta peak charger to charge the 7.2V NiMh battery pack. I changed no settings from all the other times I used it. A while later, about when the batteries should have been fully charged, I heard a loud fizzing and bang. I ran downstairs to see smoke coming from the batteries. One of the batteries had exploded. I usually charge the batteries in an old metal cash box but when the first battery exploded, it blew the pack out of the box. I quickly unplugged the charger and ran to get a towel. My plan was to quickly throw everything outside, to stop more damage but just when I was coming back there was even more explosions. In total, 5 of the 6 batteries completely exploded and melted part of the carpet it had fell on.
I didn't read anything about cleaning up NiMh exploded batteries but I took the precautions of wearing a respirator and gloves. That black soot was everywhere. Could have been worse though.
My questions are, #1. How do I trust my charger again? How can I test it to see if it is ok? #2. Does anyone have any guesses as to what may have caused this? Faulty batteries or maybe the charging settings had been accidently changed?

 

[knightmb]

Most of us are very aware of the risks with lithium cobalt, mainly when accidentally overcharged. I came across this post about a similar problem when NiMH chargers don't terminate the charge properly. I remember some discussions here about nickel batteries venting and oozing, but never outright exploding as this person describes.
http://www.rcgroups.com/forums/showthread.php?t=731212

Both of my packs have a thermal fuse inline to prevent this kind of overheating, I guess his pack had no such safety feature (unless it was bypassed). I don't think exploding NiMH is impossible, just rare. Kind of like exploding NiCd or Alkaline, it takes some extreme situations to make it happen. That kind of thermal runaway almost sounds like the batteries were packed close together and one of them melted through to the other causing a direct short circuit of some kind.

A RC 7.2V NiMH pack can't be that big on energy density can it?

 

[fechter]

I've had NiCd batteries explode quite spetacularly in the past. Little bits of razor sharp sheet metal from the battery case stuck in the wall. I had to use pliers to pull them out. Good thing it wasn't my face

Don't charge NiCds in parallel.

 

[blisspacket]

Xyster's dude who suffered an exploded NiMH says that he never changed any settings. And of course, there's a multitude of settings on the MRC, so we don't know what the electrical mix was that was injected into his battery. It'd be nice to know to either try to duplicate, OR totally avoid. TLI (too little info)

 

[magudaman]

Just a side note, I would avoid MRC chargers. I had one for about 25 minutes and it popped its insides and never worked. I have used a hyperion charger for years now and love it!

 

[amberwolf]

There is a thread either here or on DIYEC about a NiMH bike pack that did this, after having worked for a long time, but I don't remember who posted it and don't have a link. It's been a while, maybe almost a year, since it was posted.

An acquaintance of mine in the east valley here had a NiMH pack overheat and cook itself inside his bike's pack tank one night, while charging; I don't recall the reason but it was worse than just a simple overheat--it physically destroyed batteries in some way.

I've had one problem with a thermistor on the charging setup having a poor connection that I didn't notice, which resulted in overcharging my formerly very nice (if used) 36V 13Ah NiMH pack from Ianmcnally. I was lucky in that since the charger also has a voltage cutoff, it did not blow up or catch fire, but it damaged the cells so that now they only deliver around 7-8A (a bit more than 0.5C) before major voltage sag, and they now have much higher internal resistance based on how warm they get after really heavy use (unlike before).

@fechter: Maybe 15 years ago or more, I blew up my homemade tricorder prop accidentally, by hooking up the charger backwards to the AAA NiCd I'd built into the bottom half of it. It did exactly what you describe, and there were bits in the cieling, walls, etc.

 

[Hillhater]

@fechter: Maybe 15 years ago or more, I blew up my homemade tricorder prop accidentally, by hooking up the charger backwards to the AAA NiCd I'd built into the bottom half of it. It did exactly what you describe, and there were bits in the cieling, walls, etc.

Back in the early RC car days (1980 ish !) we were learning how to use Nicads and fast charging 6 cell packs from car a battery with only a resistor to limit the current .
In the chaos between race heats, car repairs, breakfast, talking sh1t , etc... i accidentally connected up a 6 cell (2Ahr, sub C )nicad pack to the 12 volt car battery in REVERSE polarity !! ... and left them for a while. !
The smoke from the vaporised shrink wrap alerted me, but by then those cells were literally glowing red hot !..and i swear you could see through them !
No explosion , but they would have made a good portable electric barbecue !

 

[chrisvw]

My 48V 12ah NIMH pack burned up about a year ago. The best we could figure there was a short between 2 cells that made a closed loop of about 35 of the 40 cells. I was running the charger in our garage, working outside when my wife informed me she smelled smoke. I unplugged the charger, but the whole thing was smoldering inside my soft rack pack. The tabs between batteries looked like a toaster. I was able to remove the pack from the bike, pulling connectors as best I could and dump it out of the pack onto a concrete slab. Because the short was all internal, nothing but some sort of in-line fuse would have stopped it.

It didn't explode, but it was a red hot mass for over an hour spewing all kinds of nasty smoke.

I've put more padding into the bottom of my pack and religiously inspect and duct tape any wear spots on the outside of my packs now. I still have an identical pack that continues to work fine.

 

[amberwolf]

Thermal breakers (such as Klixon) could be used between groups of cells; this is done in some laptop batteries I've opened up. The problem is finding ones rated at the currents some of us use. (although those using such high currents are probably not running NiMH anymore, the thermal breakers are still not a bad idea for them).

Both my 24V NiMH packs have the pack fuse between 12V halves, partly for that reason. If anything shorts from one physical half to the other (2 planar stacks of 10 cells in 2 rows of 5), the fuse should blow before anything *really* bad happens.