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BertG

NimH battery: BANG!

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So the people from our local RC shop said: NimH batteries are very safe, LiPo that's another story, but NimH: no worries. After what happened last night i am not so sure anymore... I was charging my Voltz 5300 for maybe the 10th time (charger on 5.0A). After about 50 minutes there was the sound of 'charge complete' and exactly at the same time a very loud bang, like a shotgun, it scared the badword out of me. One of the cells had burst open. The battery did not feel warmer than after previous charges.

I mostly ran this battery in cars like Frog and Super Hotshot where it was quite exposed. Apart from some scratches it looked undamaged though. The charger is an RC plus Power Plus 80.

I am now seriously thinking about switching to LiPo...

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I harbor resentment and pyro-tendencies from a traumatic childhood.  A good explosion is very satisfying.

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1 hour ago, BertG said:

So the people from our local RC shop said: NimH batteries are very safe, LiPo that's another story, but NimH: no worries. After what happened last night i am not so sure anymore... I was charging my Voltz 5300 for maybe the 10th time (charger on 5.0A). After about 50 minutes there was the sound of 'charge complete' and exactly at the same time a very loud bang, like a shotgun, it scared the badword out of me. One of the cells had burst open. The battery did not feel warmer than after previous charges.

I mostly ran this battery in cars like Frog and Super Hotshot where it was quite exposed. Apart from some scratches it looked undamaged though. The charger is an RC plus Power Plus 80.

I am now seriously thinking about switching to LiPo...

Cell was probably imbalanced compared to the others to the point where it was being overcharged whilst the others were still building voltage, so the charger didn't notice the voltage drop in the overcharged cell until it gave out. 

Maybe.

You are right in that with LiPo batteries you get to know what voltage the cells are at to avoid this issue when charged on a proper charger.

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Lipo batteries are the way to go for fast charging.

 

for myself I never never ever would charge a nimh at 5.0 A, that honestly sounds like improper use.

 

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People are very quick to say how LiPo can fail during charging but often forget that NiMH can fail when charging also.  As above, it sounds like a balance issue.  Given how chargers have advanced, I'm surprised nobody invented a balance charging system for NiMH (I used to balance mine manually by putting charge probes on each cell individually.  I know balancing dischargers have existed for a while but they are manual and only work for side-by-side race packs).

Anyhoo - the main safety difference between NiMH and LiPo is that NiMH usually goes with a single impressive bang which can split your chassis in half (or worse, if you're holding it during charge), whereas LiPo usually fails with a chemical reaction that burns at extremely high temperatures, gives off toxic smoke and doesn't stop until its chemicals are exhausted - hence it being famous for starting house fires.

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13 minutes ago, Fabia130vRS said:

Lipo batteries are the way to go for fast charging.

 

for myself I never never ever would charge a nimh at 5.0 A, that honestly sounds like improper use.

 

5,0A was quite normal back when we used NimH in racing. There were some charge programs, that even began with 10A in the beginning and a defined charging curve. (Robitronic was the charger).

But problem was indeed to have some selected and matching cells, that were quite similar to each other. As the old chargers didn´t have a single cell detection. That´s why balancing boards were used for discharging before next charging, even for stick packs there were versions available (with needles through the shrinking).

With that equipment, the use was quite safe. I say quite, because I had a friend on a race who charged his battery in the kitchen of the club (luckily) and blew up his high end selected NimH. After that, the kitchen looked quite damaged, as metal particles hit the wall etc. Everyone was scared and happy, that no one was in the room.

In the end we found out some days later, that the charger was faulty. (Don´t remember the brand, but it was o.k. before)

That said, I nowadays only use Lipos (for performance and ese of use) and charge in the recommended Lipo bags. When they blew up, at least it will be more of a fire than an explosion with particles.

But I never had a blow up so far, not even on cheaper Lipos. I had some swelled ones, but I never tried to charge them and stopped using. In our race series with many beginners we use cheap Zippy Flightmax 4000mAH hardcase types since more than 6 years, and only 1 fired up during charging, because a kid tried to charge it without Dad and got stuck in the NimH charging mode. Nobody was harmed, as it happened in the bag. But that was on the first race and never happened again. Chargers are more modern now and have better balancing modes as well as automatic cell detection.

But you always should be aware of the risks of the technique you are using. Everything can be dangerous...

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5A, although a big current, is 1C on a 5300 battery, so whilst approaching the top end of what you should do, it should be 'safe'. 

I'm still surprised that modern chargers will let you do crazy big amps without a temperature probe. 

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Suppose i had done the charge with say 3A, would the result have been the same?

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38 minutes ago, BertG said:

Suppose i had done the charge with say 3A, would the result have been the same?

If the root cause was the imbalanced state of one cell, it could have ended up the same way, in my opinion. Just would take longer time...

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OK. Thanks all for the comments. Is there a way to check if there is an imbalanced cell in a nimh pack? And a way to make it in balance again?

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2 hours ago, BertG said:

OK. Thanks all for the comments. Is there a way to check if there is an imbalanced cell in a nimh pack? And a way to make it in balance again?

There were balancing discharge boards available for saddle packs where the cell terminals were exposed. For stick packs it's hard because the cells terminals are close together and the whole thing is shrink wrapped. 

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3 hours ago, BertG said:

OK. Thanks all for the comments. Is there a way to check if there is an imbalanced cell in a nimh pack? And a way to make it in balance again?

There were also Balance boards for stickpacks available in the end. I just don´t know, if you still get them new.

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8 hours ago, BertG said:

Suppose i had done the charge with say 3A, would the result have been the same?

A thought experiment:  If you have trickle-charged it at 200mA, would it have exploded?  It's possible, but I doubt it.  

If so, somewhere between 0.2A and 5A lies the explosion point.  I'm a bit of a chicken; I am rather afraid of 5A.  So the most I would charge is about 1.5A on lead-acid battery (for 1:1 cars), 3.5A on NiMH, 4.5A on LiPo.  I also charge only when the pack is cool; never when it's warm.  If I want to charge faster after a run, I put it in freezer for 10 minutes before I start to recharge.  If you have 5000mAh, it would take 90 minutes to charge at 3.5A.  But the heat build up is that much slower.  

In your case, 1 cell was bad for sure.  Who knows, a 3000mAh cell got mixed at the factory, or it might have flipped from going too low at one point.  If charged at lower amp?  It might have degraded slowly over time, instead of going out with a bang.  That'd be my guess. 

 

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10 hours ago, SupraChrgd82 said:

I harbor resentment and pyro-tendencies from a traumatic childhood.  A good explosion is very satisfying.

That must be why people all over the world love fireworks! ;)  (sorry for Bert's battery though) 

RGRtwlz.jpg

 

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The only cells I've had go bang, so far, is a Nicad and a Nimh.

I think the fear surrounding Lipos, makes you more , involved (?) when using them. You study individual cell performance, because it's easy to do so.

I recently had a 2s lipo battery cut out on low volt cut off (6.4v) , one cell was at 3.6v , and the other 2.8v when I put it on to charge . I balanced charged (as always) the lipo, and it took almost 2hrs to balance as the cell was duff. It's since been discharged using a car headlight bulb, and waiting recycling.

I'm thinking the Nimh battery  @BertGhad was faulty/unbalanced from new. Would charging at a lower C rating have helped? Maybe, for that charge cycle the charger might have been quicker reacting, and the single good cell might have degraded to match the other 5 over time?

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