Battery Pack Burnout

scwhiteley · 1st April 2010, 10:02 PM

Hello!

I recently built a portable stereo that runs off a battery pack consisting of twelve AA size NiMH batteries (14.4V) It has worked like a dream, that is, until today.

The stereo worked this morning, then as of this afternoon, it did not. I opened it up to see what was causing the trouble, and it appears that something caused two of the batteries to overheat, hot enough to melt the ends of the plastic battery holder.

I tested all the batteries with a voltmeter, they all checked out except two, which registered a slight negative charge.

It probably is worth mentioning that I've been charging the pack as a whole unit, with a charger designed to charge a 14.4V battery pack.

So what do you suppose happened? Perhaps a short somewhere? possible, but unlikely. Is the charger or my charging method at fault? Who knows?

Andrew Eckhardt · 11:00 PM

One way or another you managed to let a couple cells get dishcharged in the string. When that happens the impedance rises. Since they can not take a reverse charge the flat cells then function as a resistor. Series charging is okay so long as cell impedance is matched. Nothing guarantees that without additional circuitry so you have to be careful. So long as your charging rate isn't too high I think you'd be okay with a bit of overcharging to make sure they are all topped off, (don't take that to the bank) but deep discharging in series is a no no (at high rate anyway). I'm not very familiar with NiMH so if you want to find out exactly how to exploit the technology to the fullest a bit of net digging could help you out. Charging individually has the obvious set of advantages/disadvantages.

Andrew Eckhardt · 1st April 2010, 11:32 PM

NiCd is exothermic on discharge, endothermic on charge. NiMh is probably similar. The effect seems strongest near full discharge / initiating charge. You could have picked up some "extra" heat that way.

Andrew Eckhardt · 1st April 2010, 11:37 PM

You may need to use an undervoltage lockout.

jkeny · 2nd April 2010, 12:01 AM

Does your charger have a balancing circuit to make sure that all individual cells in the pack get charged to the same level? If not then this is more than likely the source of your problem.

1st April 2010, 10:02 PM	#1
scwhiteley diyAudio Member Join Date: Mar 2010 Location: Missoula, Montana	Battery Pack Burnout Hello! I recently built a portable stereo that runs off a battery pack consisting of twelve AA size NiMH batteries (14.4V) It has worked like a dream, that is, until today. The stereo worked this morning, then as of this afternoon, it did not. I opened it up to see what was causing the trouble, and it appears that something caused two of the batteries to overheat, hot enough to melt the ends of the plastic battery holder. I tested all the batteries with a voltmeter, they all checked out except two, which registered a slight negative charge. It probably is worth mentioning that I've been charging the pack as a whole unit, with a charger designed to charge a 14.4V battery pack. So what do you suppose happened? Perhaps a short somewhere? possible, but unlikely. Is the charger or my charging method at fault? Who knows?

1st April 2010, 10:58 PM	#2
Andrew Eckhardt diyAudio Member Join Date: Nov 2005 Location: PA	One way or another you managed to let a couple cells get dishcharged in the string. When that happens the impedance rises. Since they can not take a reverse charge the flat cells then function as a resistor. Series charging is okay so long as cell impedance is matched. Nothing guarantees that without additional circuitry so you have to be careful. So long as your charging rate isn't too high I think you'd be okay with a bit of overcharging to make sure they are all topped off, (don't take that to the bank) but deep discharging in series is a no no (at high rate anyway). I'm not very familiar with NiMH so if you want to find out exactly how to exploit the technology to the fullest a bit of net digging could help you out. Charging individually has the obvious set of advantages/disadvantages. Last edited by Andrew Eckhardt; 1st April 2010 at 11:00 PM.

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1st April 2010, 11:32 PM	#3
Andrew Eckhardt diyAudio Member Join Date: Nov 2005 Location: PA	NiCd is exothermic on discharge, endothermic on charge. NiMh is probably similar. The effect seems strongest near full discharge / initiating charge. You could have picked up some "extra" heat that way.

1st April 2010, 11:37 PM	#4
Andrew Eckhardt diyAudio Member Join Date: Nov 2005 Location: PA	You may need to use an undervoltage lockout.

2nd April 2010, 12:01 AM	#5
jkeny Banned Join Date: Feb 2007 Location: Dublin	Does your charger have a balancing circuit to make sure that all individual cells in the pack get charged to the same level? If not then this is more than likely the source of your problem.

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