Answered: Burning Batteries!

It’s pretty normal for batteries to get hot when putting the charger one fast, but this time the battery actually began burning itself up! We’ve had the smart chargers for several months now and never had a problem with them, since they seem to be adaptable to any vex battery. For a day or two we left a 7.2V NIMH battery on the charger on safe charge. Normally the batteries, if they get hot at all, get hot when reaching their peek, but they normally don’t get any hotter. A day later when I walked into the room, I stepped on a battery that was just about as hot as a boiling pot of water: it was really hot! The battery had melted down to its the silver core and all the blue plastic was almost completely melted off. It also seared a really dark black stain into the carpet.

I’m guessing that this is a result of the charger sensor not recognizing the battery’s peek and over charging it. Am I right in assuming that the battery’s should start to cool down and stop charging once the peek is reached? How should I proceed to diagnose the problem without burning another battery? Is this a problem with the battery itself or is this a malfunctioning of the charger’s sensor? Have there been any other instances of this happening? We’ve left batteries on the charger for weeks before and they’ve never been hot for more than an hour or so, but is it save to leave batteries on the charger for a long period of time?

I have never heard of a VEX battery getting that hot!!
The first thing we need to look at is: has this charger been used with other batteries before? or was it the first time to be used?
Also was the damaged battery a 2000mAh (small) or 3000mAh (large)?

Technically it’s safe to leave the batteries plugged to the charger for a few days, but personally I usually disconnect any electronic device right after its done charging (including VEX batteries). Some things that you could try to diagnose the problem from your side would be:

  • Measuring with a Voltmeter the output of the Smart Charger (16-20V).
    -Connecting a known good battery to charger (use “safe” setting for this experiment), but make sure you keep an eye on the battery, don’t leave the room. If after a while the LED on the charger turns from red to green that means the charger is working properly and should not overcharge your batteries.
    However if after a while the LED never turns to green, and then the battery starts to get a little bit more than warm, then disconnect the charger and let the battery cool down.

Let me know about your findings, and sorry about the carpet.

Just as an FYI. The battery I previously burnt was a small 2000 mAh NIMH.


I ran the test by hooking up an old battery (the old 2000 mAh Ni-Cd ones) to the exact same charger on safe mode. After it had charged, the lights turned green, but I left it on because I noticed that 30 minutes later it was still hot. Eventually I just ended up plugging it in downstairs were we were eating. About 5 hours after I had originally plugged it in, I started to smell the burning scent of the battery (I’m guessing this is melting insulation).

When I looked at the charger, it was still green, but the battery had become extremely hot and started to smell pretty bad similar to its burnt predecessor, so I unplugged it and checked the voltage of the charger and the battery.

  • The battery gave off about 8.22 which should be well fully charged especially for an old battery.

  • The charger gave off anywhere from 16-19.9 which should be fine according to your specs. However, this calculation could very well be wrong, since I had to hold the voltmeter and a very particular position just to get a reading in between 16-19 or else it read something 0.1-ish on it. This could very well be a problem with the voltmeter, but it works fine at checking batteries.

The main problem is that the battery continues to charge even after the Smart Charger is green.

Ok sounds like we have isolated the problem, for now I would recommend you to discontinue the use of that charger and contact our technical support team ( ) for further troubleshooting.

Note: After a Battery has reached a full charge, the LED turns Green, the Charger will continue to Trickle Charge the battery. Thus the Charger is still charging the battery but the current to the battery has been limited during the Trickle Charge mode.