Disaster with my batterypack

[wurly]

Well it was good while it lasted.
My battery pack is no more, it ceases to be, it's snuffed it.

Here is the result. 30 D cells all totally knackered. It got so hot the solder melted and disconnected itself....Luckily!

It's my fault for being such a cheapskate. I didn't have a suitable charger and thought i'd manage using a cheap 36volt charger from ebay.

The charger outputs 41.4V 1.4 amp and when the pack is fully charged the output is 42V. I have been monitoring the pack during recent charges and it seemed to be ok. The one time i left it on charge while doing something else and this happens.

I am blaming the charger, but it might have been wires shorting together. I simply don't know what went wrong. Everything seemed ok when i disconnected it from the charger, the 20amp fuse was intact after discovering the pack had burned itself out.

What bothers me most about losing my pack was i never really got a chance to see how far it would last before cutout. The most distance achieved was 26miles, but i know i could have got more out of it, probably 30+miles.

It happened 2 weeks ago, so i am now more upbeat and have replaced the pack with SLA's. I will eventually get a Ping battery pack when exchange rates improve. But for now i cannot justify the expense for the amount of miles i travel.

Oh well, live and learn

Mel

 

[flecc]

Sorry to hear if this misfortune Mel.

That was a lithium-ion charger you were using and it really should have been an NiMh charger outputting more like 53 volts with temperature cut-off control using a thermistor in the battery pack.

That said, li-ion chargers are normally safe since they cannot get the 30 NiMh cells beyond about two-thirds charge due to their low output voltage. From previous experience with using cheap NiMh cells in packs I think it's more likely to have been the failure of one of the cells since exact cell matching is very critical in NiMh series packs. It was this reason more than any other that led battery manufacturers to rush into lithium and happily abandon high capacity NiMh D cell production.

What often happens in soldered packs where cell matching is less than perfect is that one cell fails with a very rapid rise in temperature, causing the soldering of one or more wire links to melt, the detached link(s) then setting off a series of shorts and a general temperature rise into destruction. As you say, you were lucky in that it disconnected before causing a more general fire.

I did some testing of NiMh pack destruction some while ago and this is an extract from a report at the time:

Having ascertained that I only needed two modules, I did destruction testing on the remaining two to check the safety position. One was wired into place and ridden on continuous battery meter red to low voltage thermal runaway, the other was charged to thermal destruction.

In both cases, very high temperatures were reached, but since the soldered connections melted and dropped away, the whole pack capacity was incapacitated before explosion was possible. Some of the wires dropping away set up new shorts, but these only involved small groups and no cell melted down completely though thermal generation was inclined to continue, so each destroyed module was cooled in a cold water bath.
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