what can you do when you over charge NiMH?

[monster]

hi

i think my socket timer broke on my battery charging and didn't turn off! thankfully two of the three (24v) batteries seem ok, protected by clever chargers.

but one was very hot smelly and shrink rap broken. the volts seem low 26v when i've seen it at 28v on full charge. i think it went 2 hours longer than it should have at 2 amp (7ah capasity).

is there anything that i can do now to save this battery? can i re-shrink rap it? or put it in the fridge to cool off? what can i expect the performance change to be? will it improve with a bit of cell cycling?

Regards, Matthew

 

[Ypedal]

Personally . i would start by totally removing the shrink wrap ( wait for it to cool of course ) and test every cell individually ( voltage ) and inspect for damage other than the current visual you now have.

report back.

did they leak ?

 

[monster]

if i disasemble the battery will i be able to get it back together? aren't the cells welded together or something?

i couldn't see any liquid any where so dont think they leaked. i could smell chemicals though.

 

[Ypedal]

The smell " could " be burnt plastic/paper..

Yes, the cells are spot welded together using metal tabs, usually, if you remove the shrink and the adhesive paper covering the tabs, you can test each cell without dis-conecting them, simply take your volt meter and probe each cell one at a time, you will be able to measure each cells voltage even if they are all conected together.

Word of caution, remove any metal rings, your watch, etc.. work on a wood/plastic surface ( no metal desks PLEASE ).. and it's not that difficult.

 

[monster]

i used the bike today and the cell seems ok. the voltage has risen to 27.8 after one charge/discharge cycle.

i'll keep an eye on it and see what happens over the next month. i'll defo be buying clever chargers for all my batteries now. i read they ballance the cells during the trickle "green" stage.

 

[Ypedal]

what charger are you currently using ? brand, model...

 

[monster]

i was using chargers designed for lead acid bats -and a timer, because that was what i had available.

but now i've shelled out for 2 more smart timers.

 

[Johnbear]

I use a Soneil 36v 4amp charger to charge my NiMH for the initial 80% roughly - they have never overcharged. It seems to cut out at the perfect time. I measure the AH's with a drain brain.
I have however used another lead acid charger and fried some NiMH's. I belive it is a combination of the lower voltage cut off and the constant current of the soniel. I do use a 1.8 amp NiMH delta V, Temperature cutoff charger, but it takes forever. I use it to top of the charge. -