can nimh bats be revived?

[turbo scorpion]

I hsve a few nimh batts that have had little use. They worked great last time i had em out and stored them charged up. Now they wont power the car for more than 2 min. They read 8.4v (7cell).
I put them on the charger in discharge mode and at 0.1amp the volts drop from8.4v to 1.0v on less than a min.

Is there any way to revive the pack or it it toast?

[Xracer]

You may have a cell that is being driven into reverse polarity under load, check the voltage and polarity of each cell individually whilst on load, not whilst on charge.
You can sometimes bring a pack back by charging each cell independently assuming you have a charger that will handle one cell.

[turbo scorpion]

Only problem is id have to disassembled the pack. The cells are in a line end to end not side by side.
Unless you know a way to measure each cill individually in that sort of pack.



One seems to be coming back with a real slow charge/discharge cycle followed by a little faster charge.....
If it comes down to it ill tear these packs apart and maybe learn to solder batteries.....

[dodgydiy]

for the voltage to drop suddenly it sounds very much like a dead cell there. you can give old packs a shock to decrystallise them, and then cycle them a few times to get the capacity back up but that isnt likely to work with a dead cell. to shock the battery, attach long leads to it and put it the other side of something solid (i use a breeze block) in case a cell pops, then connect across a car battery for ten seconds, leave for a few mins and then repeat. if after 2 or 3 mins the pack is warm then dont repeat. immediatly after, give the pack a couple of charge/discharge cycles. you are pretty safe with sanyo and gp cells, but avoid intellect cells, they go pop more easily

....always protect yourself with a solid object between you and the battery.....

[Xracer]

Hi Turbo Scorpion, you can measure individual cells in a stick by piercing the insulation on side of the cell you want to measure (its -ve can) and the adjacent cell located at the +ve end, simples!

Back in my old 1/12 days we used to skim a patch off the cell sleeving to allow us to measure each cell when charging and check for imbalance. With multi cell lipos this is now standard procedure with high tech balance chargers of course!

I have also seen dodgydiy's response and he is quite correct too but using a 12 volt you do have to be careful as he suggests, the cell may heat rapidly and vent its electrolyte or explode of course, hence hiding behind a wall!

I have done this procedure a few times in the past, drop the cell on the floor first and usually using a charged 7.2 volt pack to kick an over discharged cell back to life.
You will never recover the full cell capacity though and it will be the weakest in the pack with a risk off reversing polarity under heavy pack discharge!

Happy bomb making, seriously do be careful if using high voltages. John

[dodgydiy]

one thing i forgot, if you've got an accurate digital thermometer you can compare nimh cell temperatures when on charge, if one cell becomes warmer than the rest it is likely to be the weak point in the pack! can remember at race meetings years ago when we were running 1200 and 1400scr nicad cells, you used to see people in the pits putting jump leads from their charge batteries to the cells for a few seconds just before they went to race to give them that little extra kick! memories......

[turbo scorpion]

Originally Posted by Xracer

Hi Turbo Scorpion, you can measure individual cells in a stick by piercing the insulation on side of the cell you want to measure (its -ve can) and the adjacent cell located at the +ve end, simples!

Back in my old 1/12 days we used to skim a patch off the cell sleeving to allow us to measure each cell when charging and check for imbalance.......John

Ok. I tried to do this i dug through the plastic and Cardboard. Exposed the side of a cell and the end of the same cell and the next one. Volt meter neg lead on side....positive to either end and no reading.... When i put positive lead to the positive wire of pack got a reading.....

Do you have a picture of what your talking about or a link to something that would help me......