Rejuvenating Batteries

Submitted: Tuesday, Feb 26, 2008 at 17:13
ThreadID: 54953 Views:7377 Replies:2 FollowUps:1
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In a recent post there was mention of rejuvenating GMC batteries

"After two years the batteries wouldn't hold their charge and I was about to chuck them out. Then I decided to cycle the batteries three times - discharge at 0.1amp and charge at 0.1 amp. The batteries now have plenty of capacity and hold their charge for months"

Can anyone help me to repeat this by suggesting the practical way of

a) discharging battery at 0.1 amp current


b) using the supplied charger to charge at 0.1 amp - how do I tell when fully charged?

I am fairly theoretical OK electrically but practical advice would be welcome

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Reply By: Member - Mick O (VIC) - Tuesday, Feb 26, 2008 at 20:24

Tuesday, Feb 26, 2008 at 20:24
Don't remember the actual post but would be very interested to know myself. In the mean time try this link on battery rejuvenation. May be of some help.

Cheers Mick
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AnswerID: 289536

Reply By: Member - Phil G (SA) - Tuesday, Feb 26, 2008 at 21:51

Tuesday, Feb 26, 2008 at 21:51
What type of batteries are you talking about?
I'll assume you're talking about cordless power tools.

Theres a few reasons why their batteries don't last - they are usually garbage (look at how cheap they are) - the chargers they come with are primitive, so they can both overcharge and undercharge - and many power tool owners don't know how to look after their batteries.

If it is running NiCad cells, then yes you can rejuvenate.
NiCad cells like to be fully cycled every time they are used. If you run them to near flat, let them cool, and then recharge them to 100% you will get better capacity and more voltage out of your packs. To slip them onto the recharger when they are not fully discharged is not good.

If you cycle them several times, as you suggest, then you can return some capacity to a poorly maintained battery. Easiest way to discharge is to run the drill until it starts to slow. Best voltage to drop the pack to is 0.9V per cell (or 3/4 of the pack's total voltage), but you've probably got no way of measuring voltage. If you run the drill until it's stone cold dead, you will also damage the pack as current gets reversed in the weakest cell in the pack, so eventually that cell will die.

NiMH cells also like to be cycled, but are more tolerant to partial discharge, so they have largely replaced NiCad. They are also a bit more friendly to the environment. You may get a bit more capacity by cycling a NiMH a few times, but the effect is not usually as great as with NiCads.

The above is info gained from matching NiCad and NiMH cells as a hobby for R/C car racing over many years. The following link can tell you a little more detail and tell you about the newer LiPO cells:
Model Flight Link
AnswerID: 289559

Follow Up By: Member - Phil G (SA) - Tuesday, Feb 26, 2008 at 22:00

Tuesday, Feb 26, 2008 at 22:00
"how do I tell when fully charged?"

For both NiCad and NiMH, as you charge the pack, its voltage will rise gradually. As it becomes fully charged, the voltage will level off, and then start to drop. They are fully charged when voltage starts to drop (called the Delta V point), and at the same time, the cells start to get warm. Most automatic chargers detect DeltaV , and cut out when the voltage peaks.

If you are charging them at a low rate, its pretty much just a trickle charge, and DeltaV and temperature are not reliable. May as well calculate time, and add 10%.
FollowupID: 554868

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