Semi-dead battery pack

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I got a 9.6V battery-pack from a cordless Bosch that's in a "funny state". It doesn't seem to be very old but it wont charge to more than 4.7V. It can deliver some 2A at ~4V for 10-15 minutes and it can be used to drill small holes.

This leads me to thinking that only half the cells are good and that the rest are somehow shorted?

I've read about people "flashing" battery packs with a big cap charged to 50-100V? Any hints on this?
Do i perform the "defibrillation" once ore what?
What's "a big cap"? (10mF, 100mF, 1F?)

Thanks in advance.

Regards TroelsM
Remember that what you want to do is dangerous, so you need to be careful- one hand in the pocket at all times, but it does work. The numbers are a bit arbitrary. You need enough energy in the pulse to clear the short. That might be very little for some batteries, and a lot for others. Usually, once the shorts are cleared, the battery will work normally for a while, then develop more shorts. I use a 3.3uF film cap (because that's what I happened to have) charged up to a few hundred volts. I start low and zap the battery, say at 100V. If that doesn't work, I try higher voltages. You can zap it a good few times with no problem. I don't like the lower voltage/high current methods some use, as there's more chance of damaging the battery and less chance of really clearing the shorts. OTOH, anything that works. Use this method *only* on NiCads.
I used the low voltage high current method the other day on some NiMH AA cells. You do have to be VERY carefully with this method as the battery being revived will start to heat up almost instantly. I used a small 12v lead acid battery across the NiMH cell for around ~1 second.

I tried the high-current method as it was easiest with the parts I had. I hooked up a 400VA toroid with a bridge and 40mF of caps.

I used a meter to monitor voltage across the battery. After a few seconds the voltage rose from 5.5V to 9V. Tried again later and the terminal voltage rose to 12-13V. OK seems to be working.

Bad part is that the pack wont take charge and when loaded it falls to 5.5V quickly.

Should I try again? The high-voltage approach seems to deliver less energy to the pack, but does the higher voltage do anything else that the high-amp approach doesn't?

Thanks for the help full answers.

Regards TroelsM
Hi TroelsM .
What also can be the case is that your battery might be missing his liquids .
After a few years of use NiCad , NiMh or NiFe sometimes need to be *refilled* .
With some luck your battery has a rubber closing cap on top and you can just inject destilated water .
Its a bit of a crude method (i assume its a sealed battery) .
Discharge the battery before you do it ,nickel based battery's can produce nasty current peaks (a.k.a. youre neelde goes up in smoke)

Now a little off topic ,did you ever build the smps schematic i sended to you ?

Have a nice eve .
Hi Walkura

Thanks for the advice. The battery seems to be in a better shape now and it will charge to 8.5-9.2V and deliver some 5-8A for 10 minutes. That's OK, and I won't do more work on it at the moment.

I newer got to work on the switch-mode supply. Not enough time and to much to do :)

Regards TroelsM
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