Recgargeable Batteries

[AE4TR]

I have some electric drill batteries that will not retain a charge.  Is there anyway to restore them?  They are not really old. (about 2 yrs.)  Replacing them is really expensive.  I think they have developed a memory of their own.  Would appreceiate any help. Thanks.  AE4TR

[AG4GT]

You need to “zap” them.  Essentially what this means is supplying about 10 to 12 amps of current for a VERY short period of time, less then 1 second.  You must also open the pack and zap each cell, observing the correct polarity for each cell.  I have seen many circuits that will do this for you; most involve charging a large capacitor to about 12 volts and then dumping it across the cell.  Do a search on the net and you will find some of these circuits.  I do it manually using a 10-amp 12-volt power supply.  After each zap, check the voltage across the cell with a meter, if you have 1.25 or more volts the cell should be ok.  If you do not get 1.25 volts, zap it again.  Be VERY careful, you are dumping a lot of energy into the cell and it will get warm.  I would not zap it more then about 3 times and then let it cool for a while.  After about 5 zaps, it is probably not going to work.  Sometimes you cannot resurrect a cell but most of the time you can.  After you get all the cells in the pack to 1.25 volts or more, charge the pack as you normally would, then use it down and charge it again.  Remember, the best thing you can do with/for nicads is use them often and hard.

Steven
AG4GT
ag4gt@hotmail.com

[N2MG]

The zapping technique can achieve reasonable but inconsistent results.  Try using a big "computer grade" capacitor (say 470uF) charged to 10-20V.

Check each cell for voltage ahead of time - some might have actually "reversed" polarity - these are probably worthless. Still...

Good luck!

Mike N2MG

[WA9SVD]

One of three possibilities:

1.  You HAVE cycled them through their useful life, and they will no longer take a useful charge.  Although NiMH batteries will have greater capacity than a NiCd battery of the same size, the NiMH will not have as long a life; they may have only a 50-75%  number of charge cycles.
2.  You ran them down too far, and one or more of the cells reversed charge, which may or may not respond to "zapping" as described previously.  (But in my experience, zapping is only a temporary solution, and the cell or cells affected are on their way out anyway.)
3.  The battery pack was overcharged.  Overcharging a battery can be at least as deadly as running a pack down too far!  This is most often a problem when you have/use a "fast charge" (one hour or so) system; it's imperative to take the battery off charge when the specified time is up.  And if the "fast charge" is interrupted and later resumed, some chargers will attempt to send a full charge to the pack again, resulting in gross overcharge; this causes overheating of the battery pack, and subsequent damage.  If your charger/battery system says to recharge over night, or 14 hours, then overcharging will be much less a problem, as that charge rate will not usually cause excessive temperature rise.

    The solution?  Well,

 1.  You may be able to get the battery packs rebuilt, rather than replacing them, for less $.

2.  In the interim, (and this isn't recommended, but possible) if you have more than one bad battery pack, you can try to match enough cells with similar voltage to make up one good battery pack.  Then see Solution #1 above.

3.  Be attentive to how the batteries are used in the future:  Don't run then down until they are REALLY dead; and don't let them overcharge.