Battery Charging Question

[valterra]

I have a bunch of AA sized Nimn batteries that I want to keep charged.  

For some applications, I have put into a PVC tube to make a 12v battery pack.  (See http://www.fieldlines.com/story/2007/3/8/144911/7099 ).  Because I already had this "pack" made, I've been hooking up a 12V 1A power supply directly to the pack in order to charge the batteries.

The problem is that I have an odd number of batteries.  10 batteries makes a 12V pack, but I have something like 23 - not divisible by 10.

I was thinking of ditching the pack idea (for charging) and rigging up a charging "bus line" that ran something closer to the voltage of a single cell.  Then I'd put the cells in parallel, and it wouldn't matter if I was charging 1 or 100.  Would that work?  I figured if a cell was near dead, it would keep the line voltage below 1.2 volts, and if they were all charged, the supply voltage would keep them all charged up?

Another question, does the voltage matter that much, or is it the current that matters?  Do I have to use something that puts out only 1.2 volts, or can I use something like, say, a CD-Player adapter (4.5v), or can I use a 12v supply, or do I need to ignore the voltage and instead concentrate on getting the correct amperage?  I'm hoping to not have to babysit them.

Thanks!

[AbyssUnderground]

1.2v will not properly charge a NI-MH cell. You're looking at around 1.35v minimum to charge one. Voltage does matter because at a higher voltage, the battery will try to take more current and it will start to "burn up" due to excess current it cannot absorb.

[valterra]

So is 4.5v way out of range?  I've got a few of those "CD Walkman" power adapters.  They're 4.5v, maybe 300 mA.

[AbyssUnderground]

If you connected 3 in series you might be ok using a 4.5v charger but you would have to keep an eye on them since they can overcharge.

[kell]

Presumably you mean nimh which stands for nickel metal hydride.

Nicad and nimh batteries are charged with a current, not voltage.  the algorithm in a charger may monitor the battery voltage for -dV/dt in order to terminate charging, but that doesn't work as well for nimh as it does for nicad.  Building a fast charger is not a trivial task.  If you can't buy one, then you would have to charge your batteries at a C/10 rate, which takes about 14 hours.

Google is your friend; first link that came up:

http://www.powerstream.com/NiMH.htm

[snowcrow]

   Nicad and Nimh "AA" batteries at full charge are 1.56 Volts. You need around 2 Volts to charge them and current most be decreased as they reachs full charge, not unlike Lead Acid batteries.  I think the right charger for the job is your best bet.  I'm such Wal-Mart most sell them.