P3D Re: Nickel Metal Hydride

[Andy Beals <bandy@xxxxxxxxxxxx>]

Ah, batteries.  My favorite subject.

On Tue, 28 Sep 1999 19:46:44 -0600, "Greg Wageman" <gjw@xxxxxxxxxx> wrote:
> 
> From: Pixschack@xxxxxxx <Pixschack@xxxxxxx>
> 
> 
> >Noticed today that Fry's has nickel metal hydride batteries in the AA
> >penlight size. 

Oh, make sure that they're shaped just like regular batteries, with a *little* 
nipple up on top, not a big flat spot, like the NiMH batts I bought at a swap 
meet a few months ago...  )-:  Most AA holders require that little nipple in 
order to prevent a reverse-voltage situation should you install the batteries 
backwards.

> > I take lots of flash pictures.  I've been getting good
> >service form NiCad batteries since I got a charger that discharges the
> >batteries before going into the charge mode.  I understand that the
> NiMH
> >units don't have the memory problem.  What else do we know?  Can they
> be used
> >with a charger meant for NiCads?
> 
> 
> Modern NiCds aren't supposed to have memory anymore, though it is still
> a good idea to discharge them before recharging.  Also, don't overcharge
> them, which can damage the cell.

Overcharging drives out the water within them, resulting in what most people 
call the "memory" problem.  A good smart charger solves this one.  I haven't 
seen any good reason to go through full discharge cycles on my NiCd battery 
packs since I've gone to smart chargers only.  High-current smart chargers 
would also tend to zap the internal crystalline growth (a short) which was the 
source of the real Memory problem way back in the seventies.

Now, the Li-Ion pack in my ThinkPad does do much better if you drain it all 
the way down to "0%" on the charge meter rather than slapping it back on power 
as soon as you can or even when it hits the "5%" level and asks for its plug.  
It's the difference between a 35-5% jump and a 23-5% jump.  IBM says it takes 
three careful drain/charge cycles to get to this point.

The BIG problem with NiCd battery packs, aside from overheating them and 
driving the water out, is folks who discharge them too far down.  Once that 
happens, individual cells can reverse polarity, and basically trash the pack.  
Hence, my preference for smart chargers which simply top off the pack.

> NiMH have a higher capacity than NiCds (typically 1000mAH for AA size
> NiMH, versus 600-800mAH for standard NiCds), so you can expect them to
> last longer in the same application.

They do a real good job in the digicam.  Be aware that they do have a 
different discharge curve, so you may not be able to rely on the "low battery" 
signal.  And for a device that demands NiMH batts, nothing else will properly 
replace them, not unless you like throwing out alkaline cells after five or so 
shots.

Now, if I could only get them in the AAA size...

> I believe that due to the higher charging current required for NiMH, a
> charger intended for NiMH is required.  Radio Shack sells one that will
> charge either (switch selectable).

10-4 Eleanor.


And in case anyone is interested, lead-acid batteries [car batteries, gel-cells] do best when they're "floated" on a regulated 12.5V power supply.  Plus to plus, negative to negative.  The charging current is going to be low enough such that you won't be driving the water out of the cells, and they'll last "forever".  I have some ten year old packs that are still running strong.


	andy