[rescue] NiMH Batteries, test outside computer?

[Tim H.]

On Mon, 21 Oct 2002 09:22:34 -0400
Rob Harhen <Axthrower at fozzy-networks.com> wrote:

> Have no knowledge of the application your talking about, but I would 
> imagine you could 'fool' it into thinking it had a battery by either 
> putting a resistor  to replace the battery (same resistance of the 
> battery V=IR)

I am thoroughly baffled by that suggestion.  If you are talking about
matching the internal resistance of the battery, I don't see that having
any effect, since the internal resistance of the battery is
inconsequential until the circuit approaches a dead short.  If you are
talking about a resistor to match the load on the charging circuit, that
isn't practical, because the battery does not act as a resistor, it acts
as a variable voltage source, opposed to the charging circuit source.
The battery checking capability of the charging circuit is basically
examining the charge/discharge curve of the battery, a resistor will
have no curve.

My DUO battery has no specs on it, but the DUO says 24V at 1.5A, or 36
Watts, I would probably try to test a charged battery with a 10-15 Watt
load, If the battery is 6V (I don't know this, it doesn't say, and mine
is dead) it would be in the vicinity of a 3 Ohm load.

6V/3ohms=2amps, 2amps*6V=12watts and see how fast the voltage dropped
off.  If you know the rated amp/hour capacity of the battery, then
simple math will tell you if it is still in the ballpark.

Since most people don't have a power decade box around, the easiest way
would to find some combination of parallel resistances that came close,
remembering that equal resistors in parallelhave half the resistance and
carry half the power.  for example 4 5 watt resistors of 14.7 ohms
(isn't that a standard value) in parallel would equal 3.675 ohms, and
handle 20 watts.  that's a bit less load, a little less than 10 watts,
and be careful about touching a power resistor under load, they get
pretty warm as they approach their ratied limit.

Of course if you can't convince the DUO to charge the battery you can't
load test it.  If you try an external charger, be aware that some
batteries use different terminals for charging, and use diodes to
prevent charging on the output terminals.  If you have a known bad
battery you can break it open and probably find answers to all your
questions, because most are made of standard cells, which have standard
capacities.  In fact, my DUO battery looks suspiciously like it may
contain 2 rows of 5 AA size cells.  If that is the case (NiMH AA is 1450
mAH at 1.2V) it would be either 6V at 2900mAH or 12V at 1450mAH, depending on
configuration.  12V seems more likely for a laptop.

Tim