sci.electronics.basics

Jim Elbrecht wrote:
> I've got a couple dozen Nimh batteries that are starting to show signs
> of age. [some are 7yr old 800mAh, the newest are 2000 mAh & 3-4 yrs
> old]
>
> I googled for capacity testers and found a post that said to wire a 1
> ohm resistor in series with a aa battery operated clock. Set the
> clock to 12, insert battery, wait until clock stops.

Hmm..... I think this is where things actually went wrong. This will never
work.

> I have a box of clock parts so I set up a 4-battery tester and set it
> in motion yesterday with some of the 2000 mAh batteries.
>
> Did I wire it wrong, or did I miscalculate how long it should take to
> discharge a 2000 mAh battery? At 25 hrs, 45 minutes we're still
> running the clocks.
>
> The resistor I used is a Radio Shack- Ohm, 10w. - part #271-131.
>
> I connected one end of the resistor to the positive end of the
> battery, and the other to the positive terminal in the clock, the
> negative end of the battery to the negative terminal on the clock.

What you really need to do is to connect the 1R resistor in "parallel" with
the battery, not in series. This will put a hefty load on the battery
(about when it is full). The resistor (you said 10W) will still get
warm, so be careful. The problem with discharging like this is that the
load decreases as the battery voltage falls. This makes it a calculus
problem to determine the charge that was in the battery. If you used a
"constant current" load, calculating battery capacity would be as simple as
timing how long it takes.

With that out of the way, I suspect that you can get an approximate capacity
by using the clock as described. Assuming that it stops when the battery
voltage drops to about .8V AND pretending that the voltage curve is linear.
You are averaging a 1A load over time, so if the clock stops at 1:30, you
could sorta say that the battery had 1500mAh charge.....sorta. ;-)