piclist 2001\04\27\123446a >
Thread: NiMH battery chargers
www.piclist.com/techref/power/batterys.htm?key=battery
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face BY : David Cary email (remove spam text)



Dear battery charger people:

[When does this topic move to [EE]: ?]

Bob Blick <EraseMEbob.....spam.....TED.NET> on 2001-04-27 10:22:28 AM had several good points, then
mentioned something that triggered an old memory:
> You can also measure the peak-to-peak voltage and get an idea of what's
> going on as the unregulated supply goes on and off 120 times per second.

Measuring the voltage across the battery this way gives you
the ESR (effective series resistance) of the battery.
ESR = (charging_voltage - no_load_voltage) / ( charging_current ).

I hear that some battery chargers periodically interrupt charging to measure the
(no-load) voltage of the battery, then *discharge* the battery for a few dozen
milliseconds at some fixed current (or some precision resistance), measuring the
voltage-under-load of the battery. This also gives the ESR of the battery. (same
equation, but plug in a negative charging_current ... ESR still ends up
positive.).

I know that both kinds of ESR are very low in a freshly-charged battery, and a
completely drained battery has a very high ESR (both kinds). They are not
exactly the same, but I don't know why.

From the point of view of using a battery under high-current conditions (RC auto
racing sprints), the "ESR while discharging" is crucial -- that's the main
bottleneck in sucking the energy out of the battery as quickly as possible and
turning it into SPEED.

--
David Cary

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