Some good answers so far. I used to sell and install those things and
they ALL have issues.
We'll start with basic principles. You can get a meter that keeps track
of current in and current out. Kind of like an odometer that rolls
forwards and backwards.
The current out part is only semi-tricky. Amps X Time = Amp/Hours. If
you use 100 amps for one hour or 1 amp for 100 hours, you used 100
amp-hours. Peukerts Law (https://en.wikipedia.org/wiki/Peukert%27s_law)
screws this up. 100 amps for 1 hour takes considerably more out of a
battery than 1 amp for 100 hours. The various meters have different ways
of accounting for this, but none of them are perfect. Some have preset
constants for different battery types and some have learning functions.
Either way, errors add up and eventually your meter is way off. Current
in is much worse. Charge current can actually charge the battery, heat
it up, or boil off electrolyte. It has to go somewhere, but that
somewhere is not always stored electricity. I could put a constant 1 amp
charge on a battery for 500 hours, but that does not mean it gained 500
amp-hours. It could have gained 5 and got slightly warm instead. Knowing
when to stop counting - when the battery is topped off - can be very
tricky.
Bottom line is I found you have to reset the meter every so often to
bring it back to reality. If you are on a good charger all week, chances
are the batteries are full and you can reset. If you use 100 AH out of a
400 AH bank and the batteries are at 11.9 volts, chances are they are beat.
Joe
Coquina
C&C 35 MK I
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