"Jeff Morris" wrote in message ...
Joakim Majander" wrote in message


No load voltage is a good indicator, if you know how to use it. You
shouldn't read the voltage after charging without load and never with
a high load.

So its reliable except when charging or discharging. And for a while afterwards.

Its not usable while charging, but with a rather small or even typical
load it is usable very soon (1-10 minutes depending on accuracy) after
charging/discharging. Without any load it might take much longer for
the voltage to drop after charging. Thus you have to know how to use
it. With an accurate meter (3 or 4 numbers after decimal point) you
can easily see, if the voltage is still changing and how adding a load
affects.



How do you use a voltmeter when there's a solar panel charging at 6 Amps, or the fridge is drawing
25 Amps?

You can disconnect the solar panel for a minute or two or check the
reading when it is not full daylight. If your fridge really draws 25 A
I hope you have at least 500 Ah of batteries, which would still last
only 1-2 days (20-50% cycle and not all capasity). For 500 Ah 25 A is
a small enough load to be qive quite accurate "no load voltage". If
you have an "over-powered" compressor, you have to be carefull not to
read the voltage while it is running.


How do you know what is the
starting point (after few hours loading capacity could be anywhere
between 70 and 100%)?

Being on shore power for a day, or running the engine for several hours generally puts you at a
reliable starting point.

You have to charge at least 24 hours to get anywhere near 100%. After
"running the engine for several hours" you can be anywhere between 70
and 100%. Charging from 50-70% to 100% is very slow.


So? Life is full of inaccuracies. If you use a battery the same way every day, the capacity is not
going to vary that much. A good A-H meter compensates for the differing discharge rate with
different loads.


I think for most people a good voltmeter is accurate enough.


BTW, I currently have a dual AH meter, with one leg on the house bank, and the other on the fridge.
This gives me an accurate readout of the fridge usage. While traveling, I kept of daily log so I
could correlate with air tamp and sea temp. Usage varied between 60 Amp-Hours a day (Maine) and 110
(S FL) for a Crosby 1/2 hp holding plate system with separate fridge and freezer.

I'm planning to build my own battery monitor, which would have a log
for the voltage and current of each battery and of cource an Ah meter.

Joakim