T&T: Battery Tips

[capteric36]

 Ok.
 First NEVER fully discharge ANY battery. Try to 
recharge at about a 20% discharged point (yes, 80% 
charge still remaining).
 second, the only way to determine the condition 
or capacity of your battery is to perform a full 
charge, then an equalization charge, then another 
full charge (at least 24 hours wait time between 
the equalization charge and the final charge) . 
Then wait another 24 hours and measure the battery 
voltage under a ZERO LOAD condition. This means no 
load AND no charge was connected to the battery 
for 24 hours. Write down that voltage.
 Since every battery is a little different (I have 
two identical 40 AH AGM batteries, one measures 
12.95 volts at full charge the other measures 
12.88 volts at full charge. The one with the lower 
voltage is NEWER than the one with the higher 
voltage!) you will need to determine the percent 
of voltage difference between the full charge 
voltage and the voltage after you drain about 25% 
of the RATED amp hours out of it.
 If you look at the manufacturer's data sheets 
they will give you a guide to how much PERCENT of 
fully charged voltage should be lost at a variety 
of discharge points.
 Now drain about 23% of the charge off your 
battery and then wait 24 hours with nothing 
connected to it.
 NOW measure the battery's voltage to compute the 
percent of voltage lost. Then you can compute the 
ACTUAL amount of battery capacity.

 It is NOT worth all that effort.
 Just buy a Xantrex XBM (Xantrex Battery Monitor) 
and the optional temperature sensor and set it up 
as if the battery had the RATED capacity. The XBM 
will eventually TELL YOU what your battery bank 
capacity actually IS.

 And try to remember that the battery capacity is 
affected by TEMPERATURE as well so in the winter 
it has LESS capacity and in the summer it has more 
capacity.

 Again, not worth the effort. Just get more 
batteries if you are not happy with the 
performance of your inverter...

 Just my $0.02


Eric Thompson
S/V Procrastinator
South San Francisco
capteric36 at sbcglobal.net