"Ric" wrote in message
...
I have just bought a couple of Delphi Freedom marine deep cycle batteries
for my service bank. Is there a recommended minimum voltage below which
one
should not discharge? Also, if the nominal capacity is (say) 100Ah, is
that
measured between standard voltages or is it measured from a nominal full
charge (with full charge voltage dependent on the charging system) until
totally discharged - ie 0v? Or is there a standard "never go below"
voltage
which manufacturers use?

snip

I'll assume at first that whenever you leave the battery idle it's fully
charged. Otherwise you're shortening its life through 'sulphation' -
hardening of the arteries as far as I'm concerned. I'll also assume you
don't use very high discharge currents (50a) for periods of time that would
cause the battery to overheat.

The life of your battery will then depend on how many, and how deep, your
discharge cycles are. Typically, you'll manage about 1000 cycles if you
discharge to 50% capacity. If you discharge to only 25%, you can expect
nearly four times the number of discharge cycles. Take it down to 75%
regularly and you'll only manage a few hundred cycles at best. These figures
vary a lot between different batteries depending on how they're built
(number of plates, support structures for the plates) but the pattern is
typical of deep cycle cells. Starter batteries are much more tolerant of
high amperages, but less tolerant of deep discharges (they have many more
plates and typically a less robust support frame).

Discharge percentage is most conveniently measured with an AH meter, though
if you allow the battery to rest with no loads or charges for an hour you'll
get a pretty good idea from voltage alone how far you've discharged. Varies
a bit with temperature, but 100% is typically 12.75v, 50% is 12.25v and 10%
(may you never see it!) is 11.75v. More precise is a hydrometer - not
convenient for frequent monitoring!

My vessel has a bigger battery capacity than I need (600AH), but I rarely
allow more than 25% discharge - 150AH a day. They were fitted in 1992, and
still work fine, though checking specific gravity against AH consumed shows
that they have probably lost about 20% of their capacity.

Hope that helps.
--
Jim B, Yacht RAPAZ,
Sadly, for sale:
http://homepage.ntlworld.com/jim.bae...cification.htm
jim[dot]baerselman[at]ntlworld[dot]com