Resting state voltage readings can provide a rough guide of battery
charge state. For accurate voltage readings, batteries must remain
idle (no charging, no discharging) for at least 6 hrs, preferably 24
hrs. If loads have been light for a few hours, you can shorten these
times; for example, after running a 1 amp anchor light only, overnight,
using your 300 amp bank (0.3% of C discharge rate), turn off all loads,
wait an hour or so, then test.

Assuming generic wet cell lead acid batteries a room temperature
environment, Trojan Battery company publishes this table:

% Charge V
100 12.73
90 12.62
80 12.50
70 12.37
60 12.24
50 12.10
40 11.96
30 11.81
20 11.66
10 11.51


Decide how long you want your battery bank to last, considering the
following, per Trojan:

1. Shallow discharges will result in a longer battery life.
2. 50% (or less) discharges are recommended.
3. 80% discharge is the maximum safe discharge.
4. Do not fully discharge flooded batteries (80% or more). This will
damage (or kill) the battery.
5. Many experts recommend operating batteries only between the 50% to
85% of full charge range. A periodic equalization charge is a must when
using this practice.
6. Do not leave batteries deeply discharged for any length of time.
7. lead acid batteries do not develop a memory and need not be fully
discharged before recharging.
8. Batteries should be charged after each period of use.

So a reasonable approach from a dollars per amp-hour standpoint would
be to shoot for a recharge cycle at the 50% discharge state, don't get
worked up about the occasional discharge to 80%, recharge anyway even
if you've not gotten to 50% discharge if a few days have passed, and
leave your batteries fully charged if you'll be gone for more than a
few days. Such an approach means you would fire up the engine when you
see the resting state voltage drops to about 12.1 volts.


varnish wrote:
I've got a 300AH domestic battery bank and monitor the voltage across it
while cruising. It's usually around 12.8 when charged. What does it
have to drop to before I run the engine to recharge it?