(varnish) wrote in
. uk:

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?


Voltage doesn't really tell us what the charge left is on a battery. If a
battery is lightly loaded, for instance, it will hold quite high voltage
WAY past when it should have been charged. If it's heavily loaded, it will
show low voltage, even though it's still at 80% capacity. So, measuring
voltage is a poor place to see how much is left on the battery....damned
near useless.

Here's an excellent webpage for battery maintenance:
http://www.independent-power.com/bat...aintenance.htm

Here's another:
http://www.nwes.com/using_batteries.htm

From this last webpage, you can see the most important reading of charge,
specific gravity of the electrolyte:

"The term specific gravity describes the ratio of the density of
electrolyte to the density of water. Electrolyte weighing 1.2 times as much
as the same volume of water has a specific gravity of 1.200. The full
charge gravity of a cell is a matter of design and depends on several
factors. The specific gravity must be high enough to contain the amount of
sulfuric acid necessary to meet the chemical needs of a cell. If the
sulfuric acid content is too high, damage may result to the cell. The
standard full charge gravity for lead acid batteries used in an R-E system
is 1.250 to 1.285 depending on which type of battery you are using. Since
the acid content of the electrolyte decreases linearly as the cell is
discharged, the decrease in gravity is directly proportionate to the amount
in ampere-hours taken out. The specific gravity at any point in the
discharge indicates the depth of discharge, and can be translated into amp
hours taken out. A cell having a full charge specific gravity of 1.280 and
a final specific gravity of 1.130 has a gravity drop of 150 points.

Example: assume the specific gravity is 1.180 at 77°F at the end of a
discharge. That is 100 points specific gravity below the full charge
gravity, therefore, 100 ÷ 150=67% discharged of rated capacity. So if your
battery were rated at 1000 amp hours you would have taken 670 amp hrs out
of the battery. Use this formula and the readings from your amp/hr meter to
get a good idea as to the battery’s State Of Charge (SOC)."

So, we'll need a good, TEMPERATURE-COMPENSATED, hydrometer. I like to keep
gravity above 1.190, around 50% discharge, as the lower limit of how far
I'll discharge the cells....ESPECIALLY if I'm going to have to leave them
discharged for longer than overnight. Battery companies say you can
discharge safely to 80% of rated capacity, but I wouldn't do that unless
you're standing at the switch of the recharging equipment. Don't leave
them less than 50% discharged for any period of time and they'll last a
LONG time.

On Lionheart, we use the Xantrex Link 10 ampere-hour meter to monitor
volts/load current at the battery and ampere-hours used. It works OK, my
captain takes it as gospel, but I don't. But, you can't measure the
gravity with my hydrometer in 8' seas, so I compromise...(c;

http://www.wholesalesolar.com/products.folder/meter-
folder/Xantrex_link10.html