You post a deceptively simple question. The answer isn't simple at all.
Assuming you have flooded lead-acid batteries aboard, there are several factors to consider.
Lead-acid batteries deteriorate when allowed to sit without a charge on them. This is true even for new batteries on the dealer's shelves; they begin to deteriorate the moment they leave the factory. Nigel told me he's measured differences in new battery capacity as much as 30%!!
There are several factors involved in this deterioration, including corrosion of plates, sulfation of plates, stratification, contamination, and others. Of these, the most important contributor to reduced battery capacity is generally caused by sulfation, i.e., the formation of PbSO4 crystals which attach to the plates and, if left in place, eventually embed themselves into the plates and reduce capacity.
Lead-acid, flooded batteries have a significant self-discharge rate. That is, even when just sitting, they lose charge rather quickly. As they lose charge, the rate of sulfation increases.
Now comes the kicker: you're damned if you do and damned if you don't. Even a flooded lead-acid battery which is left on float voltage of about 13.2V (for a 12-volt battery) will lose capacity over time.
That's why it's recommended to periodically "exercise" your batteries and to "equalize" them at intervals of several months. Among other things, exercising and equalizing tends to reduce the effects of stratification and of sulfation.
A cruising boat which sits at the dock for several weeks or months and then goes cruising will normally see an alarmingly low voltage after being on batteries alone at anchor for awhile. However, after several discharge-charge cycles, voltage will tend to hold in there longer and the batteries will appear to have increased their capacity to handle normal loads.
So, what do you do to improve battery longevity?
As best I'm able to discern, through formal testing and thru personal experience, for boats at dockside much/most of the time I believe it's best to:
1. use a smart battery charger only, and preferably one which uses pulse width modulation (PWM) technology...like the Iotas;
2. leave the charger on all the time (assuming that you have reliable dock power and that you have a properly installed AC system and battery charger);
3. equalize the batteries at intervals recommended by the manufacturer. Note that the PWM chargers like the Iota MAY reduce the need for equalization;
4. exercise the batteries periodically, drawing them down to not more than 50% of capacity and then fully recharging them;
5. use ONLY distilled water to be sure the electrolyte is at least 3/8" above the plates;
6. use Hydrocaps or WaterMiser caps to reduce the need for adding water;
7. keep all connections very clean and tight;
8. be extremely careful not to introduce contaminants into the battery cells; and
9. monitor battery voltage periodically, with and without charging and load.
Sound like a lot of work? Not really, just a little attention on a regular basis.
Last edited by btrayfors; 11-13-2008 at 12:12 PM.