The other guys have given good advice.
I notice you said your voltmeter indicated 13.5 volts while charging.
Is that the voltmeter on the dash of the boat? If so use a good
voltmeter and check the voltage at the battery terminals of each
battery. If the charge voltage is really that low then that may be
part of your problem as they will never charge fully at that voltage.
You need 14 to 14.5 volts to charge.

Also if one of the batteries is weak it may be running down the good
one as the others have said. But it shouldn't do so if you do not
parallel them with the battery switch unless you have a shorted diode
in the combiner.

With the battery switch in the off position, check the voltage on each
battery with a voltmeter. You should have no need to run the battery
switch in the both position except if you have a starting problem.
Then return the switch to one.
They should both still charge through the combiner if it is installed
correctly. That way neither battery will run the other down and a load
on one will not drain the other.

Regards
Gary


On Sat, 2 Apr 2005 15:17:05 -0800, "William G. Andersen"
wrote:

How do I find what's draining the power on my battery?
I use my boat twice a week for several hours. Two weeks ago neither battery
would start the engine. Battery 1 was low, but would hold a charge, so I had
it charged for an hour before launching while I went out and bought a
replacement for Battery 2 which was several years old, dead, and wouldn't
hold a charge.
Since then, I've used the boat several times, VM always indicates 13.5 when
running.
I forgot to turn off the master battery switch yesterday. When I checked it
today it was less than 11 volts, and nothing had been left on.
I have two batteries with a 1,2,both, none switch and combiner: that's not
new, I added the second battery, switch and combiner two years ago.
Before I added the second battery, I ran separate wires from Battery 2 for a
VHF radio and radar on the passenger side of the boat, because there was no
electrical wiring there. I've since added two utility plugs to that wire.
I'm tempted to disconnect the two utility plugs to see if they've been
draining power.
But I haven't done anything electrical for several months and have had this
problem for just the last two weeks.
Is there a simpler way of finding the drain? Am I going to have to
disconnect the wires for everything on the boat and add them one at a time
until I find one that's draining power?