The two immediate possibilities that come to mind are (1)
your older battery is somehow discharging your newer one,
despite the combiner; or (2) there is a "hidden" load
somewhere in your system.

Most likely scenario is that the battery you replaced may
have ruined its partner, which is now draining your new one.

Easiest thing to do is bring the batteries up to full charge
and then disconnect one lead from each battery, taking care
to secure the leads.

Give it as along as you can, and then monitor the voltage on
each. Put a 10 - 20 amp load on each one while you measure
the voltage if you can. If one is good and the other is not,
there's your answer. (old battery somehow draining new, but
that shouldn't happen if the combiner is working properly)

If both batteries are good, then connect them as before, but
this time, make certain your switch is in the "off"
position. Give that situation as long as you can and then
check voltages (remove one battery lead so they are not
connected via the combiner when you measure voltage.)

If both are good, then you probably have a "hidden" load. If
both batteries read low voltages, then you probably have a
bad combiner. I suppose one of the diodes could be shorted.

Good luck, and let us know what you find.

Chuck


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?