Hello Nigel,

Seems you have an interesting situation to begin with. If
you are charging two 12 volt batteries in series, (with a 24
volt alternator or charger) then unless the two batteries
are identical (in age, internal characteristics, etc), they
will be charged to two different voltages. Not really the
best situation.

Now if you connect a load to just one of the batteries, that
will upset the equal distribution of charge voltage across
each battery, as Meindert has suggested. Of course, it could
actually improve the balance, as you can imagine. Just out
of curiosity, have you ever measured the voltage across each
battery while charging? It would be interesting to see what
differences you found, if any.

Ignoring all of that, it is really a matter of degree. If
you run a VHF from just one of the batteries, that would do
a lot less potential harm than if you ran, say, a
watermaker. You might get away with just the VHF. If you had
a lot of time to play with this, you could arrange to put
the same load on each battery. Say a VHF on one and
something that mimicked the VHF's load on the other. That
would require special wiring, or course, for the battery
that didn't have its negative terminal connected to the boat
ground.

Has this been confusing enough?

Good luck.

Chuck








Meindert Sprang wrote:
"Nigel" wrote in message
...

I have a 24v electrical set up on my boat, but need a 12v supply for my

VHF

(amongst other things). Is there any reason why I can't just run a 12v
supply from just one of a pair of batteries, or do I need to use a 24v/12v
converter.


You need a converter.


I release this will draw charge from just one battery , but won't
the other just top it up and there by drain them both equally


No, because the other one is connected in series, not in parallel. So one
battery gets drained faster that the other and that is the success-formula
for wrecking that battery.

Meindert