In article , chuck
wrote:

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

Ok, now listen up folks. There is no difference between, one
24Vdc battery made with 12 cells in series and two 12Vdc batteries in
series, or four 6 Vdc batteries in series, as long as the
interconnection links between cells are of similar low resistance.
Tapping ANY battery system, in the middle IS BAD. The more power
you draw off the tap as a percentage of total power available, the more
damage you will do over time to the battery system.
A single Vhf Radio will draw say 1 amp Standby, 2 amps Receive, and near
7 amps Transmit. Since most useage figures say 75% Standby, 15% Receive,
and 10% Transmit, this would not be a significant load differential
across, a battery system of like 8D's in series. That said, tapping a
battery system, is a BAD IDEA. If you are running a radar, couple vhf's,
GPS or two, Plotter, ECT, then your ASKING FOR TROUBLE, and it WILL find
you. Best Idea is to get a Switching DC to DC Converter, like a Numar
32-12-X where X is the total operational load. One could very easily
put a Group 12 12Vdc onboard, and use the DC to DC converter to keep
the Group 12 battery charged. This is a typical system, that would meet
the USCG Fishing Vessel Safety Act, Radio Power Requirement.

I have no idea where the first replyer got his information, or training
in Marine Electronics, but if he paid any money for it, he should go
back and ask for a refund.


Bruce in alaska
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add a 2 before @