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. 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
Thanks

"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

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

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
--
add a 2 before @

"Meindert Sprang" wrote in message ...

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.

No worse, surely, than it would be if there was only that battery and
a 12V system. It has no way of knowing that some of its output is
being boosted by another 12V.

I wouldn't have expected there to be any problem at all with the OP's
idea - but if he was worried, he could always swap the batteries every
season so they get roughly equal use as the 12V supply.

Ian

Ian Johnston wrote:

"Meindert Sprang" wrote in message
...

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.

No worse, surely, than it would be if there was only that battery and
a 12V system. It has no way of knowing that some of its output is
being boosted by another 12V.

I wouldn't have expected there to be any problem at all with the OP's
idea - but if he was worried, he could always swap the batteries every
season so they get roughly equal use as the 12V supply.

mm, the installation and maintenance of most boat batteries is so
sub-optimal that pulling a few watts off one battery probably won't make
any discernible difference, if done within reason.


--
Lithium ion internal and external batteries.
Internal from £30 External from £75 (trade)
All batteries factory new and guaranteed.
http://www.surfbaud.co.uk/
e-mail (www.rot13.com)

"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. 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
Thanks



Yes, you're correct. It will work.

CN

"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. 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
Thanks

INteresting replies. If the batteries are reasonable large, I doubt if the
meager
load of a VHF would make much difference. Make sure the battery you use
for the VHF is the one that has its ground referenced to the boats ground.

DOug

If you use a DC to DC converter, then you know you will be getting a
regulated 12 Volts (or maybe 13.8 for the VHF???) ... just get one with an
appropriate power output ... i.e., no need to spend big bucks for more power
than you need. I myself would go with a converter ... that way the VHF is
guaranteed to work.

"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. 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
Thanks

A VHF doesn't need regulated DC. They are designed to work off of
any reasonable voltage between 11 and 16.

"bowgus" wrote in message
...
If you use a DC to DC converter, then you know you will be getting a
regulated 12 Volts (or maybe 13.8 for the VHF???) ... just get one with an
appropriate power output ... i.e., no need to spend big bucks for more
power
than you need. I myself would go with a converter ... that way the VHF is
guaranteed to work.

"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. 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
Thanks