12v supply from a 24v system
 

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

"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

No question that a DC to DC converter completely removes the
problem. Whether it is economical depends on what "amongst
other things" Nigel wants to connect at 12 volts. FWIW,
Ample Power for one makes 24 volt chargers that are really
two independent 12 volt chargers, one across each battery.
That ensures the proper charging voltage for each.

Chuck


bowgus wrote:
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

"Nigel" wrote in
:

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



How about a nice switching (99% efficient) converter, instead of some
jackleg series regulator draining power off the 24V system as lots of heat?

http://www.samlexamerica.com/product...yID=5&packaget
ype=1&title=Stand%20Alone%20DC%20Power%20Converter

Figure 10A of each VHF radio and SMALL radar, Add up all the instruction
book 12V amperages, then buy the next higher model.

This is but one company producing what you need. Search Google for "24VDC
to 12VDC converter" with the quotation marks so it doesn't find every
converter on the planet. You'll get a whole list of them.

This guy's SD-350B-12 at 350W output generates 27A at 12V regulated solid
as a rock from ANY dc input voltage between 19 and 36V.....saving the 12V
electronics from any nasty spikes on your 24V system....$215? Who's gonna
believe that when a 10A battery charger at Waste Marine is double that!!

2:1 input range
1500VAC I/O isolation
100% full load burn-in test
Short circuit protection
Over load/voltage protection
Over temperature protection
Forced air cooling by built-in DC fan
Low cost, high reliability
2 year warranty

Looks like you'd play hell trying to destroy it, too!

Don't pay any attention to replies that talk about converters. Just run a
pair of leads from the two terminals of the 12v battery. You are simply
creating a new 12v distribution system. It is very common for boats and
aircraft to install different voltage distribution systems, and you do not
need converters to do it.

If this was just a single isolated device then you could use either battery.
But if the 12v devices involve ANY electrical connection to anything in the
24v system then be sure that the new 12v system uses the battery that is
also the battery connected to ground in the 24v system. You could help
balance the load between batteries (although I doubt that it's necessary) by
using the 12v+ and 24v+ as another 12v circuit, but only for fully isolated
items (such as lighting).
--

"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