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