Peter Bennett wrote:

On Thu, 21 Apr 2005 09:22:27 -0700, Keith Hughes
wrote:


2. If the 12V device (VHF IIRC) is a significant load, you can always
build a simple voltage divider (the VHF should be input voltage tolerant
enough to handle the voltage sag during transmission) and run from both
batteries. The overall battery draw will, of course, be somewhat larger
since you'll dissipate heat in the dropping resistor. If multiple
devices are used, however, this approach quickly becomes problematic.

A series voltage dropping resistor may work for some devices where the
current draw is constant, but it _will not_ work for a VHF radio. The
radio will only draw a few hundred mA on receive when squelched,
somewhat more when actually receiving, and a few amps when
transmitting - there is no way a simple resistor can keep the supply
voltage to the radio within acceptable bounds with that current
variation.

Keith mentioned a voltage divider. That's not a simple resistor.

Even a voltage divider is unlikely to be very satisfactory either,
since in order to keep the voltage within acceptable range for the
VHF, the resistance values will probably need to be so low, you'd
be dissipating as much power in the divider all the time as the
VHF on full-power transmit.

One possibility might be to use a zener diode in place of a simple
series resistor.

At the end of the day, having different-voltage devices aboard is
always going to be a pig's breakfast unless you have proper
fully independent systems, with two alternators, one for charging
the 24V batteries, and one for the 12V ones, or a charge controller
capable of charging a 12V battery from the 24V alternator.