[Milsurplus] 12v -- 24v Vehicle Power

[J. Forster]

Wiring batteries in parallel is not a really great idea, unless you insert
some resistance in the circuit in the charging mode.

Suppose you've used the 24 V supply for a while. One 12 V battery has been
partly drained, and the other has a nearly full charge, either because the
alternator has been running or it has much higher capacity.

Then you flip the switch. The floating battery is put across the main
battery, and, because the circuit is low resistance, large currents
circulate. This can overload the floating battery and cause failure,
possibly quite destructively.

Designing and building a 12 V to 24 V DC/DC converter is pretty straight
forward, as long as you don't need to do something requiring large
currents like starting a dyno. While high operating frequencies (10 KHz or
above) may seem attractive from the small core size needed, you get into
switching issues with the transistors that get tricky. The transistor
dissipation during switching can exceed the Safe Operating region.

I'd go with something fairly simple, like a torroid of squarish loop
material w/. decent incremental permeability, roughly 1/2 to 1 pound,
wound w/ 3 bifilar windings (connected up to make center tapped windings
for input, feedback, and output. Spread the windings out so each covers
rhe full 360 degrees of the core. Close coupling reduces switching
transients.

Pick a couple of beefy transistors (2N3055s for 50 to 100 W, or several in
parallel w/ small emitter resistors for more power) and a simle bias
network of two resistors and a diode.

The secondary is connected in a two-diode full bridge configuration.


> If the goal is to be able to operate a 24-volt radio in a 12-volt vehicle
> the simplest choice would seem to be an auxiliary battery.
>
> Keep the vehicle 12-volt system intact.
>
> Wire a second 12-volt battery (can be much smaller than the vehicle
> battery
> since it has to supply current for the radio only) in parallel with the
> vehicle battery through a switch. That way, it charges off the alternator
> along with the vehicle battery.
>
> When you want to operate the radio, throw the switch so that the auxiliary
> battery is in series with the vehicle battery. Voila: 24 volts.
>
> When you're done with the radio switch the batteries into parallel
> configuration to recharge.
>
> That's a dickens of a lot easier than building a converter.
>
> With a little extra effort the changeover process could be automated.
>
> Miles B. Anderson, K2CBY.
> 16 Round Pond Lane
> Sag Harbor, NY 11963
> Tel.: (631) 725-4400
> Fax.: (631) 725-2223
> e-mail: k2cby at optonline.net
>
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