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Automotive electrical systems are often refered to as being 12volt but can go as high as 18v with the engine running, depending on make/model.
One solution would be to use a second battery and a change over relays.
The two batteries are charged in parallel in the conventional manner then when you require your special voltage the relay connects the batteries in series to produce 24vdc then you drop this down using a conventional regulator.
Can you tell us what you are trying to power it may give us some insight to your problem.
I want to build a charger that will charge 3-4 battery packs separately. Each charger will need about 15 volts output around 1 amp max. I'm not for sure on the output voltage needed, basically I want to charge 10 nimh cells with the LM-317 chip, I'm guessing around 15 volts, still looking that one up also.