12volts dc battery output to 15volt dc

Yes, it is possible. How much current do you need? Do you want to build or buy? There are a wide variety of DC-DC power supplies that can do this (google is your friend...). Basically what the "upverters" do is to convert the incoming DC to AC, a transformer steps up the voltage, then it rectifies and regulates to DC again. Can also be done with an inductor instead of a transformer.

Yes. The commonest approach involves the use of a type of switching regulator known as a "boost converter" (DC voltage output > DC voltage input). In some cases a "boost / buck converter" may be used - these would (for example) allow 15 volt DC output, from DC sources having a fairly wide range (e.g. 9 - 24 VDC input).

Such devices are commercially available for specific purposes. For example, the MFJ-4416B is a boost converter, which is designed to provide a stable 13.8 VDC output from a 12-volt battery system, so that amateur-radio transmitters will operate properly and at full rated power when fed from battery supplies.

I don't know how easy it would be to adapt this particular product to produce 15 VDC rather than 13.8 VDC. The schematic suggests that it has some adjustment range in the control circuitry, but it also appears to have several 15 volt protective zener diodes in the output which would start conducting if the voltage rose too high.

Converters of this sort are not 100% efficient - some power is necessarily wasted as heat in the circuitry.

Yes: You need a "boost converter":

Good Luck! Rich

OOps! THIS is the one with followups-to set. :-) R