It all comes down to how much current is "high", what voltage that needs, and what battery you have.
The basic principle is that a transformer has some turns of wire on the primary, and some on the secondary. The ratio between the number of turns is called the "turns ratio". The voltage will be reduced and the current increased by the turns ratio.
If you have a 240 V input, 12 V output transformer, it will have a turns ratio of about 20:1. You will get about 20 times as much current out as you put in. The power going in and the power going out will be about the same. The 240 V winding is called the primary winding, and the 12 V winding is called the secondary, but there is nothing to stop them being used in reverse.
There are a lot more details in the design that you need to worry about if you want to make a practical transformer. However, if you need to rewind just the secondary, and keep the primary the same, that is a lot simpler. If you halved the number of turns, and used wire twice as thick, you would end up with a 6 V transformer with twice the current rating, without having to worry about lots of the other details.
There are inverter circuits that will produce AC from DC. In the switch-mode power supplies that run most electronic appliances, the mains voltage is rectified to DC first, then put into a "flyback" circuit to make it into AC again, but at a higher frequency. The higher frequency allows for a much smaller transformer.
Also in the inverters that produce mains from a battery, a similar circuit is used to produce a few hundred volts DC. That is then chopped to produce a stepped wave which is good enough for most AC appliances. You might be able to rewind the secondary of the transformer from one of those to produce a lower voltage rather than a higher one. However, most such circuits rely of feedback to control the output voltage, so you would have to get that working.
If you don't need isolation, you can use a circuit called a "buck converter" or "buck regulator" that converts a higher DC voltage into a lower voltage, but increases the current at the same time.