Another way is to convert one of those USB chargers that fit in the cigarette lighter socket so it fits on your battery. Those chargers will take anything from 12 volts to 24 volts and put out 5 volts USB with various smart charging protocols.
Getting it to physically fit the battery is the picky part - the wiring is simple.
Getting it to physically fit the battery is the picky part - the wiring is simple.