l have a 2000 Ford F250 w/ dual batteries factory. l was wondering which one is used to power the starter.

Duracell.

The owners manual should have this information.

Normally when a truck has two batteries it work together in series, so both batteries provide power to start at the same time (series conn.)

They both power the starter , but if you just want to replace one of them , then replace the one close is to the starter , (on the passengers side.)

They both work in series to start the truck. When you one goes bad you half to replace both, unlike the dumb ass not so handyman said. If you replace one at a time you be replacing batteries left and right until you replace both.

SORRY they are hooked in parallel, if in series then you would have 24volt system an that is not the case it is 12 volt, series is positive to neg hook up, they are hooked pos to pos an neg to neg. AND if you replace one you replace BOTH batteries for the new one will drop to the level of the older one left. AND yes they both supply power to the starter not just one of them. henze the need for two batteries in parallel. more amps with the same 12 volts. hope this sovles your problem