schreef:

Some large trucks use 12-volt systems and 24-volt starters.
They have two 12-volt batteries, connected to a series-parallel relay
that makes the connections necessary for starting.

Truck needn't be so large. I remember exactly such a configuration on a
Mercedes 608. The relay was right under the driver's seat, the loud
"CLACK!" always made me jump when I started them the first time after
some while.
KA