What is the second battery for on a ford E-350 truck?

if you have a seconf battery im assume it is a diesel 2 batteries are used to give the required cranking amps to start a the engine the batteries are coneected in paralal so as toboost amps but keep volts at 12 volts if batteries were connected in series it would boost voltage to 24 and burn out the electrics

Yeah, if its a diesel (I have a 1995 7.3l E-350...van, not a truck!) The driver side battery is 800-850 cranking amps, mostly for starting, plus powers some electronics. The passenger side is 500-550 cranking amps and some other electronics. Takes a lot of juice to crank over a diesel, it's a higher compression engine and the compression ignites the fuel rather than a spark plug.