did a bit more research,
typically the 2/3 rule applies, but
in a wye configuration where you are taking 208 you got only 57% or so
in wye and if you configure to take 120 off each and every leg to neutral you can get full capacity
but this will require a 3/phase panel.
delta, wye/star, and zigzag connected give you all sorts of ampacities and various voltages.
as a battery charger, where you split each pole set and rectify all and then parallel connect them for final output
you can get to near full capacity.
your still limited to the windings ampacity no matter what the voltage is, whether 120 or 48volts nominal
then there is the 1.414 thing when you rectify, so you get approx 84volts off each rectified pole, and to then drop
it to ~57.6 one will need to reduce either excitation current and/or rpm,,, and both have drawbacks in my opinion
reduce the already relatively small amount of excitation, one will need a purpose built regulator, and if you are going to
do that you may as well add the 3 stage control, temp sensor for battery temp compensation and and temp sensor
to cut back the field should the alternator get to hot.
also reduction in rpm leads to reduction in cooling fan airflow,
either way, perhaps an altered stc head for dc charging should be derated to 70-75% of nameplate kwatts?
a 12kva head would still be around 9kwatts as a battery charger, that is a huge amount of charge capability
about 156amps at 57.6vdc, hmmmm not bad?
sure would make short work of charging a battery bank, at least one sized to take that amount of current
seems appropriate for a 600-800amp/hour 48volt bank.
would be nice to see someone tackle this project even if not as large an stc head to start with?
bob g