cummins 6b5.9 alternator failures

Mine is a Prestolite 160 amp, J180 mounting in a cummins 300 HP 2003, see below. I had it rebuilt and the guy told me not to charge dead batteries with the alternator as it will overheat, the diode where toasted. He said if your batteries are low to recharge them with the gererator or shore power before starting the engine, this will save the alternator.

http://www.prestolite.com/pgs_products/specs.php?item_detail_id=25083&item=8LHA2071VA