BATTERIES- The [EFI 6.5TD] vehicle electrical systems use two 12-volt batteries connected in parallel to provide a cold cranking current of 1140 amperes at 0° F (-18° C).
Any less will not do and will quickly burn the house down... starting with switches (eg ignition, OPS), electric motors (eg starters, lift pump), sensors, solenoids, senders, etc etc. Weak batteries are most likely responsible a few PMD sales err tales too..
I would suggest purchasing a matching set of the biggest baddest highest CCA batteries you probably cant afford... Dual post batteries are highly reccommended in order to relocate the very important double gang positive post connection to the top post, slightly modified. Helps disipates the heat when the grind is on..
The double gang pos post (typically on passenger side) will get warm to hot under 'normal' start-up conditions, the slightest amount of extra cranking only makes it worse.. Excessive cranking will make it smoking hot, potentually damaging side post batteries while loosening the connection(s) as they near smeltering hot lead battery post softens releasing its grip on a most important connection... so needless to say, avoid grinding on the starter, it will quickly burn the house down..
Check out the library in my signature.. Educate yourself rookie. Dont be like most of the factory authorized pro'poser who skip the basic inspections and system checks. Assume nothing and follow proper diagnostic procedures, you'll soon be having a vantastic time..