Automotive starting/charging systems are either 6- or 12-volt. 6-volt are the very old cars. A diesel is still a 12-volt system. So you can still use it to jump start a gasser. If the batteries were wired-up in a series, it would then be a 24-volt system (like when adding 2 1.5-volt 'D' batteries to a flashlight makes it a 3-volt). It is always a good idea to replace all batteries even when just 1 fails.

 

From my experience when you replace one battery the other one usually goes shortly after. You make your own decision on that.

Batteries - parellel just the same as ag tractors and semi's.

Stevlin