Batteries have become a very misunderstood subject. Bus batteries are needed to do basically a single task. That task is to turn with a specific speed a very large and resistant load. As is known, a diesel engine creates ignition because of compression. Most bus users use batteries that will do the required work for as long as possible at the least cost. Turning a large displacement diesel engine requires lots of amperage for a short time. The amperage available in several car batteries is equal to a large bus battery. Any combination of batteries that provide the required amperage with do the same work. I prefer about any battery that is rated at about 850-950cca to spin my Cummins diesel engine. Two of these batteries are used. My favorite battery supplier is Walmart because the sores are every place and the warranty is great. Special batteries cost is expensive. Group 31, 4d, and 8d batteries are expensive and have a 18 month warranty. Making a warranty claim is very difficult. Changing and moving a heavy battery is a challenge. Many modern trucking companies are changing to 4 hd 6volt batteries. Many new trucks are being made with 24volt electrical systems and maybe cars will follow suite. Using a battery charger to start any engine is not good. The charger will be destroyed and the engine will not charge the battery. The battery must have 12 volts to make any alternator put out a voltage.
Alternators must be excited with 12 volts to put out 15 volts. The alternator uses 1/2 amp to make 100 amps charging amps. Whew, Frank in Idaho