Originally Posted by woke_up_new
**Is a marine battery enough to start the bus? From what I've read in the Boatowners Mech manual it seems like they're not really designed for turning over a motor necessarily.
"Cranking" batteries have lots of thin lead plates inside. They are designed to dump a lot of amperage in a hurry, but to only drain a small percentage of their overall capacity. I.e., you run them down 5% cranking an engine, and then they get immediately recharged. They'll do that for thousands of cycles.
"Deep Cycle" batteries have fewer, but thicker, lead plates. They are designed to dump a lesser amperage over a longer time. I.e., you run them down to 50% slowly, and then recharge them. They'll do that for thousands of cycles.
Either can be used for cranking, but when using deep cycle batteries for cranking, you sometimes need an extra battery to achieve the necessary "cranking amps", because those batteries have a bit lower cranking amps rating due to the thicker lead plates.
While you can use a deep cycle battery for cranking - you can't go the other way. I.e., cranking batteries should never be used for deep cycling. Draining a cranking battery much below 5% will reduce the number of cycles of its useful lifespan. Take it down by 50% instead of 5%, and you may only get a couple of hundred cycles out of it rather than a couple of thousand before it has reached the end of its useful life. Run it completely dead and you may only get a dozen cycles before it becomes useless.
"RV/Marine" batteries are a compromise between the two - fewer, thicker lead plates than a cranking battery, but not as few or as thick as a deep cycle. They'll do a little better at cranking than a deep cycle - though not as well as a true cranking battery; And they'll last more cycles than a cranking battery when heavily drained - though they won't do it for as many cycles as a deep cycle.
All the battery types do indeed have CA (cranking amps), CCA (cold cranking amps), RC (reserve capacity) and AH (amp*hours) ratings - but not all the numbers will be listed in the specs. For a cranking type battery, the manufacturer will generally refer to the CA, CCA and RC numbers, while for a deep cycle type battery, the manufacturer will generally refer to the AH numbers.