Originally Posted by charles_m
The starting batteries in my bus are in great shape and less than a year old. But I am considering swapping them out for either two or three deep-cycle lead-acid batteries since now I use them as storage for my 12v system and stuff.
I'd be very surprised to see 2x T105s turn over a big 6 cylinder diesel engine. If they were able to turn it, I wouldn't expect them to do the job for long due to the voltage drop.
I killed the battery in our Subaru Legacy last summer and, luckily, had a fully charged 12v deep-cycle battery in the back of the car. I used the jumper cables to connect the starter battery to the deep cycle battery, left them for 10 minutes and tried to crank the car. Just a click. It didn't turn over at all. After leaving it for 30 minutes it had transferred enough to the starter battery to crank the engine. That was a 2.2l gas engine. The starter for a diesel will pull many times the amps to get the engine spinning.
Similarly, I have boosted the bus engine twice by connecting the house batteries to the starter batteries, letting it sit for 15-20 minutes then turning the engine over. That works just fine.
Do you have the specs for the batteries you intend to use? Perhaps there is a listed maximum amp draw. I would guess that you'd need a burst of 300-400 amps to get the starter spinning.