If you find the specs for your engine and starter motor
, I would guess the starter motor
itself is rated for somewhere between one and two kilowatts of drain. All you need from a starter battery is one that can supply 1-2kW for five or ten seconds for starting then, and it becomes a question of reading battery specs to see how small a battery you can get, which will provide that a "sufficient" number of times. Even a small battery will provide that power once, twice, maybe fifty times. But if you want 200, 500, or 2500 starts...you simply need a bigger battery as your demands on it grow.
Let's say, 2kW for five seconds, that's something like 0.0015 kW-hours of discharge? So if the battery itself can withstand a brief 166-amp draw to do that...the only real question is if your diesel
cranking, putting a much greater strain on the battery.
If the lithium folks are right and you can expect 2000-5000 complete discharges from their batteries...then I'd guess even a "tiny" 40AH rated battery probably can do this. But you'd have to run the numbers to explore that. And the BMS would probably cost more than the battery!
(And don't trust my math, there's a reason I'm not allowed to practice math across state lines.<G>)