What Steve said. A weak battery will sometimes get a cylinder to
almost TDC, but not quite have enough juice. There's enough pressure built up by then to bring the piston back down, spinning the crank and sprag backwards. The sprag can't handle backwards.
Shorai and other lithium iron batteries are different from lead acid batteries. They have their advantages and disadvantages, and are better in some ways and worse in others. The biggest difference is that they put out nearly all the voltage they're capable of until they're almost fully discharged, then drop off abruptly, instead of tapering down like a lead acid battery. On the one hand, that means that it's almost impossible to kill your sprag with a lithium, but it also means no warning if your charging system's gone out without your knowing. And speaking of that, the lithiums are sold by "lead equivalent" capacity, not actual capacity. They have nowhere near the actual capacity of a lead acid battery, though they have massively better cranking power. When I had a lithium battery in my old RS and lost my charging system, the instruments were out in under a mile and the whole bike was dead in about a mile and a quarter. When I lost my charging system in my current RS with the lead acid battery recently, I was able to get home through traffic with the fan running almost constantly from 20 miles away.
Lithium chemistry is also not good in the cold. Some people on the Sprint forum have reported complete failures, but I never ran into that problem. You just do something you would never do with a lead acid battery if you want to start a bike with a lithium battery in the cold: you turn on the lights and let it sit for a couple minutes. All that would do with lead acid is waste power, but with lithium it warms the battery enough to make it perform normally.
Lead acid batteries lose a lot of power when sitting. Lithium batteries don't. I have a lead acid battery in my primary ride, but the backup bike has lithium.
Any regular smart charger that works for modern lead acid batteries is fine for a lithium battery AS LONG AS IT IS NOT IN DESULFATION MODE. Neither of my chargers, a Schumacher and a Battery Tender Junior, even has a desulfation mode, so I'm fine there. In theory, a cell balancing charger specifically for lithium batteries is a very good idea, but in practice it hasn't seemed to make any difference at all.
Edit: Almost forgot, lithium batteries do not like overcharging! The charging system on my second generation 955 puts out somewhere between 13.8 and 14.2 VDC and is eminently suited to lithium charging.(Of course, that's the one I have the lead acid battery in for various reasons.
) The automotive type alternators in T3s and first gen 955s is not well suited to lithium; it can put out north of 15 VDC and that starts to get into territory where the lithium battery gets damaged. It's not an instant disqualification; I still run the lithium in my Gen 1 955 because it sits so much, but it's something to be aware of.
Cheers,
-Kit