Coal Generator status "Insufficient Resources - Coal", 43.6t available, 0kg reserved

This means that when battery gets to 25%, the coal generator will request more coal but when battery is at 25% it probably doesn't last long until the auto-sweeper or a dupe gets coal to the generator. I usually put the generator at 100% and battery at 60-90 so when battery gets below 60 the generator will work and until it gets to 90 the generator will keep requesting refueling

Ah, that explains it. I had no idea the fuel request threshold applied to the battery capacity and not the fuel capacity of the generator itself.  Thank you!