added water to the battery ??

If you have been running a cell low for a while its probably cooked -
yur battery might put out only enough current to light lights and not
enough for the ultimate test - turning the starter.

electrical stuff is seldom that mysterious unless you have had a wiring
fire in my exp.

Did you try measuring the density of the electrolyte in each cell
(hygrometer from kmart for a couple bucks will do it) to test above thy?

ss