the controller ics for lithium ion batteries have to deduce charge from last charge time and length and amount of usage. some use temp also. this is because lithiums output voltage varies so little from full charge to no charge.
lead acid are pretty easy, 13.5 or so is full less than 11 or so is empty and go to low and yiu can fry them unless they are deep charge.
the degrade of lithium storage with age is unpredictable, or is only if you can keep track of charge history, temperature and usage over the entire history of the individual cells- kindoff tough. the tesla lithium ion powered car has an individual cpu for each of its thousands of cells.
nicad batteries build stalagtites of electrolyte meterial between the poles with use that eventually short the cell out. they can be burned off with a couple microseconds of high (110v or so) zap. that returns power to original for a short time but they usually grow back quickly.
