It's equally hard to get any useful information about Li-On or Li-Po based just on battery voltage which is why any actually useful battery indicator involves a

It's equally hard to get any useful information about Li-On or Li-Po based just on battery voltage which is why any actually useful battery indicator involves a charge counter (informally known as a 'gas gauge'). The actual coulombs of charge are counted in and out and that gives a much better estimate.