As an addition to this thread and how tricky modern battery charging has become..
I have a +/- 12 year old Philips 3 head HQ9 shaver, I have newer ones but non of them shave as well - the newer Philips have different type of cutter - which IMHO is cr4p - also tried a braun foil shaver but that is not too good either -- any way back on subject.... I decided to treat the shaver to a new battery - a 3.7 volt 700ma/h Li-ion cell - same size as a single 'AA' battery but with solder tags. I checked the voltage on the replacement battery and it was 3.6 volts, so fairly well charged when I soldered it back in. Now for the strange part, after a couple of days the red light on shaver flashed and it would not start, so i charged it up fully - after one shave the red light flashed and it would not start again***, so I took cover off and checked battery voltage - it was 3,85 volts so fully charged. I then had a light bulb moment ( literally ) and decided to unsolder one end of the battery - made a 12volt 5watt ( W5W ) bulb with a couple of wires soldered on and connected it across the battery for a couple of hours ( I checked Li-Ion battery specs and 2.8 volts was discharged voltage ) and when it reached 2.8 volts I resoldered the battery back in and recharged it in the shaver. Well since then I have used it every day for about 10 days and it is fine, the 'state of charge' charge indicator LEDs on the shaver handle ( 5 of them ) have just started dropping down. It is obvious that by fitting a fully charged battery I had 'fooled' the coulomb counter circuitry in the shaver and the only way to fix it was to deplete the battery and recharge it in the shaver so the charging circuit knew exactly how much power had gone into battery. I read on the battery sellers website that a few people have said the batteries are no good and would not take a charge etc. but it is obvious that the problem was with the shaver circuitry not the battery.... The only real way to check the state of charge of Li-Ion batteries is by coulomb counting ( as is done in phones and laptops and Dyson vacuum cleaners etc. ), so this will be the case with BEV as well, so in the case of swappable batteries for BEV the fancy electronics controlling charging would have to stay with the battery - or at least some kind of module with the battery history in it.
note *** The shaver on-off switch is not connected directly between battery and motor, it is wired into an integrated circuit and when you press it the button has to 'ask permission' to run the motor, and if the coulomb counter is not happy it says 'no'...