Hi spec,
I think most so called 18 volts Li ion tools use 5 cells. The sales department will always find a way to make the numbers look as big as possible. So they base it on 3.7 volts per cell. I've just measured the voltage of the battery from my new Aldi 18 volt drill and it measures about 15.1 volts. I then thought what do the state of charge LEDs show. The were all lit showing fully charged. I ran the drill for several seconds and it still showed fully charged. I then measured the battery voltage again. It showed about 20.1 volts. I left it for about 10 minutes and tested it again. Back to 15.1 volts again. I repeated these tests with the same results. I can only think that is the cell balancing circuit balancing the cells after use.
Les.