So, I tested the behaviour on N800, whee we know how to talk to the bme daemon directly. I sampled current consumption and charger current every second, added them together and integrated.

Short story:
N800 charged to 976mAh from first battery low. I called that 100%.
It waited until 94.5%, then activatedcharging for a minute, bringing it up to 95.5%. It kept the charge level between 94.5 and 95.5 repeatedly.

We should probably be happy with libhal and hald-addon-bme on N900. On the previous N800 and N810 it reports a constant 100% until it thinks battery is 50% full, at which point it starts saying 75%