Hey phoenix,

The reason this happens is a little complicated.

It's very difficult to estimate the amount of energy left in a battery. You can measure the no-load voltage, or a voltage under a known load, and based on that make some intelligent guesses, but they're always just that.

In the case of the n900, the BME monitors voltage and current draw over time, and integrates the values over time to estimate the instantaneous energy remaining. The BME uses some facts to aid it: fully-charged battery capacity, known discharge curves, temperature, etc.

When you reboot, the current draw spikes dramatically, and so drops the starting voltage (higher draw = lower voltage), so the initial estimate is much lower.

What you'll find, though, is that it doesn't actually make much of a difference in run-time. That last 20% will take nearly as long to discharge as the 50% it was previously reporting.

It's a difficult problem to solve, unfortunately.