installing BP without a reboot will simply f*ck up bme battery measures

a best battery drain test would be after charging N900 to 100%
then testing the battery effect by kernel-power v50 till it gets to 0%

also testing kernel-power v50 is recommended to be without having batterypatch installed, reason :
in each lock/unlock bme will change it's values according to the current profile loaded and current usage of N900

so your value will not be 100% accurated

in short : test kernel-power v50 effect to the battery drain without having BP installed

after ur finished
you can install it without being worried about kernel-power v50 effect to the battery
and remember to reboot after each install/uninstall of batterypatch

personally i have tested kernel-power v50 and noticed no different of battery drain compared to v49