My Forma's battery too become drained without any usage.
I did two test with original battery. Then I puchased a replacement battery and it seem nothing is changed.
On my tests,
WiFi was off
Auto sync was off
Sleepcover auto wake/sleep was off
Device did not have any book or collection (archived everythink on my Kobo account)
I didn't read anything. Device was sleeping all the time, and I occasionaly waked it up to take note of the battery level.
On original test 1, device was up-to-date.
On original test 2, device was manually factory resetted, so it had old firmware.
On replacement test 1, device was up-to-date.
Replacement battery is not something official. I bought a li-ion battery with same voltage value. This battery has slightly larger labeled capacity than the original.
I don't understand why there are sudden drops? I know replacement will not be perfect, but i didn't expect to see same sudden drops.
Is there any firmware settings on Kobo which survived manual factory reset? Could the problem about hardware releated not about the battery itself.
