As a lot of people coud have this problem I encountered, I decided to write this poste.
My problems I had:
When I tryied to chage my phone it did not work all the time. Some times it flikered around and said that it is charging but it did not and some times it worked (light was indicatign that it is charging) but it took a very long time (was not full after a night charging)
My phone did restart when mechanical stress was applyed (not very strong even shacking was enough to cause a restart).
I tried different cables and nothing changed.
Then I analysed the contacts and the battery and I noticed the following:
The battery was moving a littlebit when it was inserted into the phone. I started the phone and moved the battery. It did restart like it woud do with the shaking.
What I did to fix this:
I took some paper and fold it to the size of the lower part of the battery to encrease the thikness of the paper. The thickness shoud be as thick that it is still possible to inserte and remove the battery but it shoud be as thick that the battery does not move anymore. Then I glued it onto the lower small part of the battery. Now the battery is pressed against the upper contacts of the phone and does not move any more.
As a result my phone charges normally and it does not restart anymore. If you have a Faiphone 2 i would recomend everyone to check if the battery is inserted thight! realy tight! No moving! Nothing! Contact problems can cause a lot of issues like blow up when it charges not correct, wrong measured voltages of the phone and as a result wrong charging and therfore a shorter liftime of the battery. Contact is everithing in electronics!