Battery problems (wrong charging status / battery might be dying)

Maybe this has been posted before, but still:

  • replaced old battery for my Fairphone (first edition) with a new one three weeks ago
  • seems to be working just fine
  • today: phone shut itself off because battery was down; no failure here
  • just started to recharge it - first, no red light, after one minute red light appeared, I checked: big battery symbol on blue screen showed 2%
  • after 15 minutes, I checked again: recharge status showed 19%
  • charging stil going on, I immediately switched on my phone, everything started as normal, BUT battery status in recharging mode now showed 55% - !?
  • I switched off my phone; recharging status continued at 55%.
    This means: from 19% to 55% in 10 seconds!?

Maybe this piece of information helps solving the battery trouble.
Thanks!