So when battery is out and you connect ,the LED stays on and nothing else happens? Normally the phone should go in a kind of bootloop of short blinking of LED, pretending reboot (Fairphone Logo displays 1-2 seconds) and this is repeated over and over again. So if this does not happen its most likely not the battery. I recently had charging issues after my battery drained completely which were "fixed " with upgrading to Android 10.
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