@raggedyman , Dusan, If battery completely discharged try leaving on charge 24 hours. Try a couple different USB charge cables and different charge port or compatible charger. You can test to see if battery is receiving a charge by placing a DVOM on the batteries -/+ leads, check battery before plugging in then check with charger cable plugged in to see if voltage increased(which would tell you charge cable/port is OK) and battery defective and will not receive/hold sufficient charge.
Look on the battery it self for #'s etc. and google search for a replacement possibly.Good luck.
I hope this helped you out, if so let me know by pressing the helpful button.