Tried to replace the battery with two different lithiums. Both worked fine, but failed to take any charge via the mini usb. Anyone had any luck with battery replacement? What could I be doing wrong?

Has anyone tried actually replacing the battery on the portable Frekvens? I’ve tried with two batteries and neither worked. The batteries connect fine and power the unit and connect to Bluetooth, but they fail to recharge. Cannot get the recharge function to work. Very frustrating.