This small speaker apparently stopped charging and now just had a red light and refused to do anything.
It later turns out the mains charger had failed, but the user had continued to use the device until the battery was exhausted, and then it had continued below the minimum voltage level at which the device could be charged by its own USB connector. Firmware was checked and ok.
As there could have been damage from the over-discharge (although limited since it wasn’t left like that for weeks). slow and careful charging via the bench supply was performed, slowly increasing the current to get the voltage up to just over a ‘battery flat’ level of voltage, whilst measuring temperature of cells ready to abort if anything untoward looked like it was happening!
After which, with nothing spectacular happening, it had now reached a point where the devices own USB charging could take over and complete the charge successfully to a green fully charged indication!
A replacement USB power supply was used for future charging and speaker back in use.
