My 12.8v Lithium battery will not charge when connected to the MP3741 or MP3749 Solar controllers

The MP3741 / MP3749 solar controllers can not charge Lithium batteries that have been discharged so deeply that the battery's built-in BMS circuit has disconnected the cells inside.

Symptoms of this issue: The solar controller display may show a battery voltage reading of over 14v, but the attached battery will not accept a charge or deliver any current to your loads. If you leave the battery connected, but disconnect the solar panel, the solar controller display will turn off.

Testing: Disconnect the solar panel and battery from the controller. Set your multimeter to DC Volts 20V range and probe the terminals of the battery. If the voltage reading is less than 11.1v, the battery can not be charged by the solar controller. 

How to fix: You can reset the battery BMS by following the steps here. Once the battery voltage measures over 12v on your multimeter (after disconnecting it from the temporary 15v power supply), you can reconnect the battery to the solar controller, and then reconnect the solar panel. Your controller should be set to F04 in the Battery Type Menu - see here. Charging should then resume normally.