Current into or out of the LiFePO4 battery will be monitored by an INA219. This will be connected to a Raspberry Pi Zero via I2C for logging and display. Additional sensors may be added for temperature and individual cell voltages. It may also monitor the starting battery. Control outputs will be used to turn off the Converter/Charger and control the "solenoid" connecting the two batteries.
I am interested in following your progress. I am working on a similar project: https://hackaday.io/project/16787-raspberry-pi-solar-system-monitor