First some background - solar controller
I look at the solar controller as a negotiator of sorts. It uses MPPT (Maximum Power Point Tracking) to determine the best combination of voltage and amperage to pull the maximum power (Watts) from the solar panels. The MPP will continually vary depending on the panel design, sun angle and shading. The solar controller also senses what is the best voltage and amperage combination (Watts) for the battery bank. V x I = Watts. Watts into the controller from the solar panels about equals watts out to the batteries minus conversion efficiencies. So the solar controller negotiates the best power from the panels and best power to the batteries. While the solar controller senses the voltage of the batteries to determine how much power to supply to them (charge), it has no way of sensing what has been or is being taken out of them (discharge).
Some more background - LiFePO4 batteries
The discharge curve for LiFEPO4 batteries is very flat. They supply a steady voltage of between 13 and 13.4 volts up until 95% of full discharge. This is unlike lead acid batteries that reduce in voltage from 13 to 11 or less volts as they discharge. Thus, voltage only is not an accurate way to determine the state of charge (SoC) of LiFePO4 batteries.
Determining LIFePO4 battery SoC
There are two ways to determine the SoC of LiFePO4 batteries; an external battery monitor or information directly from the Battery Management System (BMS). A battery monitor (Victron makes one) has a shunt on the negative wire between the batteries and the battery chargers and appliances using power. The battery monitor measures the power crossing the shunt in both directions (charge and discharge) and computes the State of Charge of the battery bank. This technique can be reasonably accurate but most battery monitors are designed for lead acid and AGM batteries and have to be reprogrammed for LiFePO4 because LiFePO4 batteries charge so much more efficiently than lead acid. The battery monitor has enough data to compute the SoC but can be inaccurate because battery charging efficiencies vary. The diagram below may be helpful.
A well designed battery management system (BMS) can measure the charge and discharge rate and the battery voltage and accurately compute the SoC for each battery in the bank. The BMS can also compute the time to full charge and the time to full discharge. In addition, the BMS monitors and manages the SoC of each of the 4 cells within a LiFePO4 battery so it can provide diagnostic data to identify a potential balancing issue which could lead to a potential failure.
All this information can be made available through a well designed BMS Bluetooth App.
Thomas Trimmer has been cruising with his Ericson 38 sailboat on the Great Lakes for over 20 years. He has pioneered the use of solar energy for wilderness cruising. He is continually designing and building equipment to simplify and enhance the cruising experience.