Bottom Balancing

All of the 120 cells have to be bottom balanced. That process involves discharging each cell to a specific voltage level.  Usually that is accomplished with a tool like the PowerLab battery conditioner.  The time to fully discharge a 60AH cell at even 1C would be approximately one hour. Usually the discharge with the PowerLab is only 0.3C so it takes three hours to discharge.  For 120 cells that would be something like 360 hours if done one at a time!  To alleviate that time I have designed an automatic bottom balancing system that can discharge 4 cells at a time.  It uses a Arduino Due interface to monitor and control the discharge with a computer program.  I also setup a system to discharge the cells into a grid-tied inverter, that can put the energy back into my house.  The inverter is used to discharge the batteries to about 80% and then the Due system is used to do the final discharge tuning of each cell independently.  Below is photo of the actual system in use.. The load resistor is mounted with ceramic standoffs.  The resistor gets quite hot during the discharge (>200C!) but that can be greatly decreased with some air movement from a fan. The MOSFET heatsink only heats up to 90C without any airflow and the heatsink is mounted with phenolic standoffs. The Due measures the battery voltage and the controls the signal to the MOSFET. Tthe battery voltage measured by the Due is sent to a laptop running a program that logs the voltage change.  The MOSFET is run PWM from 100% down to 1% duty cycle during the bottom balancing process.  The system is designed to be isolated from other batteries and any ground point.  The stations are mounted on polycarbonate 3/8" sheet. There are also 3-1/2 digit digital displays that show the battery voltage.

 

 Bottom Balancing system