Our lithium ion battery pack derives its power from individual cells, and one of the most important factors in getting the maximum power and life out of the pack is keeping these individual cells in balance. Our pack contains 96 individual batteries which are grouped in 6 cells of 3.6 volts each (at 35 AH). In addition to the main leads which form the positive and negative poles of the entire pack, an additional wire connects from each of these 6 cells back to our Battery Management System. We are using a Mk3x8 Lithium system from Manzanita Micro, a major supplier of battery management systems for the electric vehicle community. In addition to the battery regulator board, we also have a computer interface (known as the Dongle Terminator or DT) and a charger interface known as the RROB. Together, this system allows us to actively control and monitor the voltage of each cell in the battery pack and control the battery charger accordingly. In addition to controlling the voltage of each cell, the system implements safety measures in the case of a high or low voltage situation, including opening the Gigavac GX21 main contactor to isolate the battery and disconnect all loads.
With the active battery management system, we have also greatly simplified the battery charging equipment. While conventional lithium ion battery packs need a “smart” charger with advanced algorithms for the entire charging cycle, the battery regulators allow the entire pack to charge until the first cell is full, then shunt power from individual cells as they fill until the last cell is full. Then, it passes this information to the RROB, which shuts down the charger. With this system, the charger is a Constant Current/Constant Voltage (CCCV) power supply, capable of putting out 20 amps at 25 volts. This matches the design specifications of the batteries themselves.
The image shows the elements of the battery management and charging system. Grey wires are data cables that connect the components, and the white battery pack is a test pack which represents the full battery but on a smaller scale.