Battery Management System
Lithium Ion battery cells can be easily damage by either going over voltage or under voltage. Additionally each cell charges and discharges at slightly different rates, so we can only charge until one cell reaches the maximum voltage, leaving the rest of the cells under charged, and discharge until a cell reaches the minimum voltage leaving charge in the cells we can’t use. This reduces our capacity of the battery. To address these problems we created a battery management system which monitors individual cell voltages and balances cells during charging.
To read the cell voltages we send tap lines from the battery cells into a multiplexer which we control from a PIC24 microcontroller and select one cell at a time. We send that voltage into the analog to digital convertor of the PIC24, and then send that info to the Intel Edison via I2C.
For the cell balancing we place a resistor with a switch in parallel with each battery cell. Once a cell reaches the maximum voltage, we turn off the charger and use the microcontroller to turn on the switch which then discharges the highest cells to be equal with the rest of the cells. We then turn the charger back on and charge until cells reach the maximum voltage again. Repeating this process until all cells reach the maximum voltage.