BMS Rev 3 Documentation

The purpose of the battery management system is to protect the battery cells from being over charged or over discharged and to monitor all of the cells of a battery. The BMS is also used to estimate the state of charge of the battery.

Revision 3 Objectives

• Remove BMS from PDB to create a standalone STM32-based BMS

• Fix buzzer circuit for low battery alert

• Add 7 segment display for battery percentage (send over CAN)

Problems with over discharging

In a LiPo Battery with multiple cells, if one cell discharges too low, or charged too much, that cell could become permanently damaged. Internal cell damage could lead to fires if that cell is recharged.

How does a BMS work?

The BMS makes sure that no voltage cell is lower than any other cell. The BMS makes sure that all cells discharge or charge at the same rate.

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Passive Cell Balancing

Passive cell balancing discharges all cells at the same rate of the fastest discharging battery cell. If one cell is lower than any other cell, the other cells dissipate the extra power through a resistor so that all cells remain the same voltage level.

Active Cell Balancing

Active cell balancing discharges higher voltage cells into lower voltage cells. Instead of burning the extra energy like passive cell balancing, cells with more energy transfer to cells with less energy which makes the battery last longer.

Current BMS

The current BMS uses passive cell balancing and monitors the voltages for a 12 cell battery. It is included on the PDB. The current BMS also estimates the battery percent using coulomb counting and a voltage look-up table.

This revision did not work, and thus the team bought a OTS BMS to use with the battery.

Found here: BMS Aliexpress

Future Improvements

• Get a working BMS

  • BMS Rev 3 did not end up working after testing

• Active cell balancing