Standard Lithium Cells and Voltage
A standard lifepo4 battery cell operates at 3.2V. To create a safe 12V, 24V, or 48V battery from these cells, a Battery Management System (BMS) is essential. A BMS is a circuit board connected to the cells that monitors the overall battery. For safety, the BMS ensures that the cells always operate within specified limits and takes action if these limits are breached.
Why a Battery Management System (BMS) is Needed
Charging
Overcharging lithium cells can shorten their lifespan or permanently damage them. For example, LiFePO4 (Lithium Iron Phosphate) batteries have a nominal voltage of 3.2V and should never be charged above 3.65V. A BMS ensures that charging stops when this maximum voltage is reached. Similarly, for LiNCM or LiMn204 (Lithium Polymer) batteries, which have a nominal voltage of 3.7V, the BMS ensures that charging stops at 4.2V.
Discharging
LiFePO4 batteries should not be discharged below 2.4V to avoid cell damage. A BMS automatically throttles back and shuts down the battery pack if the voltage drops below this threshold, protecting the cells from damage.
Balancing
When multiple batteries are connected, some may charge faster than others. A BMS balances the charge among all batteries, preventing overcharging of stronger cells while weaker cells catch up. This is akin to having a barrel with staves of varying lengths: the BMS ensures all staves (batteries) reach the same level. Passive BMS systems use resistor networks to burn off excess power from fully charged cells while others charge.
Preventing Over-Discharging
A good BMS keeps all cells balanced during discharge. In a balanced system, the weakest cell in a LiFePO4 battery will not drop below 2.4V, and in a Lithium Polymer battery, it will not drop below 2.8V. This ensures uniform energy storage and discharge across all cells.
Temperature Monitoring
A well-designed BMS monitors cell temperatures, preventing overheating, which can damage or shorten the life of lithium cells. While LiFePO4 cells are the least volatile and theoretically do not combust, extreme heat can still cause damage. The BMS will shut down or throttle back the charge or discharge current if temperatures become too high, ensuring safe operation. This is crucial for chemistries like Lithium Cobalt, which can spontaneously combust if overcharged or discharged.
Conclusion
A Battery Management System (BMS) is crucial for the safe operation of lithium batteries, ensuring proper charging, discharging, balancing, and temperature control. Investing in a good BMS can significantly enhance the performance, safety, and longevity of your battery pack.