A standard lithium cell is 3.2V. To turn these individual cells into a safe 12/24 or 48V battery requires a Lithium Battery Management System or BMS. A BMS is a circuit board that has a connection to the cells and monitors the overall battery. For safety reasons the BMS needs to ensure that the cells are always operating within known limits and to actively respond if these limits are breached.
Do lithium batteries need a management system?
Yes, and anyone who tells you otherwise is not telling the whole story. A lithium battery without a good BMS is like driving around with a bucket of petrol in the back seat with a hose to the engine, technically possible but not very sensible. If petrol is contained in a good fuel tank with proper fuel lines to the engine it is safe and operational. Same with lithium cells; once controlled they are safe but there is more to the story.
Lithium battery management
The most important function of a lithium battery management system is to ensure the cells remain within safe operating limits and respond if one of the individual cells is outside these limits.
The action taken depends on the lithium Battery Management System. A good one will disconnect loads if the voltage goes too low, and disconnect chargers if the voltage goes too high. It will also check that the voltage of each cell in the pack is the same, and bring down the voltage of any cell that is higher than the others. This is called balancing and ensures that cells are always operating safely.
A good BMS will also monitor the temperature of the cells and disconnect the pack long before any problems could occur.
Be aware of their functionality!
We have observed an operational flaw in many BMS’s on the market. When the voltage of a cell is too low (ie, battery is empty) it will disconnect the battery, and will not reconnect until the voltage of that cell increases again. However, in this situation many chargers will not operate, because they cannot “see” the battery and so another battery is required to jump start the original battery so that it can be charged! This is clearly not convenient.
A good BMS will allow charging even when the battery is empty. Furthermore, it will allow a user to reset the battery and allow it to be “seen” by a charger for a short period, so that the charger can begin to operate and recharge the battery.
An advanced BMS will also have the ability to send information to a screen for remote monitoring and diagnostics, and receive a reset command remotely if required.
Why Don’t Lead Acid Batteries Need a BMS?
All 12V battery types – lithium and lead acid – are made of cells in series. These cells eventually become slightly out of balance, so that one cell becomes full before the battery as a whole is full. When this happens, the question becomes what happens to the full cell when the charger is still providing energy to the battery.
Cells of different chemistries react differently to this situation. When lead acid cells (including AGM, gel etc) become full they do not absorb any more energy; rather that energy is dissipated and flammable hydrogen is vented.
However, when a lithium cell becomes full, it does not vent any gas so the excess energy from the charger has nowhere else to go – so the cell continues to absorb energy. Then the cell quickly becomes overcharged and goes beyond safe operating limits – unless a lithium Battery Management System is there to intervene.
What features do I need in a lithium battery BMS?
A lithium battery management system obviously needs to perform the basic functions of low voltage disconnect, high voltage disconnect and cell balancing. But more advanced features are needed if the battery is in a 4WD, car, RV or boat.
- Give accurate state of charge status;
- Give live calculations of time-to-full or time-to-empty based on load or charge;
- Allow loads to continue whilst disconnecting charge at high voltage point;
- Allow charging to continue whilst disconnecting loads at low voltage point;
- Auto recover of errors;
- Handle inrush currents up to 1000A so air conditioners, coffee machines, microwaves work;
- Handle 200A continuous load currents; and
- Connect to remote monitoring displays.
The LBS Battery Management System
It is relatively easy to create a BMS for small capacity batteries, at low current that do the absolute basics of simply connecting the cells. The engineering challenge was to create a lithium Battery Management System that had advanced management and monitoring. It could operate at very high currents (up to 1000A) and was robust enough to work within batteries that were suitable for cars, boats and homes in the hostile Australian environment.
The engineers at LBS undertook this challenge. After input from dozens of local engineering specialists and collaboration with electrical engineering students from the local universities our very own BMS was born. It is solid state, robust, stable and highly featured.
Simulation and Testing
LBS batteries are designed, analysed and modelled in software to ensure the BMS, cells and all other internal components fit and work seamlessly together. LBS thoroughly tests each one of our BMS boards during assembly before we install it inside a battery. A high performance BMS means that LBS batteries can easily run air conditioners, coffee machines, microwaves and other high power devices.
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