Battery Management System (BMS) Fundamentals

Battery Management System

Definition:

“A battery management system (BMS) is any electronic system that manages rechargeable battery (cell or battery pack), such as by protecting the battery from operating outside its safe operating area, monitoring its state, calculating secondary data, reporting that data, controlling its environment, authenticating it and / or balancing it”

– Wikipedia

Word of Caution

Lithium cells are much more sensitive to mistreatment than a comparable lead acid battery.

Charging

If lithium cells are overcharged, it will shorten their lifespan or perhaps permanently damage them. So with a LiFeP04 (Lithium Iron Phosphate) battery whose nominal voltage is volts you should never charge them above volts and It is best to have something in the System that will shut the charger off when they reach a maximum charge voltage of 3.85 volts.

Discharging

Conversley a LiFeP04 lithium battery discharged below 2.0 volts will damage it’s cells. So there must be some mechanism in your system which will automatically throttle back and eventually shut the battery pack down in the event of a discharge condition that goes below 2.2 volts. That mechanism needs to be in place to reduce the risk of damaging batteries in your pack.

Balancing

When they reach a predetermined voltage like 3.85 for LIFeP04 batteries then there has to be a way to shunt current from that highest charged battery and prevent it from becoming overcharged while their weaker batteries in the same string catch up. This is the third function of a BMS. A good BMS will balance all the batteries so that they reach full charge in about the same time. Cheap BMS systems do not do this they merely turn the charger off when the first battery nears 3.85 volts. Thus, you may lose as much as 20% of the battery packs full potential capacity for the sake of a small saving when purchasing the BMS system. In electric vehicles that can amount to a big loss of range.

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Protecting

The fourth BMS function concerns protecting and balancing the batteries from over discharging. If the batteries are not balanced (which a properly engineered BMS system will do) then as the battery is discharged the weakest battery will reach its full discharge shutoff point ahead of the rest of the pack. A good BMS system will keep all the cells balanced so that the weakest cell voltage will not go below 2.2 volts while the other batteries continue to discharge their energy. A well designed BMS will assure that all of the batteries are close to equal in storing and discharging their energy uniformly.

Monitoring

The last function of a good BMS system is monitoring and controlling the temperatures within the cells or the total pack. Many lithium chemistries like LIth1L1m Cobalt can spontaneously burst into flames if they are over charged or discharged. Metal fires are extremely hot and water or even firefighting foams alone will not extinguish the flames. Of all the lithium chemistries Lithium Iron Phosphate (LiFeP04) is the least volatile and theoretically will not combust. However, extreme heat can damage and shorten the life in all lithium cells. A good BMS system will monitor the temperatures within a pack and either shut the pack down or throttle back the discharge or charging current so that the cells never reach a point of overheating.

There are people who claim that you don’t need a BMS system! In fact they are right. You only need one if you are concerned about the most available power from the pack along with the health and longevity of the lithium cells that you paid a lot of money for. You will add years to your pack life if you use a good, well designed, BMS system.

SUMMARY

In summary there is an old saying that applies. “You get what you pay for!” Lithium batteries are expensive and you deserve to get a long battery storage life from every dollar that you spend on both the batteries and the BMS system. A good BMS system will pay for itself several times over the lifetime of your battery pack.

BMS Guidelines

  1. Read the entire manual of Battery Management System completely to make sure you can use the device effectively and more safely.
  2. Ensure the Battery Management System program and settings match the battery pack.
  3. Adjust those key parameters carefully, if you are not sure contact technical support for more details.
  4. Do not allow water, moisture, metal wires or other conductive material into the device.
  5. Never charge or discharge any battery that show evidence of leaking, expansion/swelling, damaged outer cover or case, colour-change or distortion.
  6. Do not try to charge “non-rechargeable” dry cells.
  7. Do not mix batteries of different types, different capacities or from different manufacturers.
  8. Carefully follow the battery pack manufacturer’s recommendations and safety advice.
  9. Never exceed the battery manufacturer’s suggested maximum charge and discharge rates.
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