BMS

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  • A Battery Management System (BMS) is a device that protects a battery comprised of multiple cells (aka battery pack) from operating outside its safe operating state and keeps the individual cells voltage balanced so no capacity is lost within the battery pack. There are many varieties with different features. A BMS is typically the only device in a system that is capable of seeing and taking actions based upon cell level conditions.

    Is a BMS needed?​

    BMSes can be used on any battery pack, but are most useful for chemistries with low internal resistances, like lithium, where small changes due to aging can prevent some cells from fully recharging or they are sensitive to over charging or temperature.

    A BMS is not strictly mandatory on any battery pack. However, considering the cost of the battery investment it is usually a good idea to have one on any system that is designed to work in an autonomous fashion. For example, charging even once at temperatures below 0° C can permanently harm lithium cells, destroying the investment (see ref for more).

    A BMS with a high-temperature cut-off should be considered mandatory on any chemistry that might suffer from thermal runaway to prevent the battery pack from exploding.


    How balancing works​

    Balancing is the most common feature of a lithium BMS. As batteries are cycled,
    the capacity of each cell becomes slightly different. This can cause premature
    aging of the cells that work the hardest. Without a BMS, the pack should be
    bottom or Top Balanced occasionally.
    1612789006024.png

    There are two types of balancing: active and passive; currently passive balancing is the most common.

    Passive balancing​

    In passive balancing, cells are voltage balanced by letting energy be dissipated as heat. Typically this is a top balance, that is the BMS waits until the battery hit's it maximum charging voltage. Then it cuts off the charging power and uses resistors to drain power from the highest cells. The system then restores charging power and the cycle repeats until all cells are fully charged.​

    Active balancing​

    In active balancing, energy is drawn from the most charged cell and transferred to the least charged cells. Despite the obvious energy savings, these systems are not as common because passive balancers are less expensive and sufficient for new cells with matched capacity as typically there is very little energy dissipated. Active balancers are most useful with used cells or cells of mismatched capacity and members suggest an amp rating of 1% the C-Rate. (ref1, ref2)​


    BMS Features​

    • Balancing - Keeps cells balanced
    • HVD - High Voltage Disconnect, prevents cells from being overcharged
    • LVD - Low Voltage Disconnect, prevents cells from being discharged to far
    • HTD - High Temperature Disconnect, prevents cells from being used when the temperature is high enough it would degrade them
    • LTD - Low Temperature Disconnect, prevents cells from being charged when the temperature (<0° C) would damage them (see (see ref for more).
    • Programmable - Allows cutoff points to be adjusted
    • Monitoring - Allows various cell parameters to be monitored, typically CanBus, bluetooth, or WiFi
    • LCD display - Shows what the BMS is doing and BMS health
    • Relay Switch - for turning secondary devices on/off (e.g., heaters/coolers, inverters)
    • Pins - General Purpose I/O pins, for e-switch/ignition key, external systems such as SOC LED, alarm buzzer

    Common vs. Separate Port​

    Although there are many varieties, a typical BMS is illustrated to the right. The difference between common and separate port is a common port (aka 2-wire or 3-wire) is the charging and discharging occurs from a single wire. A separate has a 3rd wire (e.g., P- (discharge) and C- (charge) are separated). In addition to charging/discharge wires, a BMS with a balancer will have additional wires to balance the cells.
    1613052495606.png
    The advantage to a separate port BMS is the charger and the load can be on different circuits. BMSes are typically sold by the number of amps they can pass and feature set (e.g., how many cells they can balance).

    Is this thing working?​

    BMSes are electronic devices and they do go bad. There's no magic BMS tester, to see if they're working you need to test each feature (e.g., to confirm the balancer is working measure the voltage across each of the cells, they should all be equal), test the amps drop to zero when the voltage/temperature exceeds HVC/LVC/HTC/LTC.

    Common Mistakes​

    Incorrectly setting the chemistry or voltage cutoffs. Even if you think the settings are correct, it's prudent to test with a volt-meter to validate.

    One mistake commonly made occurs when an external high-voltage cutoff is used. Many BMSes only top-balance when the battery pack is at full voltage. As not charging to 100% prolongs battery life (See capacity fade), an external cut-off keeping the batteries at 80 or 90% can prevent the BMS from reaching the top voltage and so may prevent the balance feature from working.

    BMSes typically don't have over-current-protection. That is they won't protect the battery from exceeding the C-Rate during a short. An external fuse or breaker is required for this. Some battery packs do have cells individually fused to protect the pack from a cell gone bad, but that's within the pack and not a part of the BMS.

    What's the best BMS?​

    There's no one answer, but there is a thread on that. Inexpensive BMSes exist, but they might not be a bargain as they are protecting a valuable investment.

    Projects by Members​

    Have a project to add to the list?
    Click the Report button on the OP in your thread to request this entry be updated with a link to it.

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