White Paper - Mind that BMS: Charging & Discharging limits

A BMS alone will not protect your pack - you also have to mind what it tells you...and do something about it

Sure, a battery pack with a BMS (Battery Management System) is better than a bare battery pack: it lets you know how the pack is doing, and it balances it. In a small battery (think "laptop battery") a BMS will also protect the cells by shutting down the current if the cells start getting too close to the edge of their safe operation range. But, the BMS in a large battery pack doesn't necessarily include a high power switch to allow it to shut down the battery.

So, don't think that, just because you have a BMS on your large battery pack, you're all set. You still have to do one more thing: you have to give the BMS a way to limit charging and discharging, to prevent over-charging and over-discharging.

In a typical application, there can be 3 sources or sinks of battery current:
• A charger (e.g. a renewable energy source)
• A load (e.g.: a motor DC controller or an AC motor inverter)
• Energy recovery from the load (e.g.: regenerative braking)

The BMS must be able to communicate with other components, to limit the current they source or draw, or even interrupt it. In particular, the BMS must be able to:
• Shut down a charger (and possibly tell it to reduce its output current to a level sufficiently low that it can be bypassed by its balancing mechanism)
• Limit the current draw of the load (e.g.: limit the torque of a motor driver), and even shut down the load (i.e.: through contactors)
• Limit the current from the load (e.g.: reduce the acceptable regenerative braking current)

Block diagram of standard EV system.
The BMS controls the motor driver and the charger, to protect the battery.

It is worth repeating that a large pack BMS, by itself, cannot protect the pack. All it is able to do is to requests that the current be reduced or stopped. It is your responsibility as the system designer, to ensure that the other components in the system receive such requests, and that they pay heed to the BMS' requests.

In general, a BMS can request a reduction in battery current in 2 ways:
• Hard wired (e.g.: TTL level, closed contact, D/A output...)
• Communication link (e.g.: CAN Bus, RS232 serial link, SMBus...)

The Elithion BMS uses 2 methods:
• Three hard wired open-collector lines: "Low-voltage", "High-voltage" and "Gradual Limit"
• A CAN Bus that includes CCL (Charge Current Limit) and DCL (Discharge Current Limit) messages

Devices that do not include a CAN bus nor a current limit input cannot be gradually turned down. Instead, once the Elithion BMS asserts the hardwired line from it to the device, the device must shut down immediately. For example, a wire from the BMS "High Voltage" output to a charger can shut down the charger as soon as the BMS sees that one of the cells has reached its maximum voltage. Or, a wire from the Elithion BMS "Low Voltage" output to a contactor can shut down a motor driver as soon as the BMS sees that one of the cells has reached its minimum voltage. That, of course, is not always desirable.

Devices that do not include a CAN bus but have a current limit input can be gradually turned down. For example, a wire from the Elithion BMS "Gradual Limit" output to a motor driver can request a lower maximum torque when one of the cells starts approaching its minimum voltage.

Devices that include a CAN bus can be gradually turned down in either direction. For example, by having both the Elithion BMS and an AC motor inverter connected to a CAN bus will enable the BMS to reduce the motor torque as a cell's voltage starts getting too low, and to reduce the regenerative braking current as a cell's voltage starts getting too high.

In the worst case of devices that have no control mechanism, the very least you must do is to:
• use the "High-voltage" line to drive a relay to shut off the AC power to the charger.
• use the "Low-voltage" line to drive a contactor to shut off the DC power to the motor controller.

Let your system heed the requests from the BMS, and your battery pack will be able to live a long life.

"Mind that BMS: Charging & Discharging limits" by Davide Andrea is licensed under a Creative Commons Attribution-Share Alike 3.0 Unported License. Permissions beyond the scope of this license may be available by contacting the author.

Davide Andrea, Elithion, 10/21/08