How does a BMS work?

From a distance, a BMS (Battery Management System) has a very simple task; monitor the battery pack and protect it from any excursions outside of the safe operating range of the cells which make up the pack. Generally speaking, internally, a BMS has three main “blocks” – the monitoring / computing block, and the current carrying / disconnect block, and the cell balancing block.

The Monitoring / Computing Block

The monitoring / computing block is where various parameters such as voltage disconnect points, temperature settings, and the like, are programmed.  If the monitoring section of the BMS determines that something has gone out of range, it triggers the power block to shut down.  The monitoring block of “SMART” bms units can be accessed by the operator, and various parameters may be monitored and changed.  Typical connections to the monitoring block can be made via a computer with a USB converter, and many BMS units feature Bluetooth connectivity to apply changes via  a mobile device.

The Power Block

The power block will vary by design, but typically MOSFETs or relay/contactors are used to allow current to flow in and out of the pack when the monitoring block determines it is safe to do so.  If the monitoring block determines that an unsafe region of operation is approaching, it will open the device responsible for carrying the current, putting the battery pack into an open circuit condition.  In this state, current cannot flow through the pack.

Since the power block is responsible for disconnecting all loads in and out of the battery, all of the current flowing through the system must flow through the power block.  Installing any device on the battery directly would leave you without protection.

The power block can be identified by the connections made to it.  Typically the power block has two connections, labeled B- and P-/C-.  We will cover common port BMS units in a minute.  The B- connection of the power block would be wired to the negative terminal of cell 1 in the pack, then any equipment charging or discharging the pack would be connected to the P-/C- terminal.

MOSFET VS Relay/Contactor Power Blocks

A design consideration when picking a BMS is what will actually carry the current.  Two choices are available, MOSFET based and relay based.

Relays are quite simple, either they are on or they are off.  When a relay is engaged, current can flow in and out of the battery.  This means if the monitoring block signals a shutdown, the entire system is taken offline, in both the charging and discharging directions.

MOSFET based BMS units offer far better control.  MOSFET based units can allow charging while preventing discharging.  This means in the even of a low-voltage disconnect, you can still charge your system, and in the event of a high-voltage disconnect you can discharge your batteries to bring them back down to a much safer level.

Recommended reading: Relay BMS Units – The Good, Bad & Ugly

Cell Balancing Block

Unlike Lead-acid, LiFePO4 cells that are fully charged will raise in voltage extremely quickly, which can lead to instant degradation of the battery if left unmanaged.  Over time the differences between cells may become so extreme that, even though the overall battery voltage is within limits, some cells will fail due to over- or under-voltage, so the BMS needs a way to remove charge from cells at a higher state of charge.  This is the responsibility of the balancing blocks.  Most consumer grade BMS units use a passive balancer that dissipates excess charge as heat, while some high-end BMS units use an active cell balancer.

Recommended reading: Passive VS Active Cell Balancers (Coming Soon)

Common Port VS Seperate Port

As we mentioned earlier, MOSFET based BMS units can manage charge and discharge separately.  Most commonly available BMS units are common port units, which means that charging and discharging occurs through the same terminals of the BMS.  BMS units do exist that are called “separate port” units, which quite literally have a separate port dedicated to allowing charge current into the battery.  While these are impractical for most applications, they are most commonly found on E-Bikes and scooters that have an external battery charger and do not have a need to charge through the discharge port.