Battery Management and Monitoring Systems - What are they, fundamentals

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A battery management system (BMS) is any electronic system that manages a rechargeable battery (cell or battery pack), such as by monitoring its state, calculating secondary data, reporting that data, protecting the battery, controlling its environment, and / or balancing it. A battery balancer or battery regulator is a device in a battery pack that performs battery balancing. "Balancing" means to ensure each cell in the battery pack is equally charged. Without balancing, the cell of smallest capacity is a “weak point”, it can be easily overcharged or over-discharged while cells with higher capacity undergo only partial cycle.

There are several parts to typical BMS's:

  • Monitoring the pack during charging (monitoring)
  • Monitoring the pack during discharging (monitoring)
  • Taking action to ensure the cells are equal voltage hence equal state of charge (balancing)
  • Monitoring and controlling other systems such as cooling or heating
  • Generally speaking, taking actions meant to keep the battery pack within safe operating parameters, prevent damage, and ensure battery pack longevity

A BMS may monitor the state of the battery as represented by various items, such as:

  • Voltage: total voltage, voltage of periodic taps, or voltages of individual cells
  • Temperature: average temperature, coolant intake temperature, coolant output temperature, or temperatures of individual cells
  • State of charge (SOC) or depth of discharge (DOD): to indicate the charge level of the battery
  • State of health (SOH), a variously-defined measurement of the overall condition of the battery
  • Coolant flow: for air or fluid cooled batteries
  • Current: current in or out of the battery

In charge or discharge states, the BMS will calculate things like:

  • Maximum charge current as a charge current limit (CCL)
  • Maximum discharge current as a discharge current limit (DCL)
  • Energy delivered since last charge or charge cycle
  • Total energy delivered since first use
  • Total operating time since first use
  • Over-current
  • Over-voltage (during charging)
  • Under-voltage (during discharging), especially important for lead–acid and Li-ion cells
  • Over-temperature
  • Under-temperature
  • Over-pressure (NiMH batteries)

Battery balancing involves attempting to keep the cells at the same state of charge. In practice this is imprecise and relies on keeping the cells at the same voltage. The methods for this are:

  • Wasting energy from the most charged cells by connecting them to a load (such as through passive regulators)
  • Shuffling energy from the most charged cells to the least charged cells (balancers)
  • Reducing the charging current to a sufficiently low level that will not damage fully charged cells, while less charged cells may continue to charge (does not apply to Lithium chemistry cells)
  • Modular charging

BMS topologies fall in 3 categories:

  • Centralized: a single controller is connected to the battery cells through a multitude of wires
  • Distributed: a BMS board is installed at each cell, with just a single communication cable between the battery and a controller
  • Modular: a few controllers, each handing a certain number of cells, with communication between the controllers

Resource links - Our list of known vendors making & selling battery management systems. - This is an old article (from 2009) from a site that focuses on an old electric vehicle (the Corbin Motors Sparrow) on BMS's, and primarily these are for lead-acid packs.

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