What is a Battery Management System (BMS)?

Checklist of what a BMS (Battery Management System) should do.

I start from the definition given by BUREAU VERITAS.  Then I review the functionalities that will enable a BMS to fulfill its purpose. 

The definition from Bureau Veritas:

Battery Management System (BMS): Electronic system associated with a battery pack which monitors and/or manages in a safe manner its electric and thermal state by controlling its environment, and which provides communication between the battery system and other macro-system controllers (e.g.: Battery Support System (BSS), Energy Management System (EMS) and Vehicle Management System (VMS).

For “off-grid” applications we can replace “VMS” with all the charge and load equipment of the installation

Based on that document, a BMS should be able to:

  • monitor and manage electric and thermal state
  • estimate the potential need for battery pack (dis)connection
  • provide an environment where the battery, the installation and the people are safe
  • protect against equipment faults, human errors, environmental incidents…
  • communicate with external systems and operators
  • optimise battery life-time and energy availability
  • diagnostic – record battery life history log

A BMS needs to protect the battery itself against the failure modes described in the article “What will kill your battery“.  It needs also to implement a multi-level protection as explained in the article “Think multi-level protection

Basic functionalities of a BMS

Cell voltage and temperature measure
  • voltage and temperature are the main factors that lead to battery failure
  • a small difference in voltage or temperature between cells may be an indicator of a serious problem
    • the cell voltage being quite flat in the range of utilisation, it is important to have an accuracy of 1mV
    • all cells voltage must be measured with the same reference (which excludes BMS with a separate measuring board for each cell)

Warning in case of abnormal conditions (level 3 protection)

You should be made aware of any situation that is outside of optimal conditions for the battery

  • difference in voltage or temperature across cells
  • voltage above the bulk / absorption settings of the chargers
  • voltage below your standard discharge cycle
  • pack and cell temperature outside of optimal range

A warning must be visible and audible from the main living area and they must give an indication on the nature of the fault.A warning can be reset automatically if conditions return to normal (with hysteresis)

Communication with external equipment (level 4 protection)

At a minimum the BMS should be able to communicate 3 types or command to (1) disable charging, (2) disconnect loads and (3) isolate the battery.  In addition it should be able to command external systems like generators, heaters, coolers… 

Each command may have multiple forms:

  • activate a switching devices (relay or circuit-breaker)
  • directly control certain equipment like battery chargers, converters, regulators… 

It is important to note that one command like “disable charging” may need to have different forms depending on the type of chargers you use (cut alternator field, directly control a charger or regulator, activate a relay…)

Cells balancing

This feature, often overlooked for an off-grid energy storage installation, is one of the most important to optimise battery life-time and energy availability.  

It is true that in off-grid storage conditions the cells will not rapidly get out of balance.  But it is inevitable that it will happen over time over time.  If nothing is done about it, the battery capacity will be reduced and some of the cells will age faster… which in turn accelerates the unbalance…

Alarm in case of extreme conditions (level 5 protection)

Extreme conditions are situations where the battery starts ageing more rapidly or there is a risk for external equipment or people.

Must be audible and visible from the main living area and give an indication on the nature of the fault

Must be persistant – meaning that an operator intervention is necessary to reset them

Activation of safeguard procedures (level 6 protection)

Procedures that are triggered by an alarm (with an eventual time delay):

  • Activation of a backup power supply
  • Battery isolation

These procedures can only be deactivated manually after the fault has been repaired

Internal and external self-test

This is critical to ensure that the whole protection system is operational.  It includes checking:

  • BMS internal components operation (measuring components, measure references, outputs…)  
  • proper settings of warnings and alarms
  • connection and communication between system components
  • external equipment operation

Without these basic functionalities your BMS may be useless the day you need it!

TAO BMS has all these functionalities, and
TAO Monitor takes it a few steps further…

BMS advanced functionalities

These functionalities put you in control of your installation and give you a true Energy Management System (EMS)

Personalised events

In addition to warnings and alarms you can define situations that you want to be made aware of, and can optionally trigger some actions:

  • based on voltage, temperature, SOC or current conditions
  • can be set to command BMS outputs and / or send CAN messages to external equipment


  • turn on the water-heater or water-maker when SOC is above a certain level
  • disconnect non essential equipment when SOC or voltage is below a certain level
  • activate a led when discharge current is over a set level
  • activate a fan or Pelletier cooling system when temperature is above a set level

Lifetime history log

Keep records for the lifetime of the system:

  • measures, including cell voltage and temperature, current, SOC, cell internal resistance…
  • warnings and alarms when they are activated and de-activated
  • all personalised events
  • standard events like BMS power-up, SOC reset, and many more…

In addition to fault analysis and diagnostic capabilities it enables for on site or remote support, this could save you a lot of money if your battery fails prematurely and you need to demonstrate your battery has been used within the manufacturer’s specifications

Monitor screen
  • View battery and cell status (voltage, temperature, current, SOC, cell internal resistance, cycle count, SOH)
  • Edit BMS parameters
  • Review events log
  • Statistical data and graphs

Remote access
  • Access all BMS functions remotely with a web browser (phone, tablet, computer)
  • Run diagnostics and simulation
  • connect to a Wifi network (or create an access point)
  • Access historical data and events on the Cloud

This enables you to monitor your installation when you are not on site.  It also give the ability for a technicien to diagnose some system dysfunctions and faults.

TAO Monitor has all these advanced functionalities

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