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How does a lithium battery BMS work?

2021-03-17

BMS is short for Battery Management Systems, which is an electronic system for the rechargeable battery so as to prevent the battery from hazard when the battery goes beyond the minimum or maximum SOC (state-of-charge) limit as well as the charge and discharge cycles. It monitors the battery state, calculates and reports the secondary data, and controls the battery working environment by authenticating and balancing it. It is usually related with PCM which is the acronym for protection circuit module (more details available in another article), but have more protection and features.

 


You may wonder how does battery management system work exactly, and here is the brief explanation of it without going deep into technical details. BMS is comprised of several functional blocks which can be divided into main parts of cutoff FETs, a fuel gauge monitor, a state machine, cell voltage monitor, cell voltage balance, temperature monitors, and other blocks including real-time clock (RTC), battery authentication block, voltage reference and daisy chain circuitry.

 


The monitoring of each cells’ voltage is vital for the overall battery health, and BMS for lithium ion battery usually has an operating voltage window for every cells when charging and discharging to avoid fast degradation. This is because lithium battery cells are very sensitive to overcharging and over discharging, and cell unbalancing will cause thermal runaway, and cell degradation which means reduced efficiency and life cycles, incomplete charging and use of battery pack. The operating voltage usually ranges between 2.5V and 4.2V for lithium batteries depending on the lithium chemistry used, while operating the battery beyond this range leads to significant reduction of the cell lifetime and can even damage the cells.

 

A FET driver functional block is used to connect and isolate the battery pack between the states of load and charger, and it operates according to the measurements of cell voltages, current and real-time detection circuitry. The fuel gauge functional block is to keep track of the charge when it enters or exits the battery.


Temperature measurements are not only for safety conditions, but also determine whether it’s acceptable to charge or discharge a battery. As a result, temperature sensors are applied to monitor every cell for energy storage system, and thermistors for each circuit’s temperature. The internal voltage reference is used to cut down on inaccuracies of the temperature reading V.S. environmental temperature changes.

 


The real-time clock and memory are for black box applications where the RTC is used for a time stamp and memory stores data, allowing the user to know the real-time state and operation of the battery. The battery authentication block is used to prevent the BMS electronics from being connected to a third-party battery. The voltage reference / regulator powers peripheral circuitry and daisy chain circuitry simplifies the connection between stacked devices.


Generally speaking, users need to check the BMS state on the computer after setting up the lithium battery with invertors, but a more convenient and smart method is available now. Check the world's first lithium ion solar battery with LCD touchscreen provided by UFO POWER, and you will be amazed by the user-friendly smart monitoring system and love it at the first sight!

 


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