What is a BMS anyway?

-May 20, 2017

If you have worked with or looked at battery systems, you have most likely heard of a battery management system or BMS. Since Nuvation Energy provides off-the-shelf battery management systems for large-scale applications, and has a 20-year history of creating custom power management solutions for a wide range of devices from miniature to large-scale, I thought I would share some of my experience with battery management systems.

So why do BMS prices range from $10 to several thousand dollars if they all do the same thing? An appropriate parallel would be to ask why motorized transportation systems vary so greatly in price, with a motorized skateboard at one end of that spectrum and a transport truck at the other. Let’s take a closer look at how this analogy plays out with battery management systems.

An example at the small end of BMS requirements is what is needed to protect a battery pack for a small device like a cordless drill. The typical cordless drill contains around 5 or 6 cells in series with the total cell cost of about $30. Clearly the BMS must be very inexpensive to maintain market viability; one is typically under $10 and functionality is limited to most basic protection. This BMS would be more accurately referred to as a battery protector.  These systems monitor cell voltage and pack current and open one or both back-to-back FETs in the event of voltage or current going out of range. These range limits are fixed at time of manufacture. Over-temperature protection is also sometimes included. These systems often do not include balancing and are standalone, with no communications to other systems.


Cell battery pack schematic

 

Simple BMS for cordless tools Source: Ali Express

Moving up a level would be electric bikes, which typically use 36V battery packs. A lithium-ion battery pack for this application is about 0.5 kWh and costs about $200 to $300. Unlike the cordless drill, a longer battery lifespan is important to the customer, and the bike’s total price point is aligned with that expectation, so cell balancing becomes mandatory. The BMS must also be able to connect to some form of “gas gauge” display that shows the user how much power is left in the battery. In addition to this functionality a diagnostic port for service personnel is also desirable. This BMS is around $30 to $50.

The next level up from these smaller systems to a low-voltage residential energy storage system raises the stakes yet again, this time by an order of magnitude in certain respects. Residential “behind the meter” energy storage systems are often in the 48V range, provide 7 kWh-20 kWh, and cost about $5K-$20K. At this stage balancing performance and monitoring battery parameters must be well implemented and also support the needs of installation and service companies to be able to perform remote monitoring and event logging to manage their customers’ installations (since most home owners are not technically able to manage their own systems). Service truck rolls are expensive and the availability of remotely accessible information can eliminate many unnecessary calls. This usually means an ethernet interface is required as well as software support such as a browser-based GUI that can be run remotely, and event log pushing to facilitate priority notifications in the event of faults or pending faults. A BMS for this application is typically in the $250 to $500 range.



 




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