BMS in lithium batteries: what is it and what is its function in cell balancing?
25 May 2021
When talking about lithium batteries, the term BMS (Battery Management System) often comes up: a complex electronic system that acts as the true 'brain' of the battery pack. Although it is an essential control device for safety, few people know exactly what it is and what function it performs. In this article, we explain it simply and show why it is fundamental for Battery Lifetime Extension and for enabling predictive monitoring through advanced algorithms.
What is a lithium battery BMS?
Lithium batteries are high-performing devices and offer major advantages over traditional technologies, but they also have an Achilles' heel: the cell production process cannot guarantee absolute uniformity between individual elements. This variability leads to physiological discrepancies (in capacity and voltage) that require constant, precise monitoring, managed through high-precision Cell Monitoring Units (CMU).
Although all their characteristics exceed rated values, the cells inevitably show:
- Differences in rated capacity
- Differences in internal resistance
- Differences in self-discharge
With use or simply over time, these small differences cause the cells inside a battery to become unbalanced, negatively affecting the calculation of the overall State of Charge (SoC). This imbalance can lead to a 3% to 6% difference between cells depending on usage conditions and ageing status, accelerating degradation of the State of Health (SoH) and State of Aging (SoA) of the lithium battery.
Since the cells are connected in series inside the battery, they are charged and discharged with the same level of energy. This means that without an appropriate cell balancing system, the difference between the cells would increase more and more, gradually draining the available capacity.
Let's look at the first function of BMS systems inside lithium batteries: cell balancing to maximise efficiency.
BMS lithium-ion batteries and cell balancing
How does a conventional BMS affect balancing?
To counteract this phenomenon, conventional BMS systems apply resistance to the cells with a higher charge, dissipating excess energy as heat while waiting for the cells with lower charge levels to reach the same level. This method, known as passive balancing, is often slow and limited by very low balancing currents, which are insufficient for modern high-capacity battery packs.
Let's look at the pros and cons of using this technology
How does Flash Battery’s proprietary Flash Balancing System work
Unlike traditional BMS systems, Flash Battery has developed a highly sophisticated proprietary balancing system: the Flash Balancing System. This technology acts on each individual cell through combined active and passive balancing, using high power to manage a balancing current at least 20 times higher (High-Current Balancing). This architecture makes it possible to drastically reduce charging times and ensure perfect battery pack uniformity, effectively helping to prevent Thermal Runaway.
