Why should I care for BMS at all?

You see, when your phone battery comes down to 0%, your phone shuts down. Does this mean the battery is absolutely empty and there is a single atom of energy left in it? No. When you try turning your phone on, it will briefly show you that the battery is empty and shut down again.

Actually, when your phone says 0% battery left, it can probably run for around 10 more minutes - but why doesn’t it let you? Because at that moment the battery enters an overdischarge state where the battery chemistry is damaged with every further discharge. Imagine training - once you get tired, you stop. Every further session will bring injuries before any benefits.

So who protects the battery from being over-discharged? You got it — BMS. BMS cuts off the power to your phone every time the battery reaches 0%, protecting it from further damage.

Protection

BMS is not only about overdischarge protection. It also protects against:

• battery overcharging — putting more energy into a battery is like pumping more gas into an already pressurized bottle
• battery overcurrent — every battery can deliver only so much energy in a given time. Exceeding that damages and overheats the battery
• battery overtemperature — batteries don’t behave well at very low or very high temperatures

Estimation

Does it ever happen that battery percentage goes from 100% down to 30% in half a day and then from 30% to 0% in 20 minutes? That’s because the battery state of charge wasn’t estimated properly. The algorithm running in the background isn’t perfect. Where does that algorithm sit? In the BMS.

Battery state of charge and state of health are the two most important estimations that BMS provides. There are others as well, but to get these two right, BMS has to:

• be smart (obviously)
• learn about the battery over its lifetime

These algorithms are far from simple, and we will do our best to make them as accurate as possible and most importantly - to provide them open-source.

Cell balancing

Last but not least — cell balancing. This one is more of a tech thing and it’s a bit hard to relate to everyday phone-user experience, but here’s what it is. If your battery consists of multiple cells, 2 or more, those cells need balancing. That’s because no two cells are identical. Especially over time, each cell changes its characteristics and capacity. For example, a 3-cell battery rated as 4000mAh actually has 3 cells with capacities of 3950, 4020, and 3990 mAh. In that case, charging would result in one cell charging faster than the others. To keep them balanced, a cell balancer takes a small amount of energy from the higher-voltage cells and dissipates it as heat, keeping all cells at the same voltage level.

Where BMS is used?

BMS can be found almost anywhere: phones, laptops, drones, electric cars, e-scooters, IoT gadgets, and more.

---

← Back to blog