What is a BMS (Battery Management System)

There are several different types of batteries on the market today, notably LiFePO4. You may also have heard of battery management systems, often called BMS, which are important devices for all lithium-ion batteries. In this article we will explain what BMS are, how they work and how to choose the right BMS for your battery.

What is a BMS (Battery Management System)?

Lithium-ion batteries have many advantages over lead-acid batteries. They are lighter, more efficient, faster to charge and last longer. However, they are susceptible to conditions that can damage the battery pack. Tapping into all of this potential requires lithium-ion batteries to be more complex and to contain components that help avoid these damaging conditions. In fact, this is the primary purpose of a BMS, which stands for Battery Management System.

Why do I need a battery management system?

The BMS is very important for your battery system. Without it, your LiFePO4 batteries can be permanently damaged and even pose a potential safety risk (especially for LiFePO4 batteries).

 What is the function of the battery management system Portable Power Station ?

The primary function of the BMS is to protect the battery cells from damage caused by overcharging or overdischarging. In addition, the BMS calculates the remaining charge, monitors the battery temperature, and monitors the health and safety of the battery by checking for loose connections and internal shorts. the BMS also balances the charge between cells to keep each cell operating at maximum capacity.

If any unsafe conditions are detected, the BMS will shut down the battery to protect the Li-ion battery and the user.

How does the battery management system work?

The battery management system monitors the individual cells in the battery pack. It then calculates the amount of electricity that can safely flow in (charge) and out (discharge) without damaging the battery.

Current limiting prevents the power source (usually a battery charger) and the load (e.g., an inverter) from overdrawing or overcharging the battery. This protects the battery pack from excessive high or low battery voltage, which helps extend the life of the battery.

The BMS also monitors the remaining charge in the battery. It continuously tracks the energy going into and out of the battery pack and monitors the battery voltage. It uses this data to know when the battery is depleted and to shut down the battery. This is why lithium-ion batteries don't show signs of depletion like lead-acid batteries do, but simply shut down.

Why BMS is important

The battery management system is critical to protecting the health and life of the battery, but even more important from a safety perspective. The liquid electrolyte in lithium-ion batteries is highly flammable.

Therefore, these batteries need to operate optimally and within safety limits at all times to prevent fires from occurring.

How to choose the right BMS for your application?

Most modern LiFePO4 batteries have a ready-to-use BMS built in, but if you want to build your own LiFePO4 battery, you may want to know a thing or two about how to choose the right BMS.

View of LiFePO4 battery pack with BMS board | Download Scientific Diagram

How to know what size BMS to get

When someone refers to the "size" of a BMS, they are usually referring to the maximum amount of power the BMS can handle. You need to make sure you get a BMS that can support the amount of power the load needs.

In fact, it is a good practice to increase the current carrying capacity by about 15% so that you have a little margin. After all, you don't want to run the BMS (or anything else for that matter) at the absolute maximum rating.

 Suppose you have a 1000W inverter You want to be able to operate safely at maximum load. In this example, we will consider a 7S Li-Ion battery running a 24 volt AC inverter. 7S Li-Ion batteries have a full charge voltage of 29.4 volts and a dead voltage of about 18.5 volts.

When the battery is fully charged, it takes about 37 amps to draw a 1100W load from the battery pack.

1100 watts ÷ 29.4 volts = 37.4 amps

At first glance, it may seem like you only need a 45 amp BMS. after all, your peak current is only 37.4 amps, and with 15% more current, it's only 43 amps.

But wait.

When the battery is almost dead, the voltage will be much lower. So, what happens when the wattage you need stays the same but the voltage drops?

The current rises.

1100 watts ÷ 18.5 volts = 59.5 amps

As you can see, you must plan for the maximum amount of power the battery must deliver at the minimum voltage. So, in this example, you need a 70 amp BMS.