What Is A BMS (Battery Management System)?

There are several different types of batteries on the market today, particularly LiFePO4. You may also have heard of battery management systems, often referred to as BMS, which are an important piece of equipment for all lithium-ion batteries. In this article, we'll explain what a BMS is, how they work, and how you can choose the right BMS for your battery.

What Is A BMS (Battery Management System)?

Lithium-ion batteries have a lot of advantages over their lead-acid counterparts. They’re lighter, more efficient, charge faster, and have a longer lifespan. However, they’re susceptible to conditions that can damage the battery pack. Tapping into all of this potential requires lithium-ion batteries to be more complex and include components to help avoid these damaging conditions. In fact, this is the primary purpose of the BMS, which means a battery management system.

Why do you need a battery management system?

A BMS is pretty important to your battery system. Without it, your LiFePO4 battery may become permanently damaged and even pose potential safety risks (especially LiFePO4 batteries).

What is the Function of a Battery Management System for Portable Power Station? 

The primary function of the BMS is to protect the battery cells from damage caused by being overcharged or over-discharged. Additionally, the BMS calculates the remaining charge, monitors the battery’s temperature, monitors the battery’s health and safety by checking for loose connections and internal shorts. The BMS also balances the charge across the cells to keep each cell functioning at maximum capacity.

If it detects any unsafe conditions, the BMS shuts the battery down to protect the lithium-ion cells and the user.

How Does a Battery Management System Work? 

The battery management system monitors individual cells in the battery pack. It then calculates how much current can safely go in (charge) and come out (discharge) without damaging the battery.

The current limits prevent the source (usually a battery charger) and the load (such as an inverter) from overdrawing or overcharging the battery. This protects the battery pack from cell voltages getting too high or low, which helps increase the battery’s longevity.

The BMS also monitors the remaining charge in the battery. It continually tracks the amount of energy entering and exiting the battery pack and monitors cell voltages. It uses this data to know when the battery is drained and shut the battery down. This is why lithium-ion batteries don’t show signs of dying like a lead-acid, but just shut off.   

Why a BMS is Important

Battery management systems are critical in protecting the battery’s health and longevity but even more important from a safety perspective. The liquid electrolyte in lithium-ion batteries is highly flammable.

So, these batteries need to be operating optimally and within safety limits at all times to prevent a fire.   

How to select the right BMS for your application?

The majority of modern LiFePO4 batteries come with a ready-to-go BMS built inside of it. However, if you are looking to build your own DIY LiFePO4 battery, you may want to know a thing or two about how to select the right BMS. 

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

How To Know What Size Of BMS To Get

When someone refers to the ‘size’ of a BMS, they are generally referring to the maximum amount of current the BMS can handle. You need to make sure to get a BMS that can support the amount of power that is required by your load. 

In fact, it's a good practice to add about 15% more current carrying capacity just so you have a little bit of headroom. After all, you don't want to run your BMS (or anything else, for that matter) at its absolute maximum ratings. 

Let's say you have a 1000W inverter that you want to be able to safely run at max load. In this example, we will consider a 7S lithium-ion battery running a 24 volt AC inverter. A 7S lithium-ion battery has a fully charged voltage of 29.4 volts and a dead voltage of about 18.5 volts. 

Drawing a 1100W load from the battery pack will require around 37 amps when the battery is fully charged.

1100 watts ÷ 29.4 volts = 37.4 Amps

At first glance, it may seem like you just need a 45 amp BMS. After all, your peak current will only be 37.4 amps, and adding an extra 15% to that comes to just 43 amps. 

But wait.

When the battery is nearly dead, the voltage will be much lower. So, what happens when the amount of watts that you need stays the same but the voltage goes down? 

The current goes up.

1100 watts ÷ 18.5 volts = 59.5 Amps

As you can see, you have to plan for the maximum amount of current that your battery will have to provide at its lowest voltage. So, in this example, you would want a 70 amps BMS.