LiFePO4 BMS: What is it, How to Choose?

By now, you are likely quite familiar with all the different battery types out there, and in particular the LiFePO4. 

You may have also heard of a battery management system, more commonly referred to as a BMS, of which is a vital piece of equipment for all lithium-ion batteries. 

In this article we will explain what a BMS is, how they work, and how you can choose the right one for your battery. 

Understanding LiFePO4 BMS

15S 48V LiFePo4 Battery 3.2V 50A BMS Battery Protection Board – Electromann  SA

A BMS or battery management system is an important part of any lithium-ion battery system. 

You can think of it as the brains of your system. It essentially makes sure your battery stays healthy by controlling the discharge and charging process. 

In addition, it also monitors the battery cells and measures parameters such as voltage, current, and temperature.

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 happens if I don't use a BMS?

A BMS helps prevent your battery from:

Overcharge

Overcharging a lithium battery can cause increased pressure, and cause thermal runaway.

Overtemperature

Overheating a LiFePO4 battery greatly reduces their lifespan. However, in extreme circumstances can potentially lead to them catching fire and exploding.

Cell imbalance/Shorter life cycle

Cell imbalanced is essentially caused when the two above points happen. Once your battery has a cell imbalance, the total lifespan of your battery system will have been reduced. 

This is especially critical in LiFePO4 batteries as they are often bought due to their longer lifespans. 

How to select the right BMS for your application?

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

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. 

A BMS greatly depends on the size of your battery system. Most importantly its rated voltage and capacity.

A few important terms you want to better understand before deciding on the right BMS are: 

  • Voltage
  • Amperage
  • Capacity
  • C-rating

Any LifePO4 BMS should be compatible with your LiFePO4 specs. For example if you bought a 12V battery pack, you should be using a BMS rated for 12V.

Most importantly, you want a BMS rated with the correct amperage. 

To calculate this, you need to estimate the maximum power (in Watts) that you will be drawing from your battery.

Remember: Power (W) = Voltage (V) x Amperage (A)

Here's an example, let's say you wish to build a solar system with a 3000W inverter to power loads of up to 2500W. In addition, you’re thinking about getting a 100A BMS to connect to your 12V LiFePO4 battery pack for this system.

At first glance do you think this will work? 

That's right, it won't.

That's because: Power (W) = 12V x 100A = 1200W

With this sort of system, you won't be able to power any loads reaching over 1200 watts. 

However, if you use a 250A BMS, your maximum power output of your system becomes 3000W: Power (W) = 12V x 250A = 3000W

Now you have a compatible BMS size to your 3000W system.

Do keep in mind though, that we are using an example with a battery pack rated at 12V. If you were using say a 24V or 48V battery your power output would look something like this:

24V battery pack: Power (W) = 24V x 100A = 2400W max power output

48V battery pack: Power (W) = 48V x 100A = 4800W max power output

With a 100A BMS in a 24V battery you come close to being able to power the loads you are after. 

Do keep in mind though that this particular 100A BMS you are after will need to be rated at the same voltage as your battery system. In this case, 24V or 48V. 

Another way of figuring out whether your desired BMS will be compatible to your battery is using the rated capacity and C-rate.

Here's example:

Your battery pack has 200Ah of capacity and a maximum C-rate of 0.2C. Then your BMS should be able to handle at least 40A because: 200Ah x 0.2C = 40A max, delivered for 5 hours

Final thoughts

Although sizing the correct BMS for your battery is not necessary when buying a ready made solar generator, if you are thinking of building your own LiFePO4 battery pack, you need to make sure you have the basics down before choosing a BMS.  

In summary, you want to make sure you choose a BMS with a continuous discharge current larger than what you’ll be using your battery for and a charge current larger than you’ll be using to charge your battery.

We hope you found this article informative, albeit shorter than usual (we want to make a sometimes complex topic less taxing to learn).

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