How To Choose A BMS For LiFePO4 Cells?

By Matt •  Updated: 09/18/22 •  7 min read

If you have any ideas about batteries, I’m sure you know that BMS is an essential accessory for LiFePO4 battery cells.

But how do you choose the right BMS size? Well, you can size them based on some factors. I’ll discuss that in this article. So, let’s get into it.

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These are the elements based on which you have to choose your BMS size:

  1. Voltage:  Measurement of potential electrical difference between two points in a circuit.
  2. Amperage: A measurement of electrical current flow.
  3. Capacity: A total amount of current supplied by a battery for 1 hour.
  4. C-Rating: A discharge rate of a battery against its rated capacity.

Let’s assess your LiFePO4 battery Specs

You will have to measure the above 4-key components of your LiFePO4 battery cells and size the BMS by that. For better understanding, I will discuss them in detail.

The BMS for your LiFePO4 battery should be compatible with your battery cell specs. Let me put it this way if you have a 12V battery pack; your BMS should be rated for 12V. It entirely depends on your battery size.

Infographic for selecting a BMS for LiFePO4 Battery
Infographic for selecting a BMS for LiFePO4 Battery

The voltage of your LiFePO4 battery

I’ve already mentioned that voltage is the potential electrical difference between two circuit points. Its unit is written as Volts, or simply V.

Let me explain in simpler terms, voltage is a pressure or force in which direction current moves from point A to point B in an electric circuit. 

Your LiFePO4 battery pack is an accumulation of multiple cells, and the total amount of cells in the battery pack will determine its nominal voltage.

Generally, each LiFePO4 battery cell has a 3.2V nominal voltage. So, if you take 4 of these battery cells, the total nominal voltage will be 12.8V.

Amperage Value

Those who were attentive in their chemistry classes should remember that amperage measures electrical current flow rates. In other terms, it is the rate of electron flow in a circuit.

It shows the number of electrons passing through a certain point at a given time in an electrical circuit. It’s expressed as an amp/ hour or Ah. Normally, a 12V battery can produce a maximum of 100 amp per hour. 

Full Capacity of the Battery or Battery Pack

Battery capacity is crucial for BMS sizing. It shows the total amount of current a battery pack can supply for 1 hour until the voltage drops to a specific value. The value depends on the battery type.

Battery capacity is shown as amp-hour. The more your battery capacity, the bigger your Battery Management System will be.

C-Rate Value of the Battery Cell

It measures the discharge rate of a battery pack against its rated capacity. Every battery pack has a rated discharge rate, but C-rate measures the rate of discharge of your battery in reality.

If your LiFePO4 battery has a 1C rate, it means the current of your battery will be discharged in 1 hour. So, if you have a 100Ah 1C rated LiFePO4 battery, you will get 100 amp for 1 hour.

In another scenario, if your battery pack is 100Ah but 0.5 C rated, you will get a 50 amp current for 2 hours.

This is How You Select a BMS For LiFePO4 Battery Pack

Now that we know what determines the size of a BMS, now you can easily calculate the correct size.

I have already said that a BMS must be compatible with the battery pack. Most importantly, you must use a BMS rated with the correct amperage.

You have to calculate the maximum amount of power your LiFePO4 battery pack will produce to size its BMS.

Power (W) = Voltage (V) x Amperage (Amp).

This is the formula to calculate maximum power generation from your battery pack. Now, putting numerical values will give you the power output.

Say you have a 4-cell LiFePO4 battery pack and 100A BMS, but you want to draw 2000W from your battery.

We know that 4 cells of the LiFePO4 battery pack produce 12V, which can produce a maximum of 100 amp. This is the amount your 100A BMS can handle.

Now you will get maximum, Power = 12V X 100A = 1200W.

So, with this system, you can only produce 1200W, which is far less than 2000W. That’s why a 100A BMS won’t suit your battery pack. However, a 200A BMS will be compatible with it.

Because it will produce, P= 12V X 200A = 2400W.             

That’s how you can size your BMS according to your battery pack size.

Similarly, if the nominal voltage of your battery pack increases, a 100A BMS will be suitable for larger power output.

For example:        

With a 24V battery pack, the maximum power output would be 2400W. So, a 100A BMS will easily take 2000W output. But remember, in that case, your BMS will also have to be rated for 24V along with your battery pack.

What Are the Types of BMS available?

According to how they are built and the technology of the battery packs, there are 4 types of Battery Management Systems. Let’s take a glimpse at them.

Centralized Battery Management System

In this type of BMS, all the battery packs are directly connected to the central BMS.  It is a compact BMS with clear feasibility.

Modular Battery Management System

This type of BMS is grouped into many duplicate modules. And each module has its separate wire bundle and connects to the adjacent assigned part of the battery pack.

Primary Battery Management System

It is somewhat like the modular topology. However, the primary modules here only transfer the measurement info, whereas the main module will compute, control, and run the external communication.

Distributed Battery Management System

In this BMS, there are modules and hardware & software acting like modules, which are attached through wiring. Every component is monitored and synchronized by a control board.

Here’s a table showing the advantages and disadvantages of all 4 BMS types.

BMS TypeAdvantagesDisadvantages
Centralized BMSCompact DesignExcellent FeasibilityMore batteries require more ports.
Modular BMSOffer more space for other options. Less computational attempt.Incurs higher costs.
Primary BMSLess CostlySimple functionNot compatible with scaling operations.
Distributed BMSFewer cables Automatic communicationDeep routine and difficult troubleshooting.
Pros and Cons of different BMS system types

What happens if a BMS fails?

According to a study in the journal of energies, failure of a Battery management system will stop it from controlling the battery pack, which will lead to a series of issues for the battery pack.

If a BMS does not work properly, it won’t compute and sense the battery pack. As a result, the battery cells might get over-charged or start over-discharging. It can lead to a total failure of the whole battery system.

However, since LiFePO4 batteries are extremely safe, the chance of catching fire is virtually zero. But your battery might get damaged in other ways.

Final Thoughts

BMS is crucial, but even more crucial is selecting the right size for your battery pack. So, you can confidently size BMS for your LiFePO4 battery pack after today. Just make sure you select the right BMS type.

References

1. P. Krishna R et al. (2020). Battery Management Systems—Challenges and Some Solutions.

2. A Complete Guide on the Components of Battery Management System. 2022.

Matt

I’m Matt, a computer and electronic embedded system developer. Currently, working as an electronic engineer. In my leisure time, I do love to blog about technology stuff. Specially, new advance high tech devices. Portable solar systems are one of topic that I love to investigate. They are becoming more powerful day by day which keep me alert.

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