Which BMS to select for a lithium battery?

Which BMS to select for a lithium battery?

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The BMS &#;Battery Management System&#; is a term frequently used when talking about batteries, especially those using lithium technology. This electronic card is a fundamental pillar of lithium battery management due to its complexity. It continuously monitors the cells and provides key information about the battery&#;s condition.

In order to benefit from all the advantages offered by the BMS it is necessary to select the most suitable solution for your lithium battery.

The BMS: 2 main functions

Guaranteeing the safety of the battery

The classic function of the electronic management card is to protect a lithium battery or battery pack from any risks or dangerous circumstances. It provides protection against overvoltage, undervoltage or overcurrent by measuring and monitoring cell temperature, voltage and current.

Some boards have an integrated temperature sensor that allows them to cut off the charge or discharge of the lithium battery, depending on the temperature reading, to prevent thermal runaway, i.e. the degradation of the cells due to intense overheating, which can lead to a battery explosion.

Optimises battery performance over time

Regardless of the electrochemical lithium technology, the BMS ensures that all cells are correctly balanced, i.e. they have the same current intensity and temperature. This function allows it to maximise the capacity of the battery or battery pack and avoid wear and tear in the long term.

Good management of the temperature and balance of the cells by the BMS results in a longer life for the battery.

The different categories of BMS

It is possible to classify the electronic management boards of a lithium battery into 2 categories:

• BMSs with simple functions, commonly called PCMs (Protection Circuit Modules), which provide standard protection against overvoltages, undervoltages and overcurrents with a very basic balancing function for the card&#;s electronic components;
• Smart BMS or &#;high level&#; BMS. In addition to the above functions, they can manage a greater number of configurations (calculate the state of charge and the state of health of the battery, provide self-diagnosis functions, etc.), while potentially communicating with other electronic elements (smart chargers, remote display, etc.).

Among the many differences between these two electronic management cards, the most important one to highlight is the lack of security of PCMs. Indeed, by definition, PCMs are limited in that they only perform a small number of functions to protect the battery or battery pack.

PCMs do not have the software intelligence of Smart BMSs and their microcontrollers to ensure accurate measurement of the battery cell temperature. This means that there is a risk of overheating the cells and therefore of thermal runaway.

The choice of a Smart BMS is therefore recommended to ensure the full safety of a lithium battery or battery pack.

Three questions to ask yourself when choosing your Smart BMS

The choice of a BMS depends mainly on the application in which the battery or lithium battery pack is integrated. Indeed, the electronic card selected for the lithium battery pack of an embedded solutions (e.g. electric vehicle) will not be the same as the one intended for the management of a battery of a stationary application.

Three questions can guide you in the choice of your solution:

What is the maximum current that my battery or battery pack can deliver?  

This will allow you to choose the maximum amperage at peak load, particularly for the BMS. Some electronic boards are specially designed for applications that require a lot of power, such as electric vehicles (buses, trucks, powerful vehicles, etc.). Others are more suitable for light applications or those with lower energy peaks (electric bicycles, stationary applications etc).

What is the voltage of my battery?  

The higher the voltage required for the battery, the greater the number of cells it will contain. It is therefore important to choose a BMS that is adapted to these key characteristics of the battery.

Do I need to optimise space in my application? 

The issue of space optimisation in BMS refers to the notion of the powerbox. As part of the BMS, the powerbox is a set of electronic components that supply energy to the BMS and measure, monitor and control the current flowing through the battery.

Some BMSs have an internal powerbox, i.e. located on the board, while others require an external powerbox. The use of an internal powerbox allows for space optimisation as it does not require an external installation to the BMS.

However, it is important to note that the choice of powerbox is linked to the current intensity that the BMS must handle. For batteries of more than 50A continuous and 100A discharge, the use of an external powerbox is necessary because it is more powerful.

The internal powerbox will therefore be present on BMSs dedicated to small mobility (electric bikes, scooters etc.) or applications requiring little power.

Smart BMS designed by BMS PowerSafe according to your application

As an electronics expert, BMS PowerSafe designs BMSs adapted to each project. We develop the most reliable and efficient solution to ensure the safety of your application while respecting your technical constraints.

Some examples of applications and the customisable PowerSafe BMS electronic board to select from:

Would you like to know more about our BMS? Please contact us.

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

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

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Additional resources:
How do you trickle charge a NiMH battery?

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?

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 W inverter to power loads of up to W. 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 = W

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

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

Now you have a compatible BMS size to your W 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 = W max power output

48V battery pack: Power (W) = 48V x 100A = W 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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