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What Is A LiFePO4 BMS?
Pubdate: 2024-04-22
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What Is A LiFePO4 BMS?

A Lifepo4 BMS is an integral part of any lithium-ion battery system — it’s responsible for keeping the cells within the battery pack healthy and performing optimally.

Every battery has a specified range of voltage, current, and temperature in which it can safely operate. If one or more of these parameters varies considerably from their specified range, your battery pack can get permanently damaged (due to degradation of its components) and even pose safety risks.

Most batteries come with a built-in BMS to prevent this from happening.

But how does a BMS prevent you from damaging your battery pack?

A LiFePO4 BMS controls the discharge and charge processes of LiFePO4 battery packs. So if anything goes wrong during these processes, the BMS protection immediately kicks in and adjusts the charging parameters or cuts off the power flowing to and from the battery pack entirely.

Moreover, a BMS monitors the battery cells and makes sure they’re all working together correctly. It also measures parameters like voltage, current, and temperature to ensure that the battery is healthy and safe.

This way, a BMS helps prevent your battery from:

  • Overcharge
  • Overvoltage
  • Overcurrent
  • Overtemperature
  • Cell Imbalance
  • Shorter life cycle

Additionally, a BMS optimizes your battery capacity and overall performance in every charge/discharge process. This way, you can get the most out of your LiFePO4 battery pack regarding performance and lifespan.

 

LiFePO4 BMS Main Functions

1. Control operating conditions

Measures voltage, current, and temperature signals and controls these parameters to achieve cell balance and prevent damage to the battery.

2. Perform Diagnosis

Monitors cell-to-cell variations over time; diagnoses errors, detect safety risks, and sends warning signals to the driver (which can then initiate the proper measure to avoid this safety risk).

3. Collect and Store Data

Records the pack’s and the individual’s cell signals; stores data related to the battery’s life cycle history.

4. Estimate Parameters

Determines cell and pack levels such as State of Charge (SOC) and State of Health (SOH); communicates with controllers for cell balancing.

 

How Does A LiFePO4 BMS Work?

A LiFePO4 BMS consists of several hardware and software functional blocks programmed with functions that protect the battery pack by monitoring and controlling its charge and discharge conditions.

A good BMS should offer protection against:

  • Over and under-voltage
  • Over and undercurrent
  • Cell Imbalance
  • Over and under temperature

The same applies to Undervoltage: when the voltage monitor detects a voltage lower than the minimum charging or discharging voltage, the cut-off transistor kicks in, stopping this charging voltage from reaching the battery, thus avoiding any damage to the battery pack.

 

Cell Balancing

Another primary function of a LiFePO4 BMS is ensuring that all cells in a pack are correctly balanced.

Balanced cells mean that if you were to measure the voltage of each cell individually, you’d find the same value for every cell. This helps prevent the battery from charging/discharging erratically.

Here’s how a BMS balances a battery pack: by changing the charging current for one or more individual cells in the pack, making it different from the pack current, in one of the following ways:

  • Removing charge from the cell charged the most provides “space” for additional charging current to prevent overcharging and allows the less charged cells to receive more charging current.
  • Redirecting the charging current (partially or entirely) to the less charged cells, allowing them to receive a charging current for an extended period until cell balance is reached.

 

Temperature Control

A temperature sensor sends the battery’s temperature signal to the BMS’s monitoring unit. If a potentially dangerous charging or discharging temperature is detected, the BMS automatically cuts off any power to and from the battery, preventing any safety risks related to over or under temperature.

 

Cut-off Voltage And Current

Battery management systems have current-driven and voltage-driven cut-off transistors that can cut off the power from the charger to the battery or from the battery to the load.

These transistors act as switches: when the cell voltage monitor detects a voltage higher than the system can handle, the switch is turned off, protecting the battery from overvoltage.

 

Do Lithium Batteries Needs A BMS

Lithium-ion batteries do not require a BMS to operate. With that being said, a lithium-ion battery pack should never be used without a BMS. The BMS is what prevents your battery cells from being drained or charged too much. Another important role of the BMS is to provide overcurrent protection to prevent fires.

BMS modules are not expensive (compared to the rest of the battery pack) and they are relatively easy to install. So, there is really no reason to not use a BMS. There are a few super special and rare situations where it may make sense to not use a BMS, but they are outside of the scope of this article.

 

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 amp BMS.

 

What Happens If You Build A Lithium Ion Battery Pack Without A BMS

Lithium-ion battery packs are composed of many lithium-ion cells in a complex series and parallel arrangement. Many cells are needed when building a battery pack in order to provide the right amount of voltage, capacity, temperature, and current-carrying capacity characteristics.

The ways in which lithium-ion cells have to be arranged inside a battery pack depends on the cells and connections to the cells being exactly the same. In the real world, however, this is never the case. Even if cells are the same brand and type, and even if they are from the same production run in the factory, no two are the same. Also, no two spot welds and no two solder joints are the same.

So, because of this, there needs to be some active circuitry living inside the battery pack that monitors its internal state so corrections to natural imbalances can be made. The BMS actively monitors each cell group’s voltage and provides a balanced current to compensate.

The BMS shuts things down when they get out of spec. If anything unusual happens, a BMS will do what it can to resolve the problem. If it cannot, it will effectively put the BMS into sleep mode which shuts the battery pack off. Building a battery without a BMS is dangerous and can result in fires, injury, and loss of life.

 

How to choose a Right BMS for liFePO4 battery cells ?

If you are looking to build safe-high performance battery packs, then you are going to need to know how to choose a BMS for lithium batteries. The primary job of a BMS is to prevent overloading the battery cells. So, for this to be effective, the maximum rating on the BMS should be greater than the maximum amperage rating of the battery.

When choosing a BMS for a lithium-ion battery, the most important aspect to consider is the maximum current rating of the BMS. In addition to that, you need to make sure the BMS supports the correct number of series cell groups. Also, wireless connectivity is important to you, make sure the BMS you are looking to buy has Bluetooth because most BMSs do not. A BMS’s discharge current, charge current and balance current.

When choosing a Battery Management System (BMS) for LiFePO4 battery cells, there are several important factors to consider.

Cell Compatibility: Ensure that the BMS you choose is specifically designed to work with LiFePO4 battery cells. Different lithium chemistries have different charging and discharging characteristics, so it’s crucial to select a BMS that is compatible with the specific chemistry of your battery cells.

Voltage and Cell Count: LiFePO4 batteries typically have a nominal voltage of 3.2 volts per cell. Determine the total voltage and cell count of your battery pack to ensure the BMS you choose can handle the voltage range. The BMS should be capable of monitoring and balancing the cells within the desired voltage range.

Current Capacity: Consider the maximum current rating of the BMS. It should be able to handle the charging and discharging currents required by your application. Make sure the BMS has an appropriate safety margin to handle peak currents without any issues.

Balancing Function: LiFePO4 battery cells benefit from regular balancing to maintain cell voltage consistency and prolong overall battery life. Ensure that the BMS has a balancing function or supports external balancing to equalize the voltage across the cells. Balancing helps prevent overcharging of some cells and undercharging of others.

Protection Features: Safety is a critical aspect of any BMS. Look for a BMS that provides protection features such as overvoltage protection, undervoltage protection, overcurrent protection, short circuit protection, and temperature monitoring. These safeguards are essential to prevent damage to the battery cells and ensure safe operation.

Communication and Monitoring: Determine if the BMS provides communication capabilities such as UART, CAN, or SMBus protocols. This allows you to monitor and control the battery pack remotely, gather data on cell voltages, temperatures, and other parameters, and integrate the BMS with your overall system.

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