BMS in an eBike Battery

What is the BMS on an eBike Battery and What Does it Do?

The BMS system on an eBike is a critical component that should be understood. If you take care of your battery properly, you will already be taking care of your BMS system. If something catastrophic goes wrong with your battery, the BMS system is a potential culprit. Continue reading to learn more about the BMS system and how to manage it.

What is BMS?

The BMS, or battery management system, does as the name implies: manages the batteries in the battery pack used to power the eBike. EBike batteries are made up of multiple cells, the BMS monitors the state of charge and discharge of each individual cell to deliver the best performance and safety. The BMS has a low voltage and high voltage cut-off to preserve cell life and keep the “pack” safe.

What an eBike Battery BMS looks like
What an eBike Battery BMS looks like

A BMS is a small electrical component inside the housing of an eBike Battery. The BMS does several things: balances the power level of individual battery cells and provides battery safety by using high and low voltage cut-off.

BMS are specified by the number of cells they manage and the amperage rating.

A single battery cell is not enough to meet the requirements of an eBike motor. An eBike battery works by combining multiple battery cells together to form a larger battery. Stacking batteries end to end increases the voltage needed to meet the watt requirements. Connecting batteries side by side increases the power needed to increase the amp requirement.

Battery cells in an eBike battery
Battery cells in an eBike battery

Watt requirements are met with what is called “series” connections, while amp requirements are met with “parallel” connections. Enough series and parallel connections form a single battery comprised of dozens of cells able to power the eBike motor.

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Do I Need a BMS?

The purpose of the battery management system is to manage the individual performance of every battery cell. The BMS ensures that each battery cell performs, acts, and drains at the same rate throughout the battery. This creates a uniform battery cell performance that allows you to have full operation and consistent performance out of your eBike motor.

BMS also increases the life of your battery. A system without BMS could allow for individual battery cells to overcharge, making them explode or drain too much such that they cannot recharge. This would increase the overall battery deterioration rate. A faulty BMS is one of the most common causes of failed batteries on eBikes, so it is important to have a BMS that works properly.

How Do I Take Care of My BMS?

There are a few tips to keep your BMS system running properly and smoothly. First, do not use a battery charger that is not rated for your battery. The BMS system on an eBike is in place to handle the charging of each battery cell.

Too much voltage running through a charger can cause the BMS system to overload, removing the limiters built in to keep the batteries from overloading. Do some thorough research when shopping for a replacement charger.

Second, do not leave the battery in extreme conditions including wet, hot, or cold. Extreme temperatures, both cold and hot, can damage the individual battery cells and the BMS system. This can lead to performance issues and battery issues.

Maintaining an eBike Battery for the longest life
Maintaining an eBike Battery for the longest life

The BMS system is designed to not let the batteries completely drain. Even if the battery level is so low it requires a charge, there is still an electrical current running through the battery. Minimize the battery’s exposure to extreme conditions and water exposure whenever possible, and ensure it is never fully submerged in water.

Lastly, refresh your BMS system after periods of inactivity. When batteries are unused for long periods of time, the BMS may enter a negative charge state. This means the system reads that the batteries have more power than they do. Refresh the BMS system by leaving your battery on the charger for 12 to 24 hours (depending on the length of inactivity).

The same goes for installation of a new battery. Extended full charges when you add a new battery refreshes the BMS system and verifies that the battery is charging properly before being out and stranded on your eBike. (Source)

RIDE e MTB Pro Tip: With eBikes growing in popularity learning all the terms and definitions can be a bit overwhelming. Let me help with this comprehensive glossary. 👉 eBike Glossary

How Do I Know if My BMS Has Gone Bad?

If the charging system on your eBike has failed, it could be several things. It could be a supply failure where the charger is broken and the voltage necessary to charge is no longer being delivered. It could be a mechanical failure where a connector wire inside the battery is broken. It could even be a battery cell failure because eBike batteries consist of multiple battery cells. Lastly, it could be a BMS failure.

Testing each of these components is the only surefire way to determine the loss of charging power. Testing the charge is not difficult. Ensure that your charger is outputting at least 58 Volts for a 52 Volt battery, and 54 Volts for a 48 Volt battery.

RIDE eMTB Pro Tip: Check out this video on YouTube. Luna tests an eBike battery 👉 Testing a Bad BMS

Using a multimeter, you can probe the battery to read operating voltages. You can open the battery and visually inspect all wires and solder points. eBikes can get beat up and it is possible that a joint will fail, or a wire will come loose.

Testing the internals of the battery is the last step. After you visually inspect all wires and connection points, you can do a BMS battery reset. You can also test voltages on all the pins to ensure each battery cell is operating properly.

The most common causes of BMS failure are the components that get overloaded due to too much current, a solder point fails, or a connection breaks.

Bafang Mid-Drive Electric Bike Conversion
Bafang Mid-Drive Electric Bike Conversion

Did you Know You Can Convert a Regular MTB to Electric?

Bafang a leader in DIY electric bike conversions has a mid-drive kit that will fit on most regular bikes. I’d recommend the BBSHD kit, it’s a complete setup with a huge capacity battery and a 1000 watt motor. Why this setup?

  • Heavy duty 1000 watt motor (built for cargo bikes)
  • +50 volt 17.5 Ah battery for speed and distance
  • Easy installation with provided tools.

If you’re considering a conversion, do what I’m doing get the BANFANG BBSHD 1000w eBike Conversion with Battery

Can a BMS System Be Replaced?

Before replacing the BMS system, there are things you can try. Try jumping the battery. Different BMS systems and batteries have differing protocols for this, so consult the circuitry diagram that came with your battery or consult the store where you purchased the battery. You can also check the core of the battery and test individual cells to ensure they are functioning properly. These cells can be replaced if you can spot weld.

BMS systems can be purchased online. The only hang-up is ensuring you choose one that is rated for the size of your battery pack. Once you have done this you can remove the connector at the top (usually 10 to 14 pins plug-and-play type connectors), use a low power soldering iron to desolder and resolder the connectors at the bottom. Now use the soldering iron to connect the new wires. (Source)

Green Bike Kit has a great site with various BMS systems to choose from. Learn more here:

The battery on your eBike is one of the most expensive parts
The battery on your eBike is one of the most expensive parts

Finishing Up

The BMS system on your eBike battery is a vital component of keeping your eBike working properly. Ensure you maintain and take caution with this system so your eBike battery keeps functioning well for a long time.

David Humphries Author at Ride e MTB

Hi David Humphries here the guy doing all the pedaling behind the scenes with this blog. I’ve been in the MTB world for a while and recently started getting into eMTBs. You can check out more about me HERE and on my other passion project –