ebike bms


Why you need a quality ebike BMS

ebike bms

One of the most important components onboard any ebike is the battery management system, or BMS for short, it does exactly what the name describes: it manages the batteries used to power your ebike. For the casual rider, you don’t really need to know anymore about BMS systems, just remember that if your ebike battery is having serious performance issues, it’s likely due to a faulty BMS. For the more dedicated ebike owners who want to understand every component of their ebike, here is our simplified BMS guide

How Does my Motor Get Power?

ebike bms

Your ebike has a motor powered by electricity rather than traditional gasoline or other common fuel sources. While the motor is compact, it still requires substantially more power than what you find within your average small household battery. Not only that, but your motor requires two electrical thresholds to be met for successful powering - both a voltage requirement and an amperage requirement.

A single battery cell on its own is not enough to meet your motor’s power requirements, but by combining multiple cells together we form a single, larger battery pack able to power your ebike motor for extended ranges. Connecting battery cells in series increases the voltage required to meet the voltage requirement, while connected batteries in parallel increases the amperage to satisfy your motor’s current requirement. The volt requirement is met with ‘series’ connections, while the amp requirement is met with ‘parallel’ connections. With enough series and parallel connections, you form up a single battery pack from dozens, sometimes hundreds of cells able to power high wattage electric motors such as the 1hp geared hub motor laced into the rear wheel of your Unconquered Custom.

This is how your motor receives the current necessary for its operation, from a single battery made up of many individual battery cells. However, individual battery cells within the pack do not drain, deteriorate, or perform at the same rate. Some cells will last longer than others, some cells will drain slower than others, and some cells sometimes glitch out and cause more problems than others. Without a BMS, and with so many individual cells acting independently you would quickly encounter multiple battery pack issues, since the individual cells would soon become unbalanced with one another. Soon your pack would be unable to power your motor at all.

You’re now ready to understand the necessity of a quality BMS integrated within your ebike battery pack

Why Does my ebike need a quality BMS?

ebikie bms

The battery management system exists to manage the individual performance of each battery cell. Instead of allowing for individual performance, the BMS ensures that each battery cell drains, performs, and acts the same as all other cells throughout the battery. By having uniform cell performance, you will have consistent performance and full operation of your motor that would otherwise be impossible without it. The BMS also extends the life of your battery as well. Without a BMS, the individual cells can overcharge, causing them to explode, or drain too far, making them impossible to recharge or greatly accelerating their overall deterioration. If you’ve had an ebike battery explore or fail out far sooner than promised by the manufacturer, the cause of failure is having a faulty BMS in your battery. In fact, a faulty BMS is one of the most common sources of failed performance in your ebike. Therefore, at Unconquered Customs we integrate only the highest quality ‘smart battery’ BMS’ into each of our packs. Each of our BMS’ include a circuit protection module to protect the battery from over current, over charging or over discharging.

How a BMS balances an ebike battery pack

The way that a BMS works is that it allows a simple bulk charge to pass through it to the battery until it reaches the programmed “full” charge. Then it stops the charge and takes a moment to sense what each of the sub-packs’ voltage is at. Most sub-packs will be at 4.20V, but one cell might be at 4.15V

Most BMS’s then drain the other sub-packs to the voltage of the lowest pack (in this example, 4.15V). and then the BMS allows the bulk charger to send another full charge to all the cells. This drain and charge happen several times until the BMS senses that all the sub-packs are all close enough in voltage to be considered “balanced”.

Due to the varied individual cell resistances, when the low voltage cutoff cuts power at the end of a ride cycle, each of the sub-packs will be at a slightly different voltage (which is natural), but…as long as they are not too far away from each other, the BMS can then manage getting them to a balanced charge state without it taking so many drain/charge cycles that…it seems like it takes forever for the pack to finish charging.

Out of all the different ways that a piece of electronics could fail, the sub-pack “drain” function (as part of the drain/charge cycles at the top-of-the-charge for balancing) can fail, and completely drain that sub-pack down to zero. If you have a 13S / 48V pack, and the packs highest voltage is 4.2V less (50.4V instead of 54.6V), you have a dead sub-pack that will no longer take any charge. If a BMS uses the “drain the high cells” method to get the pack balanced, that is called a “resistor bleed”.

How to take care of your ebike BMS?

So that it will perform manage your ebike battery for optimal performance? Here are a few key tips to make your your BMS, and ebike battery in general, continue to perform well and serve your transportation needs for years to come:

 Top Tips to Manage Your BMS System:

  • Don’t Use a Charger that Isn’t Rated for Your Battery: Your BMS system is in place to handle the overall charge each cell in your battery receives. At too high a voltage, the BMS system can be overloaded, removing limiters, and exposing your cells to overcharging as well. When using a new charger, make sure it is compatible with your battery before charging.
  • Don’t Leave Your Battery In Extremely Warm, Cold, or Wet Conditions: Extreme heat and cold will damage both your individual cells and your BMS system, leading to serious performance issues. While your battery is sealed, the BMS system is also designed to never let your battery fully drain, meaning that even when ‘empty’ it still has an electrical current within it. Try to minimize battery exposure to water as much as you can, and especially make sure it never becomes submerged in water.
  • Refresh the BMS System After Long Periods of Inactivity: When a battery goes unused for a long time, the BMS system has a habit of entering a negative charge state, where it thinks that the batteries have more power than they do. To help refresh the BMS system, you will want to leave your battery on a charger for 12 to 24 hours, depending on how long it’s been since you’ve last actively used the battery. You will also want to perform an extended full charge when you first receive a new ebike battery, as it will not only refresh the system but also verify that your batteries are able to recharge properly before you start riding your ebike.

 Taking care of your ebike battery is taking care of your BMS

Your typical BMS system is built right into the battery itself, which means that if you take care of your battery properly, you’re already taking care of your battery’s BMS. However, when something does go significantly wrong with your battery the BMS is usually the most likely culprit. BMS are very complicated systems though, and even if you can access them, they are best managed and repaired by certified technicians and engineers experienced with complex BMS systems. So, if you encounter a charging or range issue, and you can’t seem to figure out what the cause of the problem may be, use what you know and check with your bike technician to see if the BMS may be the issue!

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