What is electric bike 18650 BMS? What do they do

You hear a lot about BMS and it is kind of mysterious part in your electric bike. You know it must be something very important as everyone talks about it but you never see it. It is like gluten, even you don't fully understand what it is, you just know it is something to pay attention to. 

Let's explain what is Gluten ? lol 

BMS is short of Battery Management System. As you may guess they manage and control the battery pack in a way that all cells inside your battery work smoothly and in coordination. 

Generally, if you have a dead electric bike battery, most common cause of it is using a cheap BMS. We hear much lower end electric bikes and conversion kits face it. 

Before we dig deeper let me explain certain things about your battery. 

Your battery consists of several battery cells which are produced by companies like Samsung, Panasonic, LG or similar manufacturers. So when an electric bike company says they use Samsung Battery actually they mean they use Samsung Cells in your battery pack. 

So your battery pack is made from parallel and serial connection of those cells. Each of these cells has 3.6V nominal voltage. Recommended Max voltage per cell is 4.2 V and the lowest is 3.0V. If your battery cell charge goes lower than 3.0 V it's internal chemistry changes and won't be able to keep charge like before. We call it lazy battery. If it goes very low it is called a dead battery and you won't be able to charge at all.  

Even it is possible to charge your cells to 4.2 V each, it is not recommended as fully charged battery cells create stress inside the cell chemistry. The sweet spot for full charge battery cell is 4.05V. In this way you can use your battery for longer durations. 

To summarize we need to keep each of our cells within 3.0V - 4.05 V range. 

Ebike BMS's main role is arranging each cell to work within these safe ranges. An electric bike battery will have around 52 or 65 cells inside like below one. 

If one of those cells goes above 4.05 V or below 3.0V it shuts off the battery for protection. Because if you have one deadcell or lazy cell it affects the whole battery pack. Ebike batteries won't tolerate any weak links.  

What are other functions of E bike Battery BMS?

Besides protecting your cells being overcharged and over-discharged, A 18650 bms will also try to keep all your cells voltage to be at the same value. This is called cell balancing. At each charge, a good quality BMS like we use at Ariel Rider will balance your cells. 

If your BMS don't have cell balancing feature and one of your cells higher voltage than others it will reach 4.05 V earlier than other cells. In that case, BMS will stop charging to prevent overcharging. AS you can see your battery isn't fully charged and you will start to see a decrease in your range. In the long run that can cause an up to 50% drop in your range just because your BMS don't work well.  

Also, BMS will check these parameters too; 

• Temperature
• Short Circuit
• Over Current: Won't let your batteries to be charged at un-safe current.
• Discharge Over Current: Won't let your motor suck juice at high rates which may damage inside chemistry of the battery.
• Arrange communication between the controller of your electric bike and battery. It is done in two common ways; UART communication protocol and CANBUS communication protocol. You can learn more about them here

For the communication part, BMS always send SOH (State of Health) and SOP (State of Power) information to your e-bike controller too. In this way, it can know how much capacity is left in your battery pack and how long you can use it based on your current power consumption. 

Where is BMS? 

Bms is located inside your battery pack. 

WHY BMS LIMIT CURRENT DISCHARGE?

This may sound little weird as your controller has a certain max limit of current that can suck from the battery. But in certain cases, some users bypass it and they manage to suck more juice or current from the battery. If it gets too many amps from the battery it can damage inside chemistry of your batteries and can cause serious problems. 

We at Ariel Rider take battery subject very seriously and get every protection possible so you can ride your e-bike safely. 

About the batteries, you can read more about What is Cell Balancing Article