BMS’s, what the hell do they do?

January 6, 2016

The Battery Management System (BMS) on an ebikes battery pack is one of the least understood, and yet most important components on an ebike. Most new ebikers easily grasp that a quality battery will provide better performance and last much longer than a mystery pack made from counterfeit cells that don’t live up to their advertised performance claims, and also will wear out quickly.

But…quality costs more, and a high-quality battery pack is likely to be the most expensive single component in an ebike build. There will be an incredible amount of anger and frustration when someday you discover that your expensive battery has died and early death, and after an investigation…the most frequent cause of an early battery death is either a cheap charger that failed, or a BMS that screwed up. And between the two, the BMS’s are the culprit much more often than the charger.


Where is the BMS located?

A friend of ours in Spain named Damien Rene built up an ebike battery pack from spot-welded 18650 cells, and documented it. There had been many projects like this over the last couple of years, but Damien took some great pics of the entire process from beginning to end, which allowed us to write an article on building DIY custom battery packs.



The pic above is the main spot-welded block of series and parallel sub-packs, and the main power cables only need one fat red wire, and one fat black wire. The bundle of thin black and red wires shown are the balance wires, which allow the BMS to monitor and adjust each paralleled sub-pack. Here, the small balance wires have not been soldered onto each parallel sub-pack yet.



In this next pic (above), the green board with electronics on it is the BMS. Damien has just soldered the BMS onto the packs main power cables, and then plugged in the many thin balance wires. You can see here he also added a digital voltage meter, and an ON/OFF switch.



Here’s a slender “shark” pack with a hard shell, and filled with 52 of the 18650-format cells, and the BMS is shown mounted on top of the pink cells.


Series and Parallel

If we take a look at configuring two individual 18650-format cells, it will help new ebikers to understand why batteries are they way that they are. Each cell has two electrodes, a positive cathode end, and the other end is a negative anode. If we attach the positive of one cell to the negative of the other, that is a “series” configuration, and it doubles the voltage, but keeps the Amp-hours (Ah) of range and current-output capability the same.

If instead, you took those same two cells and attached them side-by-side, and then connected both cathodes together with a wire, and both anodes together with another wire…that would be a “parallel” configuration. Doing that results in a 2-cell sub-pack that has the same voltage as a single cell, but…the Ah of range and the current-output capability has been doubled.

The majority of lithium-based cells that you will find in ebike battery packs have a “nominal” or average voltage of 3.7V per cell (and per paralleled sub-packs of cells). They can be fully charged to a voltage of 4.2V per cell. They are commonly considered to be “empty” when they get down to 3.0V per cell.

[as a side note, there is a significant increase in the cycle life of a pack if it is large enough for you to be able to only use it between 3.3V as the empty setting, and 4.1V per cell as the full setting, between charges]

It is possible to group a bunch of your cells into series sub-packs first, and then connect the series sub-packs into parallel groups. However, almost all battery pack builders group them into parallel groups first, and then they series those parallel sub-packs. The reason is that…if you do it that way, it is electronically easier to treat each parallel sub-group as a single large 4.2V cell.

Just as an example, a common battery pack might be a 48V / 15-Ah pack. If you use a 3000-mAh cell, then 5 of them per parallel group (5P), would result in 15-Ah. Then, you would need 13 of those 5P sub-packs to make a pack that is 13 sub-packs “in serial” (13S). The resulting pack would be 13S / 5P

Since each parallel sub-pack electronically “performs like” a single large 4.2V cell, a BMS with 13 channels can monitor and adjust each of the 13 “5P” groups to keep them healthy.


Bulk Charging and Balance Charging

Each cell in your pack (and each of the 5P sub-packs) has a slightly different resistance to the electrical charging and output-current. If our theoretical 13S / 5P pack was fully charged to 4.2V per cell, the pack voltage would be (13S X 4.2V =) 54.6V

An inexpensive $30 13S bulk charger would charge the pack to 54.6V, then…you would drain it down on your rides to roughly 39.0V, which is where the Low Voltage Cutoff (LVC) in the controller will disconnect the battery. This is so that the battery doesn’t get drained down so low, that it is damaged.

The problem is this…with each cell (and each parallel sub-pack) having a slightly different internal resistance (measured in Ohms, or milli-Ohms). One sub-pack will reach to 4.25V, while another only reaches 4.15V or even 4.10V…

The same thing happens at the low end…when the pack reaches down to 39.0V, one sub-pack would be at 3.10V, while the next sub-pack is at 2.90V (the controller only reads the whole pack voltage, not the individual Paralleled sub-packs.

Over time, the sub-packs can get farther and farther out-of-balance. The bulk chargers job is to simply, reliably, and affordably…provide the 54.6V input that the pack needs to end up at.

Remember how I said the cells should never be allowed to go below 3.0V per sub-pack? and also they will live much longer if they are only charged up to 4.10V per sub-pack? After a few months of daily use, a battery pack without a BMS (or one with a malfunctioning BMS) can get VERY out-of-balance. The pack voltage may be the correct 54.6V, but…some sub-packs are too low, some too high, and that means that…there is extra heat when charging and also a short life-cycle.

If one sub-pack dies (or loses a significant amount of its capacity), most people are not going to dis-assemble the pack and swap-in a new paralleled sub-pack to repair it. The BMS might have malfunctioned, and it is the most likely culprit. With no BMS, the pack is guaranteed to go bad soon. And with a BMS, it is vital to have a reliable unit.


What about RC LiPo packs with no BMS?

The DIY garage-built LiPo packs (from Hobby King) almost never have a BMS. However, RC chargers are designed for a smaller pack, such as a 6S pack. These 6S “bricks” are common, and already have thin balance-charge wires connected to each 3.7V nominal cell. Instead of having a BMS, most RC packs have a balance charger.


In the pic above, the yellow plug with the two fat wires (red/black) carries the main current from the battery to the controller. The white plug with the 7 thin wires is the balance plug. The thin black wire is the negative for all six the cells, and the 6 multi-colored thin wires are the positive wires for each of the 6 flat foil-pack cells

The LVC of the ebikes’ controller will cut power on an RC pack at the same 3.0V per call as an 18650 pack with a BMS, which can can result in the RC pack being slightly out of balance at the end of the ride (one cell is 3.1V, another cell is 2.9V), but…when the RC pack is charged up, each cell is charged individually to the same voltage (an RC LiPo charger bulk charges through the two fat wires for 90% of the charge, then the thin wires are used at the end for the low-amp balance charging).

Most ebikers have voted with their dollars by deciding to buy bulk chargers, and buying battery packs that have a BMS, in order to keep the pack healthy and in balance.

If you were to decide that you wanted to balance charge your pack every time (just like the RC packs), there are two problems. First, the balance wires are very thin, and they are only designed for sensing the voltage of each sub-pack, plus their thin-ness means they could only charge at a very low amp-rate.




The second problem is that RC balance chargers are typically designed to charge 6 cells (6S), so if you have a 12S battery pack, you’d need TWO chargers. Our theoretical 13S pack would require THREE 6S chargers. These RC chargers are designed to be able to run off of a automobiles charging system in the field, so they typically use a 12V input, and that means when you are charging your ebike battery at home, you will also need a 120V AC “power supply” that converts house current to 12V DC.

It can be done, but…most ebikers just want a quality bulk charger paired with a quality BMS to keep their pack healthy. 90% of the battery charge can be done at a higher amp-rate to get the pack charged fairly fast, and the final “topping off” phase can be done at a slower rate while the low-amp topping charge is controlled by the BMS.


How a BMS Balances the Pack

If your charger shows a green light (indicating that its full), our theoretical 13S pack will read 54.6V when its hooked up, but if…as soon as you unplug it from the charger, the battery pack plug reads 53.0V on a voltage meter? This means that one or more of your sub-packs is not taking and holding the full charge.

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 happens 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 cells’ resistances, when the LVC 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”.


BMS’s limit current in and out

Not all BMS’s have the same features. Almost all of them limit the amount of current that can be used to charge the pack, so the pack is not over-heated by charging with too-high of a current.

Most of them also limit how many amps can be drawn out of the battery pack. Most controllers have a fixed max amp limit, and the exact amount of amps you are actually drawing is dependent on the throttle. Other controllers can have the max amps adjusted, and…if a customer adjusts the controller to allow very high amps to pass through to the motor, it can draw so many amps that it hurts the battery pack, by getting the cells very hot.




By having a max amp output limit, BMS’s try to protect your battery pack. The amount of amps that a battery pack can safely put out is dependent on the type of cell you use, and also by how many cells are in each paralleled sub-pack. For instance, if you use an authentic Samsung 25R cell, each cell is rated to be able to output 20A. If we go back to our theoretical 5P pack, a 5P pack made from 20A / 25R cells can safely put out 100A! Of course, that is a high-performance cell.

High-amp BMS’s are not common, and because they need to carry a higher amount of current, they are physically larger. It is possible to find a 50A BMS, such as the batteries from Luna Cycle.


Do BMS’s have other features?



Most BMS’s balance the cells in the pack, and also limit the amount of current that be used to charge the pack, and how much current can be pulled out of the pack for the controller to run the motor. But…not all BMS’s are the same, and some have upscale features, such as:

  1. Temperature monitoring the cells. Even if the pack is limited to 30A, most ebikers only use max amps when the light turns green, and even then for only a few seconds. If you ride up an extra long and steep hill, with no “cooling off”  cruise-phase, the battery pack could actually get pretty hot. A temp probe can warn the BMS that the pack needs to be amp-limited, or…maybe even shut down…until it cools off.
  2. Bluetooth connectivity. There are bluetooth antennas that can be added to a BMS that allows a smart-phone to wirelessly track whats going on in the battery.
  3. ON/OFF switch. If you have a handle-bar mounted ON/OFF switch, it likely controls that feature on the controller. Some enthusiasts like having an ON/OFF switch on the BMS, or on both.
  4. Individual parallel sub-pack LEDs. The cheapest BMS’s might only have a single LED to indicate its done charging. And one that’s slightly better might have a LED for each paralleled sub-pack. Some even have a second LED per sub-pack that indicates that each sub-pack in in the “drain” phase of the topping-off cycle.


Written by Ron/spinningmagnets, January 2016


Grew up in Los Angeles California, US Navy submarine mechanic from 1977-81/SanDiego. Hydraulic mechanic in the 1980's/Los Angeles. Heavy equipment operator in the 1990's/traveled to various locations. Dump truck driver in the 2000's/SW Utah. Currently a water plant operator since 2010/NW Kansas


  1. This is one of the best articles I’ve read so far and has cleared up so many of my beginner battery/BMS questions. You are an excellent writer. One follow up though, you talk about balancing cells and I read elsewhere about charging that one should not charge a battery to 100%, but rather up to 80% for longevity. Do 18650 Li-ion cells get out of balance like other cell types? When does the BMS balancing kick in, only if you charge to 100%? If so, then is the proper procedure then to charge to 80% most of the time and only charge to 100% for balancing say once per month?

    • Yeah even I heard that do get the most of the battery life we shouldn’t charge more than 80% and that will be like 3.36 Volts per cell.
      As far as what I have read cells do charge quickly upto 80% and after that they take a while for balancing and top off at the end

  2. Great article Ron once again.

    One thing you didn’t mention is that with the Hobby King chargers you MUST MAKE SURE that it is set to the correct battery type and configuration. You can blow up packs if you plug in a LifePo4 pack and accidently have the charger set to Lipo. Those cheap brick chargers tend to have really crappy and confusing user interfaces to boot.

  3. I don’t know…! But all the talk about BMS I think I need to use the restroom as I have a BM coming on …! Lmao…!

  4. useful and informative. Keep it up

  5. where can I buy a bms for a 36v 10ah battery? mine died. I have looked all over the internet.

    • ebay?

  6. Hi, thank you for the article ! I just have a question : What about using a RC Lipos and add a BMS ? What can be the problem doing that ?

  7. Nice article! Would it be possible to connect a bms to HK lipos and then charge it with a bulk charger, let’s say 16s 67.2v charger on a 16s lipo (4 x 4s lipos)?

  8. Thanks for the great article.
    I am looking for a bms for a 13s configuration with thermal management i.e. temperature monitoring. Can I buy such a pcb?

  9. From one Navy guy to another Great article SPINNINGMAGNETS!


  10. Great article. But I have a doubt. what if one or more sub packs are dead? SInce the bms balances each sub pack by bleeding it to the level of the lowest sub pack will it also kill the rest of the sub packs? Or does it have a mechanism to cut off that particular sub pack fro the rest of the battery while charging and balancing?

  11. Ron,
    Amazingly well written and informative . Is it reasonable to assume one could take a Luna triangle pack 20s/3p, 72v /- 10ah and reconfigure to a new shape and change to 14s/4p, 52v/ …Would the BMS be ok ? Adjustable to new voltage ?..I would probably need a spot welder to do would I be better to start from scratch ? Parts from whom ? Ebay ?

  12. Good article. I think in the part: “The white plug with the 7 thin wires is the balance plug. The thin black wire is the negative for all six the cells, and the 6 multi-colored thin wires are the positive wires for each of the 6 flat foil-pack cells.” it should be like “black is the negative of the first cell” because all cells are connected in series and thus black can only be connected to one cell.

  13. My pack got way out of balance and quit on the first long ride this spring. I opened it up and found voltages from 3.6 to 4.15 per bank of cells. I was able to charge each bank individually to 4.15+. Subsequently the fully charged battery voltage is dropping about 1/5 volt per charge. I assume it is getting out of balance again. Should the BMS be correcting for this? Is there a way to tell if the BMS is actually working?


  14. This was a great article with lots of amazing information. It was very useful. One thing that was slightly unclear to me was your preference to either run cells in series first or in parallel first when making your batteries. Except for the fact that you showed images of running series first and then combining those in parallel that would lead me to believe you’re in favor of that. But then the writing to me says otherwise. I believe there must be a reason why everyone runs there batteries in parallel first and then in series. I can think of a few reasons very quickly. You’re going to make a lot better connections when you’re running them parallel and then connecting those in series with multiple connection points to the next group that is run in parallel. Otherwise you end up with large groups connected with one small wire. That in my opinion would cause the battery to run a little bit hotter. And then the second thing that comes to mind is when you have cells run in parallel they are naturally balancing each other. If you run them in series and then connect them in parallel you lose that natural balancing effect. I would love to hear other people’s opinions on this one.

    • In my commercial battery every cell is connected both in parallel and series. It is a 10s5p 36volt battery. Each group of 5 parallel cells is connected in series to the next group with 5 connectors.

  15. Yes in response to John that is how I build batteries and I believe that is actually the more efficient way of doing it versus the way described in this article matching up with the pictures. But then after the pictures they seem to contradict running cells in series and then connecting those groups parallel. I think there’s major flaws with that scenario. And the reason why professionals don’t ever do it that way. By running your cells parallel and then in series you’re getting natural balancing effect on all cells that are run parallel. And you’re getting much better connectivity. I am interested to hear anyone’s opinion that may or may not agree. I am still somewhat new to building batteries(5years). What I can say for sure is of the hundreds of batteries that I have torn apart I have never seen one that was run in series and then parallel like shown in the pictures. Again I don’t think that would be advantageous. And unless I’m missing something the BMS wiring would be a nightmare because you would no longer be wiring one wire to one location you’d have to splice that wire and go to multiple locations. Am I missing something here?

    • Brandon, I think that ‘SpinningMagnets’ used the ‘S’ diagram to illustrate, to the novice, how batteries are connected in a series configuration, explaining S vs P. He tells us it can be done either way, but most do it your way.

      I agree with you that “P” groups should be wired in series, mainly because the other way round makes bus bars almost impossible to design. And I do like the idea of running fuse wires to the bus, like Tesla is doing now. He DOES give the ‘full disclosure” that he has never built a battery pack himself. Consider him a most excellent tech writer/researcher that works for you, for free!

      PS, at (now) 6 years experience you’re no longer “somewhat new” to battery building 🤗

      • Thanks for the vote of confidence, Thom. I do my best to provide the raw information for novices to commence their research…

  16. how do you know when balancing is fully finished after charging?

  17. I ran into an odd scenario recently. I have a 13S4P 48V pack that has been behaving well enough but voltage sagging a little. When I pulled it off the charger it would sag some and then hold it’s voltage. It behaves as if the BMS was not working and now it is working. Charger never did a balance charge, it would just get to the right voltage and stop. Now it has started doing balance charges. Battery seems to have grown in AH capacity and I arrive at my destination with battery to spare where previously I would arrive with half a battery left.

    Not unhappy naturally. Just confused why this is happening. No, I haven’t ever opened the battery. If it returns to its old behavior I might.

    Great article. As usual I learned alot!

    • Figured out the battery – flaky charger. Charger died today. Charger told out at 14 V now. Battery is still fine. Ordered new charger. Would like to repair the old charger.

  18. Hi, a problem someone may be able to help me with?

    I have been building an e-bike and I bought a Li-ion battery from a chinese dealer on aliexpress. I specified I wanted anderson connectors on the discharge cable as that is what I have on my controller. When I got the battery, I plugged black to black and red to red anderson connectors and then turned the battery on. I immediately heard a small ‘pop’, green smoke started seeping through the casing and the plastic on the bottom of the casing started melting. I quickly disconnected everything and got the hell out of the way. I came back a while later to inspect – the battery appears completely dead. I have the feeling the anderson connectors on the battery may have been installed wrong and I ended up connecting positive to negative and vice versa – would this have caused the battery to ‘blow’ like it did or is it more likely a battery fault? Is there any way to salvage this battery? And is there a chance I could have damaged by controller or motor?
    Thanks for any input!

  19. So if the BMS regulates the current in and out for charging and for load, you only need a cc/cv power supply to charge the batteries because the BMS keeps everything balanced… or am I reading too much into this… ???

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