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


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