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BMS = Battery (Blowup) Management System

Writer's picture: DaveDave

If you've had any experience with ebikes, you may have heard the term BMS. This acronym stands for Battery Management System and the job of the BMS is ultimately to keep your battery pack from exploding. Hi power lithium battery packs, like those found in most ebikes, are made up of groups of lithium polymer battery cells. The typical size of these cells is either the 18650 (18mm x 65mm) or 21700 (21mm x 70mm). The packs are described in terms of "16s 8p", which indicates that the pack has 16 cells in series and 8 cells in parallel. As each fully charged 21700 cell is 4.2v, we can quickly see that 4.2v x 16p = 67.2v, which is the fully charged voltage of a 60v pack. Similarly, we can get to the Ah capacity by multiplying 4800mAh x 8p = 38.4Ah (the 4800mAh is the charge capacity of a single 21700 cell in this example).


Now, back to the BMS. For each series group of cells in the pack, the BMS has an electrical connection to that group. During and after charging, the BMS monitors each group to make sure that one group is not drastically different than the others. If this was the case, the BMS can bleed off a bit of current from the high group while simultaneously charging the lower groups. In this way, the BMS maintains the "balance" of all of the cell groups in the pack. This also prevents the cells from overheating and possible explosion as the charger just keeps pumping current into the cell groups and doesn't know that one group may not be able to take the same charge as the more healthy cell groups. While balancing is the primary function of the BMS, another important function is monitoring the pack temperature as well as the amount of current flowing through the BMS. In the event of overheating, the BMS can shut down the flow of current from the pack to prevent explosion. Without the BMS monitoring (and limiting) the discharge current, the pack would deliver as much current as each individual cell could produce, which may exceed the safe duty cycle of the cell. This is why "bypassing" the BMS is not a good idea as there simply is no limit other than what the cell will deliver, which in some cases can also lead to an explosion.


We had a Talaria MX3 battery come in the other day with a toasted BMS. Apparently, a screw that holds the output connector in place was missing and the bike was hosed off. The water went through the missing screw hole and PSSTT went the BMS! We replaced the BMS and got the customer back on the trail!








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