E-bike: Battery Management System BMS

(Linked from Conversion to eBike)

Table of Contents

What is a BMS

If you have 48.1 V, then that is the sum of 13 cells in series of 3.7 V each. If a cell is charged, you do not want more than 4.2 V per cell, otherwise the cell will be damaged. The charger delivers 13 times 4.2 V, a total of 54.6 V.

If you put the 54.6 V charger directly on the ends of the cell block, then that goes well in principle. However, it is not the case that all cells end up at 4.2 V, some cells will be a little less and some cells will be a little more. The cells that are above 4.2 V are suffering at that time.

It is not wise, but in case of emergency you can recharge cells in this way. Keep measuring between cells and if you measure 4.0 V for example, simply stop charging. Do yourself a favour, charge outside and take care of personal protection, just in case…

With a BMS there are wires between all 3.7 V steps. If there is a higher voltage somewhere, the excess current is sent to a cell with a lower voltage via the wires. BMSs are available in many types, but this is the main task in a bicycle. In addition, the negative main current of the bicycle, from battery to controller, tends to run via the BMS, mainly to prevent the battery of low voltage damage.

What are the options

There are two common models, with charging wire and without. Additional, it seems possible to use the BMS for charging only. See for example here.

I have two bikes and one BMS malfunctioned before even placing the battery pack, resulting in a destroyed new battery pack of a few hundred dollars because the balance wires sucked out all current. The other BMS failed within a few hours. Those are good reasons to bypass BMSs as much as possible, only using them for charging. A normal engine controller unit will protect against an undercharged state, that seems not to be problem.

So it boils down to this:

If your BMS works fine, there is no need to make changes. Considering that Voltage balancing is only done during charging, the process of charging can be split in two sub processes: main current charging and balancing. So it is possible to do balancing outside the battery box by using, for example, a 14 pin GX-20 connector set. I haven’t gone so far… yet. It also means that extra checking of Voltages is easier, giving you insights of battery health at the same time.

One of the important points when building your e-bike is modularity. I made the mistake to integrate the BMS with the battery pack. Storing your pack for a longer time means switching the main current off and disconnecting the balance wires by pulling out the JST connector. 14 pin JST connectors aren’t made for regular connect and disconnect cycles. And considering that it is smart to disconnect even for short periods, say a few weeks, it is not a bad idea to go for a proper 14 pin connector, as an alternative to keeping your charger plugged in permanently.

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