BMS

BMS stands for Battery Management System. It’s a small circuit card that manages the charge and discharge of your battery’s cells, keeping them in good health and hopefully making them last longer.

It also means you can charge batteries together without the hassle of a balance charger.

Key considerations when choosing a BMS:

  1. The voltage and current the BMS can take
  2. The number of cells it will manage

1. Current and Voltage Handling

Do you want your BMS to balance both the charging and discharging of your batteries?

If yes, your BMS needs to be able to handle the high current draw the motor takes from the batteries. It’s much higher than when charging; charging tends to be less than 5A but discharging can draw up to 80A. This will care for your batteries best and make them last the longest, but BMS’s that have such high current protection can be expensive.

You can bypass discharge and use the BMS for charging only.

This way your BMS doesn’t need such high current protection as charging is done with a smaller current. If you wire your BMS so it is only in use when charging your e-board, and gets bypassed when the batteries are being discharged, you can get away with this.

2. No. of Cells

You need to make sure the BMS can handle the same number of cells as you have in your electric skateboard. If your e-board has two 5S battery packs and you buy a 6S BMS, you will be missing out 4 cells for balancing. Wherever you buy a BMS from should state something like 6S, 10S or 12S for example, which should be the same as the number of cells in your board.

Note: you may want to get a BMS with balance leads and a connector so you can connect the balance cables.

Understanding the wiring

Please look at the diagram your BMS comes with, either in the box or from the place you bought it from. Not all BMS’s are the same.

Most BMS’s have three ‘solder spots’ which should be labeled CH- (or C-), B- and P-, and a balance cable connector. Some BMS’s may differ to this.

The BMS won’t connect to any positive wires.

Let’s start with the solder spots.

Each of the three solder spots needs to be wired into different parts of the circuit, that’s why they’re labeled differently.

  • B- gets wired to the negative end of your battery pack (positive end of the battery will go to the positive end of the components (VESC) you are powering).
  • C- gets wired into the negative end of your charging port (positive end of the charging port will connect directly to the positive end of the battery).
  • P- gets wired to the negative end of the components (VESC) you are powering.
BMS Wiring

Next is the balance cables.

This is where you need to look at the given diagram for your BMS. You need to make sure you get the balance leads in the correct order.

  • For a single battery, you should have one more balance cable than the number of cells e.g. 4 balance leads for a 3S battery.
Single Battery Wiring Diagram
  • For two or more batteries in series, you will get extra leads.
Multiple Battery Wiring Diagram