Your cells must be configured in the appropriate configuration. For example, a 4s BMS must be connected up to a battery pack configured in a 4s (12 volt) configuration. This is accomplished by connecting 4 cells (or blocks of cells) in series to form 12 volts. For additional help connecting cells in a 4s configuration, refer to an article covering connecting batteries in series.
Mount the BMS near the batteries, while being mindful that the balance harness for the BMS must be able to reach the positive terminal of each cell (and negative terminal of cell #1). If you can avoid mounting the BMS to the batteries, it is recommended to prevent the BMS from heating up the batteries, however, you can mount the BMS to the cells if no other options exist.
Shorten or lengthen the balance leads so that they can reach the cells. Avoid having large rats-nests of wires as this can pose a danger to your system, as these wires are directly connected to the cells. The balance wires do not need to be equal length.
Optimally, the balance wires should connect to the bus bars interconnecting the cells, while not being a part of the mechanical joint between the battery screw or stud connection to the bus bar. If you can drill and tap a screw hole for a #6-32 screw in the bus bar, and utilizing a small ring terminal, you will have a far superior connection as compared to just putting an oversized ring terminal under the fastener attaching the balance lead to the battery.
Ring terminals that fit 24awg wire are tough to find, especially with a M6 or M8 hole to fit onto battery terminals. We recommend if you absolutely must use an oversized ring terminal, strip back extra length of the wire and fold it in half. We also recommend that you utilize a high quality heat-shrink style connector that can provide additional strength to the connection.
When connecting the balance harness, take careful note of the order of the pin-header. The black wire at the far end of the balance harness header is Cell #1 negative. This wire should connect to the same terminal as the B- cable is connected. The wire adjacent to the black wire is the Cell #1 positive terminal. Continuing down the harness, the next pin is cell #2 positive, and so on, continuing until all of the wires have been connected. If you have a 4s BMS, you will have 5 wires on this header to connect, 8s will have 9 wires and 16s will have 17 wires.
Connect the B- connection of the BMS to Cell 1 negative terminal. Do not connect any loads directly to the battery cells, they must be connected to the P- (power negative) terminal of the BMS. Any loads connected directly to the battery will bypass the protection of the BMS and can lead to damaged cells.
Using a digital multimeter, measure the voltage between adjacent pins. You should have around 3-3.5 volts. With the black probe of your multimeter measuring on pin#1 (black wire) of the balance harness, measure with the red wire on pin#2. You should have the previously mentioned 3~3.5 volts. If for any reason you have a negative number, you have the polarity of the balance harness mixed up, which could fry the BMS if left alone. Repeat this process between pin#2 and pin#3, you should have a very similar reading, again making sure nothing goes negative.
If for any reason you connect up the multimeter and do not read a voltage, go back and check your wiring. If you have a break in the wire, the BMS will not be able to function.
Once you have verified the balance harness voltages, you may plug it into the appropriate port on the end of the BMS.
After all connections have been made, plug the small black probe included with your BMS into the “NTC” port. If you do not connect this probe, the BMS will not be able to activate.
All connections to this point should be ready, you may now proceed with activating the BMS.
The easiest way to activate the BMS is with a light board or activation button depending on the model of BMS you have. The Light board is compatible with BMSs 5s and larger, while most of the 3s and 4s models only have an activate button. Simply push the button, then the unit is ready for use. BMS units bought from CurrentConnected include the light board/activation button as a standard accessory.
If you need additional help activating the BMS, check out this article specifically covering the various different methods that you can use to activate the BMS, as well as troubleshooting help if you have a difficult time activating the BMS.
After you have activated the BMS, we recommend fully charging the batteries while monitoring each cell closely to ensure that the BMS properly shuts down at the appropriate voltage setpoints. While this is not required, it is a great idea to have confidence that a safety system, protecting your battery investment, is properly functioning.