How to wire a BMS and make a battery pack

Following a BMS wiring diagram can be quite confusing, so we are going to make a battery pack to see how it’s done.  Normally, you would need a sport welder to connect the 18650 Lithium-Ion cells between them. However, if you don’t have an improvised one, those can be quite expensive. No worries. I’ll show you how I did it without the welder. Not so good looking, but working decent.

The setup

First of all, you will need the components for the battery pack. In our case we are going to make a 7S battery pack. This means 7 cells wired in series. This will give us a combined voltage of 29.4V when they are fully charged. To extend the battery life I wired 2 cells in parallel. This doubles the current capabilities of the battery pack and the overall battery life. As a result, instead of having a 3400 mAh, we are now working with 6800 mAh. What do we need?

This is pretty much it for this pack. Of course, you will need some wire and a soldering iron. But that is to be expected. I want to mention something. When you pick the BMS, if you need 7 cells in series. You need precisely a 7s BMS. You can’t get a 10S BMS and wire only 7 cells to it. It won’t work. I will swiftly contradict myself, because a I have seen 14S universal BMS’s that can accommodate any number of cells as long as it’s higher than 3 and lower or equal to 14. But make sure it says so in the description of the BMS.

If you only need a 12V battery pack, you could save yourself the hassle, and buy a plug and play pack from Dayton Audio. You can find more in this article.

Battery pack without spot welder

So I presume you don’t have access to a spot welder. In this case we will use the battery holders and be a bit creative. Each holder will wire the 2 cells in parallel. This means we disregard the polarity indicated by the holder and place the cells both in the same way.

18650 cells in parallel

Furthermore, we snip the 2 wires and solder them together. Effectively we have wired the 2 cells in parallel.

18650 holder in parallel

Now, all we have to do is wire them in series and connect them to the BMS. If you look at the BMS wiring diagram, you will see that each battery (in our case 2 parallel ones) is connected to the BMS via the positive terminal. As a result, we want to place the cells in a position which favors us.

BMS wiring diagram

If you take a look at the holder, you will see that it has 2 rivets exposed on one side and only one on the other. Place the positive terminals of the 2 cells towards the part with the 2 rivets. One will connect to the BMS and the other will be used to connect in series to the next pack. To where? You guess it. To the single rivet on the other side, which will be the negative terminal.

Battery holder wired to BMS

It’s like these holders were made for this. When you doing the soldering, make sure you are hasty and don’t keep the iron too long, as you will burn the plastic in the near proximity. By doing this the springs might come loose, and simply it doesn’t look clean.

Connections to the BMS

At the previous step, we basically wired all the positive leads to the BMS. Next, we have to wire them in series. Use 6 pieces of wire to connect the packs in series. Anyway, by looking at the BMS wiring diagram we can clearly see that we are not done yet.

  • The negative of the battery pack goes to the B-
  • The positive of the battery pack goes to the positive of the charger, and to whatever we want to power (in out case an amplifier for our portable speaker)
  • Solder a wire to P-. This will be the negative terminal. You will wire the negative side of the charger and of the equipment you want to power in this spot.
  • Some BMS’s have C- point. You will wire the charger there. However, in our case P- stands for C- as well.

Battery pack without spot welder

Basic rules when doing all of this :

  • Do not do any soldering with the batteries connected. You will unavoidably have to do so at some point, but leave those for last. It reduces the risk of doing short circuits by mistake.
  • Don’t play with the batteries when the connections are made to the BMS. It’s safer to unplug it, and after everything is wired up, you can plug the connector into the BMS.
  • Make sure the cells have fairly close voltage values.
  • Make sure you wired all the cells in parallel correctly. You can verify that by measuring the voltage of each of the 2 cells. It should match or be very close to one another. When you wire 2 cells in parallel, if they have different voltages, the higher one will discharge into the lower one, until the voltages are equal.

Double connection

Even if the connection is good, making a double connection will be beneficial. Those skinny wire increase my anxiety levels. Though they are quite short, for my piece of mind.

Conclusion

Well, first of all the BMS provides protections for the cells. Some provide balancing between the cells also. People look at batteries and they want to know the percentage of how they are charged up. Simply put, that is translated into voltages. The spec sheet of this 18650 cell, indicates a voltage of 3.7 Volts. But that is the nominal voltage. When they are fully charged they have 4.2 V. Also, when they are dead they have 2.75 V.

Having said that, when you are picking a charger for your BMS, you need a charger that has 7 x 4.2V = 29.4 V. Be careful when picking the charger. Some BMS won’t even start, and enter directly into protection if the input voltage is higher than (in our case) 29.4 V.

Battery pack 18650

You can wire a charger directly to the cells. It will charge them. But you might run into problems if they overcharge. They can get hot an literally catch fire. Li-Ion cells are no joke. Check the BMS wiring diagram and make sure you wired everything correctly.

Please note how the BMS works. If you have one battery at (let’s say) 4.0 V and the rest at 3.6 V, it will stop charging all of them when the first battery reaches 4.2 V. So, you have to discharge them at the same level and then start charging the up together. Use a resistor 10W-20W 10-20 Ohm to deplete the cells to equal levels. The lower the Ohms, the higher the watts need to be, the faster it will drain.  You can also connect a speaker to the battery. The cone will move into one direction and stay there. It will drain the battery over time.


References

  1. Image source : link.