DIY LiFePO₄ Battery Pack - 4S 3P - 12.8V 18Ah (Low Cost)

Recently, I wanted a powerful battery pack in order to power my devices and other electronic stuff outdoors. Using a lead acid battery would be the cheapest solution, but it is very big and heavy to carry around. Lithium polymer batteries are well known to catch fire, explode and whatnot. Lithium ion batteries however offer the best energy density, that is they pack a lot more energy while also being small and light. Plus, their chances of catching fire, explosion, etc are way lower compared to Lithium polymer batteries. The only problem was that the cells I am comfortable working with, which are 18650 cells, are more prone to scams. Like the ones here . Now, I could buy the cells from trusted brands like Samsung, LG, Sony, etc. But they are too expensive in my taste. A single 3.7V 3Ah cell from Samsung costs 9$. For me it would take 18 cells or about 162$ for a 11.1V or about 216$ for a 14.8V pack. This yields us about 0.82Wh/$ which is very bad. Plus, neither 11.1V nor 14.8V are close to 12V. Hence I used Lithium Iron Phosphate batteries. These cells are the safest among lithium batteries. They also, cost a lot less. Plus, the voltage is suitable for 12V devices. Hence, I went with a 4S, 3P configuration of 32700 cells each with 6Ah of battery capacity. Totalling to 12.8V and 18Ah or 230.4Wh, which is definitely enough for me.

In order to build this battery pack, I used the following parts:

1. 12 x 32700 3.2V LFP cells 6Ah. (Though they are differently coloured, they are the exact same cells. Hence I used them, if you are making a similar battery pack, I strongly recommend you to also do the same. If you have no other option than to use dissimilar cells in parallel, keep an eye on each individual cell as the cells with the least capacity wears a bit faster than the one with more capacity. In my build, even though they are the same type of cells, their manufacturing date still varies. Hence, I connected all the similar cells in series and the non similar cells in parallel. Keep in mind that I had already tested this setup for more than a month. I had used this setup only because there were no issues. If there is an issue, stop using/making the battery pack immeadietly.)

1. 2.5sq.mm wire (This wire is the one I used to connect all the batteries in series.)

1. 1.0sq.mm wire (This wire is used to connect all the cells in parallel)

1. XT60 connector (This is the connector that I used for the input / output of the battery pack)

1. LFP battery BMS (This circuit is used to protect the cells from overvoltage, undervoltage and overcurrent)

Before doing this step, I measured each cell's voltage to make sure they were same. (I had to charge some up). This is highly recommended to prevent an unhealthy amount of current from going from one cell to another when connected in parallel. There are also some issues when connecting batteries with different voltages in series. Due to the dissimilar voltage, the one with the lowest voltage would be damaged by a reverse voltage by the other three cells during a discharge cycle. (For example, in a 4S pack, if one cell is at 3V and the other cells are at 3.2V, 3.3V and 3.4V, the one that has 3V would receive -9.9V [during short circuits, but you get the idea.]. This kills the battery.)

In order to properly solder wires to the batteries, I first sanded the positive and negative terminals of all the batteries. This is important as sanding roughens the surface, removing the oxidised layer on top, allowing us to solder the wires firmly. I set my soldering iron temperature to 370°C. This is because as these terminals are huge and my iron's tip is small, the temperature of the tip drops quickly upon contact. It is worth noting to do the soldering job as briefly as possible as these cells could be damaged on overheating the cells too much.

I soldered the 2.5sq.mm wires on top of the cells and began connecting them in parallel like shown in the diagram.

I then glued the cells together at a low temperature on my hot glue gun.

Once I made the pack, I repeated this procedure two more times with the other eight cells.

Once I got all the three packs ready, it was time to connect them in parallel. I once again checked their voltages to make sure they were good before I used some hot glue to glue all of them together while ensuring all the terminals are on the correct side (like in the 2nd image). I then taped them together. Now, it felt a lot more secure and stiff.

I then grabbed 1sq.mm wires (colour coded by the way) and began connecting each and every cell of each pack in parallel. (For example, 1st cell of all the packs are connected together). This process took more than two hours as it was a lot of terminals to connect. I also had to make sure that I did not short any terminals during this process. I also soldered two 2.5sq.mm wires for the main positive and negative terminals of the battery pack. Once I was sure that everything was fine. I double checked the connections with my continuity test feature in my multimeter. As everything was connected properly, I did some wiggle test on all of the connections just to make sure they don't cause any problems.

Once that was done, I first hot glued the battery BMS board on the cells before I soldered the two main wires of the battery pack to the B+ and B- of the BMS board. I then continued by connecting 1sq.mm wires between B1 and 3.2V, B2 and 6.4V and B3 and 9.6V. After doing that, I measured the voltage of all the four cells.

After that, I soldered two 2.5sq.mm wires to the XT60 connector and used heat shrink tube to insulate the exposed terminals. Then, I soldered the wires to the battery pack, and we are done!

Now, I have a 12.8V, 18Ah battery pack with a maximum current of 54A peak and 40A continuous. This gives us 230.4Wh of energy, which is great for outdoor uses. For example, a portable power station.

The total cost of this came to around 65$. This yields us around 3.54Wh/$, which is really worth it for the price.

I tested this battery pack with various different loads where it held up pretty well. I used this pack to power my oven which I had modified it to work with 12V. It drew 10A from the battery and lasted for around 2 hours. If we do the math, we can see that the capacity is slightly above the ratings. I was even able to go above 18A, which is 1C for the battery. During this time, the battery voltage dropped from 13.04 to 12.82. This was partly due to the resistance of the cables, transistors of the BMS and the internal resistance of the battery. Though I did not have a 54A load to test it with, I am sure it can supply 54A (peak), which is around 690W of power. Charging it was also not a big deal. I usually charge it with 1A of constant current with 14.4V set in my lab bench power supply. Even though its too low, it puts the least strain on the batteries. When I need to charge the batteries fast, I set the constant current to around 9~10A.

There are quite a few additions that I could do to this pack:

The first and the most important one is a case. Which is not really necessary currently as I am planning to build a more powerful portable power station than my last one, which has a dedicated battery compartment that ensures the safety of the battery. However, if you want to use the battery pack by itself, a case is strongly recommended as it can prevent the batteries from being short circuited, and the surroundings from catching fire.

The second is to add a 5 pin balance connector to balance charge the cells when needed. As my BMS has battery balancing built in, I don't really need one, but I am adding one soon to it. This is because, though the BMS can balance the cells, the time it takes is very high. (In my case, the resistors are 100Ω, so around 32mA or more than 16 full hours to discharge 100mV.) This is where battery balance and chargers come into play. They have a much higher current rating for discharging / charging (usually 2A max) reducing the balancing time significantly.

If you have any additions that you would add to your battery pack what would it be? Comment them.

Anyways, that's another interesting Instructable of mine, stay tuned for my next one.