DIY 3S Lithium Ion Battery Pack

Recently, I built an awesome RC car that requires about 30W on average to run, turn, light up, and perform other functions.

Initially, I used a lithium-polymer battery with a protection accessory, but the need for extra care despite the protection, made it less practical.

I wanted a setup similar to a real car’s fuel tank, where refueling is straightforward, rather than removing the battery for balance charging.

A safer alternative is a lithium iron phosphate (LiFePO4) battery, which would require four 32700 cells in my case.

Another option is the 26650 lithium-ion cell, which has a 5Ah capacity and requires only three cells to replace a lithium-polymer battery.

Since a 4-cell LiFePO4 pack (12.8V) is farther from 11.1V than a 3-cell lithium-ion pack (11.1V), I chose the 26650 option. It also saved me cost as I only had to use three cells instead of four.

In this Instructable, I will guide you step by step to create one yourself. So, without further delay, let’s get started!

1. 26650 Lithium Ion batteries - 4 No. : I used 26650 lithium-ion cells with a voltage of 3.6V (or 3.7V, as both are cross-compatible) and a capacity of 5Ah. I tested these cells beforehand, and they met their rated capacity. Three pieces costed me about 14$.

1. 3S Lithium Ion battery protection circuit - 1 No. : Since we are using lithium-ion batteries, a protection circuit, also known as a BMS (Battery Management System), is mandatory. It is recommended to use a BMS with a cell balancing feature. This feature discharges cells with higher voltage to match the lowest voltage cell using high-value resistors. Without balancing, battery mismatch could lead to reduced lifespan—one cell may charge to 100% while another barely reaches 50%, effectively cutting battery life in half. Balanced cells charge and discharge evenly, preventing degradation and preserving battery life. It costed me around 3$.

How to find one?

You can recognize a BMS with a cell balancing feature by inspecting the board. In the image (the third one), the blue board only has battery monitoring resistors, a battery monitoring IC, transistors, and a few passive components. However, on the green board, apart from the four large output transistors, there are three sets of transistors and three sets of resistors. These resistors are connected to the batteries via transistors, which discharge them when necessary. Generally, BMS units with higher current ratings and prices include cell balancing.

1. Female XT60 connector - 1 No. : The XT60 connector allows easy swapping of the lithium-polymer battery with the new lithium-ion battery. It is also highly versatile, as most RC batteries use XT60 connectors. It costed me 1$.

1. 3S (4 pin) battery balance (JST) connector - 1 No. : Though our BMS board offers a cell balancing feature, it is still wise to balance charge the cells externally once in a while. For this, we remove the battery cover in the RC car and take out the battery, similar to changing a car battery. It costed me 0.5$.

1. Wires - Along with these components, we would also need some wires. Namely 1 square millimetre wires of black and red and some standard ribbon wires used to connect the batteries to the BMS board. The 1 square millimetre wires could easily handle 18 A of current, which was definitely enough for me. The ribbon wires, were rated for a current of 1 A. Which, was enough for me as they were only used to monitor the voltage and discharge the cells when needed. This would have less than a milliamp of current flowing through the wires. They cost about 1$

1. PLA filament - 50 g : For making the case for this battery pack, I used PLA pro plus filament for better durability and strength. It costed me 1$

All of these total to 20.5$, yielding us about 2.6 Wh/$.

*Warning: This method of connecting the batteries is performed by someone with around 10 years of experience in electronics. As a layman, you solder the batteries entirely at your own risk. An easier alternative would be to use a spot welder. Since I don’t currently own one, I went with this method without any issues, but the same can't necessarily be said for you.

I first started by gluing the cells in an alternating manner using my hot glue gun at a low temperature.

It’s important to use a low temperature, as hot glue guns can reach up to 250°C, which is too much for the cells to handle.

Now, you might wonder, “A soldering iron tip heats up to 400°C. Wouldn’t that also harm the batteries?” The key difference is in the duration and amount of heat applied. When soldering, we briefly hold the iron tip on the terminal, applying fresh solder until it sticks, typically within 6 seconds. In contrast, hot glue flows at 250°C and takes longer to solidify. Solder cools faster than hot glue because we use only a small blob, whereas glue is applied in much larger amounts.

In order to solder the batteries, I firstly sanded the terminals of the batteries in order to roughen remove the oxide coating on them.

I then prepared my soldering iron to exactly 375°C.

This temperature is the perfect balance between usability and safety as the heat is easily transfereed without causing harm to the battery cells.

I connected all the 3 cells in series with 1 square millimetre wires.

My advice is to take your time and do this job patiently to prevent damage to the battery cells.

After successfully soldering the cells in series, I glued on the BMS board on top of the batteries.

To wire up the BMS to the batteries, I first soldered the main positive and negative terminals using 1 square millimetre wires, ensuring they can handle up to 15 A.

These wires will later serve as the main power connections from the battery pack to the circuit.

For the 4.2V and 8.4V battery tabs, I used ribbon wires, which are thinner but sufficient for this task.

It is crucial to connect these correctly, as incorrect wiring can lead to malfunctions or even permanent damage to the BMS.

Before soldering, I carefully checked the polarity and verified all connections to prevent short circuits.

I then later double checked the 4.2 V, 8.4 V and 12.6 V levels with my multimeter.

We can't have a 54 Wh energy source exposed to the air right? So, I had designed a case for the battery pack.

It measures 82 mm x 34 mm x 78 mm, making it extremely portable. It also features cutouts for my wires and balance connectors.

I firstly soldered the output terminals to the XT60 connectors and used shrinking tube to insulate the exposed contacts.

I then got the output wires and balance wires through the front panel of the case and then soldered them to the BMS board's corresponding terminals.

The wiring is done according to the scheme shown in the image above.

After soldering the wires, I added two small foam pieces in order to insulate the terminals just in case. I then tried closing it with the top cover but it did not close. The thickness of the foam piece was apparently too much.

So, after a bit of hammering the sheet with a hammer, it became thinner. So, I placed the foam piece and then closed it firmly.

The end result really satisfied me. It was so portable that I could easily fit in my hand.

The volumetric energy density of this battery pack came to around 248 Wh/L, which was noticeably low compared to other lithium ion batteries and a gravimetric energy density of 150 Wh/kg. Though low, it was pretty high than standard lead acid batteries with a maximum of 100 Wh/L or around 70 Wh/kg.

Now, blindly marking it 3C, 15 A on the battery pack and calling it a day is not my option. So, I used my induction heater which I had modified into a resistive heater along with my variable lab bench power supply to test the battery pack.

I tested it at various different current ratings and got this graph (given above).

The only problem I had was that no matter what I tried, I never reached 15 A.

I was only able to go up to 14.7 A maximum when the voltage slowly falls below 10.6 V.

I know, my bench power supply is to blame with its minimum input voltage of 11 V but the voltage dropping from 12 V to 10.5 V at 15 A load is not acceptable for me.

So, I can claim a maximum current of 10 A where the battery runs fine.

I don't really know if it is normal for these lithium ion batteries to drop voltage like this. Maybe it is caused due to the internal resistance of the batteries? or the internal resistance of the transistors in the BMS? The voltage drop on the cables? I don't really know. Please let me know in the comments.

With this, I had completed another successful build that upgrades my DIY RC car.

I am still satisfied with the results despite the current capability of the battery pack.

Still, 10 A at 11.1 V yields us about 111 W of power, which is really good for a battery pack this size.

Now, I can just install this in my RC car and add a DC jack for the fuel refuel port.

For charging, I can simply set 12.6 V in my lab bench power supply with 2 A constant current and the battery would get charged up within 2 hours.

In my RC car, it lasted me roughly 3 hours with light accelerations and drifts.

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