First assembly of a 36-volt electric bike battery. What to do and what not to do

This year, for the first time in my life, I assembled an electric bike battery myself. I’m sharing my experience, diagrams, and tips with you.

It all started with the understanding that the batteries, which are about 3-4-5 years old (and some of them I bought already used). The time has come to replace them. Previously, I did it according to the scheme sold here – I bought the same ones on Ebay. But firstly, this scheme did not want to work, and secondly, I found a place where you can order high-quality batteries. So between a battery with a bunch of used Chinese cells and a completely fresh battery with understandable cells, I chose the second.

ATTENTION! The article shows the complete experience of assembling a battery. Therefore, the information in it will be presented in full, that is, all the correct and incorrect steps will be demonstrated so that it is clear what errors you may encounter in assembling a battery.

Elements

The most important part of the battery is the cells from which it is assembled. Just as previously, memory was bought for computers, which came out in one series from the assembly line, so with battery cells – you need to take those that will be closest in parameters. It is quite difficult to do this, because each seller is on his own mind. But before that, I had already ordered cells from the Dutch store Nkon several times . I stopped at the  EVE INR21700-50E 5000mAh 15A cells . As can be seen from the name, they have a capacity of 5A at a constant current of 15A. These are quite good indicators for such cells. Moreover, assembling a battery from them will allow, by increasing the weight of the battery by about 1.5-2 kg, to get a 2.5 times larger capacity, namely 25A at a voltage of 36V.

The price of the cells played an important role. The store offers these (I note, new) batteries at a price of 1.39 euros when buying 20 or more pieces. In our case, 50 pieces will be purchased for the assembly and, surprisingly, it turns out that there are exactly that many cells in the box with them. That is, we get exactly what we wanted – the cells came off the assembly line with the slightest difference in parameters and release date. Considering that all the cells were without any traces of soldering or anything else, we can assume that they are really completely new. And given that they often disappear from sale, I think they just came from the factory. The cells have been selected. Let’s move on – the case.

Case

The desire to assemble a battery on 21700 format cells appeared at the moment when I realised that with a slightly larger size than 18650, we get approximately 1.5-2 times more capacity. Most often, the cases offered for 18650 are also available for 21700 cells. For myself, I chose the POLLY DP-2170-5C model. This model has space for 52 cells. In our case, 50 holes will be enough, since the battery will have a 10s5p format (10 serial connections of 5 parallel cells). It is important to note here that you can purchase the case in different versions – the case itself with mounting on the frame and a key. You can buy a case with the addition of nickel-plated plates for welding. The wider configuration additionally has a BMS for full battery assembly. I purchased the case for one battery at the link, and I took the second one somewhere in Ukraine. Additionally, we will go through the components for the case.

Welding plates

The plates that come with the kit are quite thin and have a thickness of 0.15 mm. However, I want to draw your attention to something else. For the same case (purchased here and in China), I received two different sets of plates.

This set of plates was from China. In addition to the positive and negative contacts, it also has 9 outputs for BMS cables.

In the case of these plates, the arrangement of the elements is slightly different, the contacts come out from the ends, and there are only 6 pins under the BMS. When I asked the seller about the fact that there are only three pins, he told me that all the others should be soldered directly to the plates. Considering that the inscriptions on the picture are in Chinese, you can get this type of plates from there. I would advise ordering with the first ones, because for a beginner there is a diagram of the arrangement of the elements, plus it is more convenient to solder. For the first scheme, the elements should be arranged like this:

I will note that if you place the elements incorrectly, you will have a short circuit. Considering the currents that these elements can give, I would advise you to be very careful when assembling, because I myself managed to short a pair of BMS contacts.

BMS

The quality of the entire assembly depends on the quality of the BMS. I would say even more – it is more important to take the right BMS than quality cells. Because in the case of a quality BMS, it will give the maximum from what the cells have in terms of capacity and power. If the BMS is the cheapest, you may encounter a problem in operation. In addition, it is desirable that the BMS has at least a basic cell balancing function. Does the BMS that comes with the case have such a function? I think not. So I did not order it. A complete BMS for 30A costs about $18. What if I tell you that there is a cheaper and more interesting one?

The last time I bought a BMS called Jiabaida SP14S004 at a price of $13.33. This is much cheaper than the “cased” one. In terms of functionality, it already has a basic balancer, built-in Bluetooth for viewing the battery status, and the power it produces is already 50A. It can be criticized that there are more branded and high-quality models, but there is one nuance – the dimensions.

This picture shows three BMS. The second model in the photo is known as Daly Smart BMS 8S-17S 40A BMS CAN 1A Active Balance and the third is Jikong JK BMS 60A 8-17S 0.6A, Built in BT . The Daly cost $35.45, the Jikong cost $30.74. Yes, they would have been better because I was using Jikong to build a battery for backup power at home. But the size of even the Daly model did not allow them to be placed in the battery case. Therefore, unfortunately, I had to limit myself to the Jiabaida SP14S004, which is probably one of the smallest with such functionality (and cheaper too).

I will discuss the specifics of connecting this BMS a little later. Also, at the end of the article there will be links to documentation and other useful things.

Welder

Our next stop is a welder for elements. This was also one of my first experiences, and for this I chose a model…um… 8000W something like that . This whole miracle of Chinese technology looks like this:

In my opinion, this is a very good option for a spot welder. It cost me about $39, and for its price it is quite possible to solder your own battery. According to the seller, it is designed for plates up to 0.3 mm thick. I welded plates with a thickness of 0.2 mm with it without any problems, it copes perfectly with the plates that come with the battery. The device is completely autonomous, for ease of operation it has a pedal.

Process

The assembly of the battery itself does not cause any problems, but there are certain points regarding insulation and in general. It all starts with the arrangement of the elements, as shown above. Then we mark where and which plates we will have, along the way checking not to mix anything up anywhere.

At this stage, there is nothing wrong, but after welding the elements on one side, I advise you to expose them gradually on the other side so that nothing is shorted anywhere. Also, keep in mind that after soldering, the battery should be separated from any metal elements so that nothing is shorted anywhere. Pay attention to the free windows – we will also use them in the future. In terms of current strength on the welding machine, I would advise you to set it to about mode 6. The fact is that when welding with each subsequent welded point, the contacts of the device will heat up, which is why the current will flow more. The metal will burn more. That is why I started welding at about mode 6, gradually reducing the current strength to 4. After several points, the handle becomes uncomfortable to hold and you should let the device rest. So I welded each plate in two steps.

For reliable fixation, I welded each element at six points. I would advise using protective cardboard rings on the positive contacts. This is exactly what I did on my second assembly:

The problem is that there is a place on the positive contact where the plus can meet the minus and there will be a short circuit. So it’s better to be safe.

When all the plates are welded, it’s time for insulation. At first, I went the wrong way, deciding to use… reinforced tape for it. Yes, yes. I said to myself, “I don’t want this PCB, self-adhesive cardboard, and so on.” And I took what was in the nearest hardware store.

For the sake of objectivity – I think it is correct to wrap the elements around like this. In this case, it will be easier and more reliable to glue insulating cardboard to the elements later. So there are no questions here. After gluing the battery on the sides, I decided to solder the BMS.

At this stage, I made a mistake by not looking at how exactly the balancing cable is connected here. This point will be corrected a little later. The correct connection of this cable is necessary so that the battery application correctly sees how many groups of cells our battery consists of. Usually I am used to the fact that if the BMS is designed for a larger number of cables, then all the others are ignored. In this case, everything is a little different. The connection features will be discussed a little lower.

I also decided to cover the sides of the battery with reinforced tape, and this is not entirely correct, because this tape can be easily damaged, and things like tweezers can easily short-circuit our battery.

Connecting balancing cables to BMS

As I specified above, this BMS has an unusual (in my opinion) connection of balancing cables. The connection diagram is at this link , but it didn’t open for me without a VPN, so here’s a saved copy of this page  on Google Drive . In short, the diagram is as follows:

We start connecting the wires from zero (black) and sequentially solder B1, B2 and so on. Having soldered the last tenth element to the plus, we also solder all the wires that follow it to the plus. In my case, I cut them, connected them and threw the whole thing with one wire. There is no difference how you do it, but there was a difference in the application. While these wires are not soldered, the BMS believes that the battery consists of more elements than there actually are. If everything is done correctly, 10 elements will be displayed in our application.

About the app

The application comes in several versions. There are so-called “white” elephants, there are “blue” elephants (the colour of the application icon). I am a simple person, here is the link . We download a file called Xiaoxiang App from it, or we do it from Google Drive . There are many versions, moreover, it updates itself. But there is a nuance. Many people are faced with the fact that someone has a limitation on the maximum current that the BMS gives out (25A instead of 50A). Someone else says that they simply cannot change the settings on a newer version. Still others say that they were able to change the settings only after contacting the seller. In my opinion, the most correct theory is that for full configuration the application requires registration (it warns that without it access will be limited). After registration, access to the settings opens. But settings can be made (only?) on the first device to which the BMS was tied. Perhaps if you remove it from the profile, the settings can be changed on another phone. But there is such a moment in the work.

Returning to the specifics of soldering, it all looked like this for me after that:

By the way, looking at the photo, I remembered that the link to the app should still be included with the BMS on a small sheet with a QR code.

There is a place for the BMS board in a small recess on the case. This is one of the good points. Of the bad points, we have very limited wires coming out of the BMS. This causes some inconvenience. However, I wanted to write this article to show exactly how to connect the BMS to the negative contact and install a non-standard BMS in this case. This is where we use the holes of the missing elements and push the black wire through them. Before this step, I would advise you to solder the wires to the place marked on the BMS board as SW. This is necessary so that the board responds to the switch that is present on this case.

Switch implementation

At this point we are faced with a “fork”. The fact is that the switch can be used either to de-energize the entire battery (to break the negative or positive contact), or to turn off the power to the controller. The first gives us a complete de-energizing, and only in the case of immersing the battery in water will the contacts close and we received short circuit. The battery may fail. In the case of connecting the button to the SW mark on the board, when it is turned off, our bike will not receive power to the controller, but battery charging will still be available through the connection connector. In my batteries, I have encountered both the first implementation and the second. But the problem is that on those batteries where I encountered this, the switches were designed for a higher current, and the battery itself, according to the manufacturer, can receive no more than 2-3A of current and give no more than 15A. With a basic battery capacity of 25A, it is clear that we will want to remove a little more than 15A from the battery and charge it not for half a day, but a little faster. A switch that has characteristics of 3A at 250V will not allow us to receive from the battery and charge it with a large current. So I decided to use it from the switch from the BMS, having limited but useful functionality.

Let’s go back to the battery. We connected the negative contact from the BMS (blue) to the negative of the battery. I had to cut it a little, I used insulating tape here. It would be wiser to use thermal tape, but as it is. 30 meters of thermal tape are already on their way to me from China, so the next battery will be assembled more correctly.

The soldering/connection procedure for me was that first I soldered the negative contact from the BMS (black) to the negative contact of the battery case connector. Then I soldered the blue contact from the BMS to the negative of the battery. The next step is to solder the positive contact from the charging plug and the battery case connector to the positive contact on the battery assembly itself. After that, we connect the balancing cable to the BMS.

Another lesson I learned during this build is to not use sealant if you don’t know the properties of its interaction with a particular material. The fact is that the case is not sealed from the factory in the place where the charger is connected and the power button is located. I wanted to add a little sealing. As a result, 3 days after applying regular plumbing sealant, I realized that it…did not dry. That is, almost not at all. Because it was not designed for this. What to use?

While searching the internet, I came across two products that can help with this issue. The first of them is Kafuter K-704B.

It is very popular with people who make “packaged” assemblies of elements. That is, those that are inserted into a PVC heat-shrink sleeve and seal the space between the edge of this sleeve and the PCB. And this is the best option, provided that you can find it and do not regret about 4-6 dollars for it. The second option is Soudal FixAll Flexi like this:

It can be purchased for about $7. Don’t pay attention to the colour – it comes in white, gray, and some other colours. In my case, it seemed more practical to take black.

The first of these two sealants is more fluid, which has a certain advantage if we are interested in sealing. Moreover, it should definitely adhere well to PVC film and other surfaces. Earlier, a sealant from Soudal arrived at my place. I will not say for its sealing properties, but it is very good to use as “liquid nails”. In this sense, what is sold here under the name “liquid nails” showed itself very poorly when I used it primarily. But this sealant held the cables that were attached to the insulating cardboard well. It was removed together with the cardboard, so I think this is already a good result.

Battery re-made

When the insulating cardboard and sealant arrived, I decided to do some work on the battery. I started by peeling off the reinforced tape from the sides.

I had to be very careful during the conversion so as not to shorten anything. I recommend cutting out the shape for the side parts of the battery in advance.

I have the temporary elements fixed on black electrical tape. In my opinion, they should all be placed on sealant, fixed in the case, but the battery will be for my own use, so it will be fine for me.

Here you can see the black sealant (Soudal) in action. And this is what the sealant from Kafuter looked like:

Overall, both performed very well. So I recommend both.

The positive contact from the battery was thrown directly to the housing connector (common plus of the battery) with two wires with a cross section of 2.5 mm (I specially took a higher-quality wire for such purposes). I think that the price will fully withstand both charging current up to 20A and discharging within 20-25A with rare use for higher current.

This picture shows perfectly how inconvenient it will be to solder the SW connector when the BMS negative contact is already soldered to the power connector. Therefore, I highly recommend taking care of this in advance.

The positive contact was additionally inserted into the thermo-tube. The photo clearly shows that it is of more than sufficient diameter.

Short circuit protection

When assembling a battery, you think about its weak points. What are they in this battery? First of all, it is short-circuit protection. I remembered that in old, larger batteries we had a fuse. Yes, it could be of low power (there the calculated maximum current is 15A), not quite correct (as an option, an automobile one, not protecting against an arc that can occur in the event of a short circuit), but it was there. There is no place for it in this case, and in every sense.

I spent half the morning with Google and ChatGPT to find out what kind of fuses there should be. There is a lot of colourful information on the Internet, but I still haven’t found a specific answer to the question of what kind of fuse should be for an electric bicycle battery at 36 volts and 25 amps. Someone advises to install a regular circuit breaker. Someone interrupts, specifying that the circuit breaker should be more expensive and for direct current. Others simply install a switch. Others install glass fuses “so that they heat up less”. Others install a car fuse. So which of all these options is ours?

In fact, we need a fuse with arc extinguishing. Usually these are special automatic transfer switch, but you can’t connect such switch with a compact battery. So you need something smaller. After much thought, the GPT informed me that the Chinese manufacturer Tomzn (he is known to me from the time when I was assembling a LiFePo4 battery and was looking for a fuse, again, with arc protection) has fuses in the 10×38 format. They belong to the Solar series, are marked DC 1000V PV and are suitable for currents up to 32A. Of the features, the presence of sand in the capsule, which, when the fuse is closed and blows, should fill the capsule, thereby removing the arc. Half the trouble is finding this fuse, it’s harder to find a case for it. There are three (four) variants of compact cases. In our market there is a model CQ215 , which in one store costs $4.2, but is not available. In China they can be found under the name R3-41 fuse holder or R3-18 Fuse Holder . Both look like this:

There is also a slightly different version with a transparent case and it looks like this:

The first two are shorter but wider, the transparent version is a little narrower but longer. For testing, I ordered the two smallest versions to see which one would fit better in our case. Considering that the fuses are currently on their way, the article will be updated when they arrive and it will be possible to complete the assembly of the battery in the final version.

Charging

The issue with protection and assembly is closed. How to charge all this? The official documentation for the elements gives us the following information:

That is, it is desirable to charge and discharge cells with a current of 1A, the maximum constant charge current should be 5A, the maximum constant discharge current is 15A. Our battery consists of 10 groups of cells with 5 cells in each. Technically, we can constantly charge and discharge with a current of 5A without any problems, and we can charge with a maximum of 25A and discharge with a current of up to 75A. Of course, no one plans to use these maximums, but I think that the battery can be charged with a current of 10-15A without any problems. I found two options for more or less high-quality chargers. The first is a charger from the manufacturer Wate with parameters 42V/10A . The second, more interesting, is a charger from the manufacturer SouthROL for 10-93V and 3-20A, which cost me about $95. This charger will close the question with its versatility, allowing you to choose which batteries to charge and with what current. The pluses are versatility, the minuses are that all settings are made via Wi-Fi in the phone app.

The battery will be charged either with a standard charger (2-5A) through a regular connector (5.5×2.5 can be used for charging with a current of up to 5A), or with a powerful charger (fast charging) with a current of 10-15A through a special connector. The plans are to remove the sealed connector from the battery mount on the bike, as I came to the conclusion that there is no comfortable place on the battery itself to install an additional connector for fast charging.

Materials

Connecting balancing cables .
Battery configuration application .
Application instructions (in English) .

Cost

The final cost of a finished battery depends on many factors. When I did the calculations, a “basic battery” (without a fuse, without purchasing sealing, insulating paper, and so on) came out to about $170. If you add the cost of a fuse, welding machine, insulation, charger, the price of the battery will increase significantly. So if you just need a replacement battery, then it is probably cheaper to buy a ready-made one locally. If you (like me) plan to replace the entire battery fleet (I have eight of them), this option will only pay off in this way. However, I would like to emphasize that self-assembly of a battery is a very dangerous process that can end tragically. Therefore, you should assemble a battery yourself only if you clearly understand what you are doing and why. 

The article will be updated soon, so see you soon! 🙂

UPD from 06.08.2025

I have finalized the assembly of my battery and can now add to the article. What was done?

My fuse housings and the fuses themselves arrived. And immediately I encountered a moment I was used to – what you see in the photo and what you get will be different. Not least in their size.

The first model fell out immediately, as it barely fit in length. I would rather not say anything about the width.

The second model more or less fit, but it was also not without its peculiarities. I had to grind off the top of the cover so that the fuse would fit in the only free space for it.

It turned out to be a corner where the battery cell used to be. The plastic had to be removed to fit the fuse in its place.

Finally, the fuse was placed in the place where the power contacts come from the battery. We connect one plus of the fuse to the positive terminal of the battery, and the second contact is brought to the terminal and to the plus of the charging connector. Thus, our fuse will protect both in case of problems during charging and during a short circuit on the controller side. Please note – I replaced the regular cable with a 12AWG cable. This will provide additional safety (the cable can withstand heating up to 200 degrees) and, in general, this is a more profile cable for batteries.

I reworked the positive and negative contacts and routed them in different directions. Again, from a safety point of view, this is the best option.

In the previous version of the battery, I couldn’t figure out where to put the temperature sensor. In this version, everything fell into place – I placed it near the fuse. There are two options here – in the first, the fuse will get very hot and the battery will turn off due to protection. In the second, it will be additional protection in those conditions when it would be better if the protection worked than if the battery, conditionally, started to melt. So far, my wife has been driving on the new battery without any complaints or problems. The driving dynamics have become much better compared to the old battery. After some time, we will make a trip of 50 kilometres and see how long the battery will last and how it will behave.

I replaced the electrical tape with thermal tape. I think it will do its job better.

Just in case, I sealed the connectors and button with Kafuter K-704B sealant.

The connector for connecting the battery to the bike has also undergone some changes (I didn’t talk about it before). Now it uses the same 12AWG cable, and at the output we have an XT60 connector. To be more precise, the connector is called Amass XT60U-F . The peculiarity of this connector is the presence of a groove.

Thanks to it, this connector fits perfectly in this case in place of the wire output. Add a little sealant and voila! We have a connector that can work with currents of 60A. This will be more than enough to use our battery in any scenario. Moreover, it is because of it that I decided to make such a thing as fast charging. That is, it will not be some separate port. At home, you can make yourself a docking station from a battery mount for fast charging. The “mother” connector was used for a reason – I don’t like having a chance to short the contacts on the “father” contact of the XT60 connector. So it will be a little safer this way.

We fill the whole thing with sealant, close the lid and the issue is closed!

I have already managed to assemble two full batteries with fuses. One bicycle out of three has been converted and is already running on a new battery. I am currently converting two more. So far, there have been no problems or complaints about the battery. I hope it will continue to be like this. May the roads be smooth for us and the war end soon.

UPD from 18.08.2025

If you (like me) are fed up with the glitchiness of your native “White-Blue” elephant, I advise you to pay attention to an alternative application called Overkill Solar. It is not as buggy and also allows you to monitor and change the settings of this BMS.

Author: Sa Crea

Hellmin