A few years ago, I bought an old electric bike for 50€, yes 50€ ! Why such a low price? Its battery was almost dead, allowing only 1.5km of electric assistance and the cost of a new battery, 400€, was too high considering the age of this bike. But why replace something you can fix and improve? ;)
Before diving into this instructable, a few words about me: I’m Alex, a 31 year old french folk using my bike everyday, for everything! I always wanted to try an electric bike but such beauties have their cost, generally more than €1200, a bit costly for something I'm not sure to like! Building a (big) battery was something i have never tried before this project but hey, if some guys manage to create their own, why not trying?
The idea was basic: keep the battery case, remove the old NiMh cells and replace them with shiny new lithium cells allowing to bring more power with a lighter weight! And if you are familiar with lithium cells, you know there is also some security circuitry to add if you want to use your battery safely. I’ll cover this part too.
Step 1: Parts and Tools
For each part, i tried to link the corresponding ref to major suppliers, do not hesitate to compare price, they can vary A LOT especially on batteries !
- 2 lipo battery packs 4S 14.8V 10Ah : Amazon / Hobbyking / Ebay
- A set of XT90 connectors (5 mâles + 5 femelles) : Amazon / HobbyKing / Ebay
- A serial cable with XT90 connectors : Amazon / HobbyKing / Ebay
- A serial cable with JST-XH connectors Ebay
- A JST-XH 8S cable with a female socket at one end (the other end will be cut so it can be any type of JST-XH cable…) Amazon
- One BMS board adapted to your battery Ebay / Aliexpress
- 12AWG wires (i recommend at least 2 different colors, e.g. red and black, to prevent accident !) Amazon / HobbyKing / Ebay
- One power supply adapted to the tension of your BMS (33.6V here) Amazon / Ebay
- Heat shrink tube Amazon / Ebay
- Foam pads (I salvage them from the packaging of a few heavy goods i purchased)
Depending on discounts and shipping cost, you can buy all the parts for around 200€. Added to the initial bike cost (50€), you get a new ebike for 250€: pretty sweet deal!
Step 2: Disassemble the Original Battery
This is not the most complicated part of this instructable but you need to follow some precautions. Even with old batteries, you need to handle them carefully to avoid short circuit and potential injuries. Unscrew the battery case to access the NiMh cells. In my case, there were 24 elements, split into 4 groups of 8 cells. Using cutting pliers, cut the wires going to the cells groups (the closer the better), adding insulating tape on wires after each cut to avoid electrical shock.
Once the cells are removed, the case should be empty with just the switch, a fuse, and the charging port disconnected.
Step 3: Choose the Proper Lithium Battery Packs
To build a new battery pack for an ebike, you have several paths you can follow. I choose to use some lipo battery pack used in RC and assemble them to build a 10Ah 29,6V involving 8 lithium cells. Knowing the specs of the original ebike battery (28,8V), you should pay attention to the battery voltage : it should be in the same range if you want to keep the controller of the bike (the controller is the brain of an electric vehicle, managing power and driving the engine). A lithium cell is rated to provide a nominal tension of 3,7V. Using 8 of them, it will be close to the original value (8x3.7V = 29,6V for the new battery, 28,8V for the old one). To assemble 8 cells, you can choose the combination of your choice :
- 8 packs of 1 cell in series
- 4 packs of 2 cells in series
- 2 packs of 4 cells in series
- 1 pack of 8 cells
The price of a pack grows with the number of cells. In my case, a good compromise was to go for 2 packs of 4 cells: they are not too expensive, and they will limit the quantity of cables in my battery case.
Step 4: Mount Your Battery Packs in Series
To achieve this part, you need to buy (or build) two cables. The first one with thick wires (10 to 12 AWG) and XT90 connectors, the output cable, will be used to power the bike when you will use it. The second one, with thin wires and JST-XH connectors, will be necessary to charge and balance the battery (balancing is the process to charge equally each cell of the battery to preserve it and increase its life-time).
Connecting them is not difficult, but you need to pay attention to some details to prevent accident.
Start connecting the balance cable (the thin one with lots of wires). You can not make polarity error thanks to the JST-XH connectors.
Then, before connecting the output cable, check which pack is connected to the red lead of the output cable: you MUST ABSOLUTELY CONNECT THE PACKS IN THE SAME ORDER as with the balance cable. If the positive pole of the pack #2 is connected to the positive pole of the balance cable, then it will be connected to the positive pole of the output cable too. PLEASE double check the schema and your setup before connecting the cables.
Step 5: Add a Connector in the Case
Go back to the battery case. At the place where the old Nimh were connected, I added a XT90 female connector. Do not forget to add heat shrink tube before welding it. This connector will be connected to the output cable from our battery packs.
Step 6: (Optional) Charge and Test the Battery
At this stage, you can test your new battery pack connecting the case to the battery. If you want to test it, do not go for a long ride: brand-new packs are generally sold half charged, and if you are not equipped with an RC charger, you probably won’t be able to charge them until your build is completely finished. Again, battery packs are fragile so handle them gently when you place it in your case. Some foam spacers can help reducing vibration and protecting your cells.
Step 7: Choose the Propre BMS and Associated Charger
If you’re not familiar with lithium batteries, you may not know the BMS boards. BMS stands for “Battery Management System.” It’s a piece of electronic built to protect and monitor your battery during charging and discharging phases. Use a lithium battery without BMS is dangerous: you can destroy it and it can explode too!
Without going too deep into explanations, a lithium cell voltage can go from 2.7V to 4.2V depending on the charge level. If you go too low, the cell will die, if you go too high, the cell can burn and explode. The BMS is here to stop the charge at 4,2V and to prevent the discharge to go lower than 2,7V.
To choose the right BMS, you should consider 3 parameters: The number of cells it can handle (8 cells in our case) The current output allowed (this bike wont draw more than 20A) The battery technology handled (we need in our case a li-ion or li-po compatible BMS)
Step 8: Build BMS Cables
As the BMS is there to protect your battery during charging and discharging, you need to build/buy some cables to connect it to 3 elements: the battery, the case and the charging port. A schematic is generally provided with the BMS, but here is one i drew for my case:
To be able to mount/unmount/replace the BMS easily, i chose to solder the same connectors as on my battery pack : XT90 for the power leads and JST-XH for balancing. Be careful for the JST-XH cable : in my case, there were not the same number of wires on the BMS end as on the battery end. The main reason was the battery negative pole is already connected to the BMS through the “B-” pad on the circuit. That’s why I had to build a JST-XH 8S to JST-XH 7S adapter, omitting the last negative wire.
Step 9: Connect Everything ! and Close the Battery Enclosure
When you’re done with the soldering steps, it’s time to package this mess of cables and circuit. I use some foam pads to prevent the battery packs from moving. For the BMS, I taped it inside a thin plastic sheet to prevent unwanted electric contacts. Add tape and foam if needed to wedge it
Step 10: Choose the Good Battery Charger and Adapt the Charger Output Plug If Needed
Your needed power supply depends directly on the number of cells in your battery and its technology. With an 8 cells lipo battery, i needed a 33.6V (4,2V x 8) power supply. If you can not choose the corresponding plug to the socket of your existing battery case, you’ll need to adapt your power supply end. Your soldering iron and heat shrink tube are your best friends for this step. Check carefully which socket pin is connected to the positive and negative side of your system, and solder your male plug on your power supply output (i reuse the plug from the old NiMh charger).
If you followed all these steps, you are now the lucky owner of a very cool electric bike. In my case the full charge takes between 2 and 3 hours and I’m able to travel on 30 to 40km with a fully charged battery.
Step 11: Conclusion
The bike is now powered by a 29.7V battery of 10Ah. Enough power to cruise around 35km with a single charge, more than enough for my 6km everyday commute :) It was my first contact with an electric powered vehicle and now I’m furiously thinking about building another one bigger (a scooter ? a car ? who knows !)
So far, I traveled over 6500km with my ebike and I’m very happy with it! Until now, the electric parts never failed and my only troubles were with the cycle parts of the bike! Maybe the subject of a future instructable! I hope this guide will be useful to you guys and will lead to some ebike resurrections!
If you are able to read french, you will find there the two articles I initially wrote covering this subject:
Finally, i wrote this instructable for the "Fix it contest". If you like this tutorial, do not forget to vote ;)
Runner Up in the
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