In our previous project, we explored creating a sensored ESC to power an electric bike wheel using low voltages. However, the achieved rotation speed was insufficient, and not everyone is inclined to build their own ESC for an e-bike conversion. So, we decided to revisit the controller included in the kit and focus on customizing a Li-ion battery pack suitable for an e-bike.
How to Custom Li-ion Battery Pack
1. Testing the Controller
To test the controller, a new lab bench power supply capable of outputting up to 60 volts was acquired. By connecting its outputs to the controller’s input and gradually increasing the voltage, the controller started working correctly at around 40.6 volts. Increasing the voltage further made the wheel rotate continuously faster until reaching the power supply’s limit.
This experiment indicated the need for a battery pack that can cover a voltage range of at least 40.6 volts to a maximum of 61.5 volts. A suitable choice for this application is 18650 Li-ion battery cells, known for their excellent volumetric and gravimetric energy density and ability to deliver sufficient current. Moreover, they are commonly used in e-bike battery packs.
2. Determining the Battery Configuration
While searching for a compatible battery pack, the high prices prompted the consideration of building a custom one. By inspecting the datasheet of typical Li-ion cells, it’s evident that they have a nominal voltage of 3.6 to 3.7 volts, a maximum charging voltage of 4.2 volts, and minimal capacity at 3 volts when discharged.
This means the voltage range per cell is 3 volts to 4.2 volts. For our controller’s voltage range, connecting 13 cells in series (13S) makes sense, resulting in a battery voltage range of 39 volts to 54.6 volts, with a nominal voltage of around 48.1 volts—which aligns with the controller’s advised voltage.
Next, the maximum required current of the controller needed determination. Although the lab bench power supply test didn’t provide an exact figure, the product page stated 1000 watts at 48 volts, equating to a current of approximately 20.83 amps.
To be cautious and double the battery pack’s capacity, it was decided to use two cells in parallel (2P). This configuration results in a 13S2P Li-ion battery pack with a capacity of 5 amp-hours, a nominal voltage of 48.1 volts, and a possible constant output current of 40 amps.
3. Assembling the Battery Pack
A total of 30 Li-ion cells were acquired from a trustworthy seller. Upon receiving the cells and visually inspecting them, their voltages were measured and found to be very close to one another—a crucial factor when connecting cells in parallel to prevent large current flows due to voltage differences.
To assemble the 26 cells into a neat battery pack, plastic spacers that hold two cells each were utilized. Thirteen of them were connected in series through their interlocking system. Cells were placed with the same orientation in the first row, and the orientation was alternated for the next pairs while filling all the spacers.
For connecting the cells, a 7mm wide and 0.3mm thick nickel ribbon was used, capable of handling up to 30 amps. Twenty-six smaller pieces of nickel ribbon were cut to connect all the parallel cell pairs.
To create the actual connections without soldering, a battery spot welder was employed. By pressing the electrodes onto the metal with a distance of roughly 3mm and activating the welder, solid connections were formed. Two welding spot pairs were created for each battery terminal, resulting in a total of 104 welds.
Next, another 24 nickel strips were measured and cut for the series connections, connecting them to the parallel batteries in the arranged configuration. This process added another 96 welds.
With that completed, the 13S2P battery pack was basically complete and delivered a voltage within the previously calculated range.
4. Charging the Battery Pack
Charging the battery pack required careful attention to prevent overcharging and maintain cell balance. The datasheet of the Li-ion cells recommends a constant-current constant-voltage method with 1.25 amps and 4.2 volts per cell. Scaling up for the 13S2P battery pack, this translates to 54.6 volts and 2.5 amps.
By setting the lab bench power supply’s current limit to 2.5 amps and the voltage limit to 54.6 volts, and connecting it to the battery terminals (with thicker 10 AWG color-coded wires soldered beforehand), the charging process worked effectively.
However, as the target voltage approached, the charging process was interrupted to measure the voltage of each battery pair. The voltages remained close to one another, but over multiple charging cycles, differences could grow, potentially leading to cell damage.
5. Adding a Battery Management System (BMS)
To prevent such issues, a Battery Management System (BMS) was incorporated. It not only keeps all cells at an equilibrium voltage but also adds overcharge, over-discharge, and short-circuit protection.
The BMS was connected by soldering its balance connector wires to the battery according to its label: B1- to the ground potential, B1+ to the 3.71V potential, B2+ to the 7.4V potential, and so on, up to B13+ connecting to the 48.1V potential. The ground wire of the battery was connected to the B- terminal, and two more black wires were added to the P- and C- terminals.
To charge the battery pack, the positive supply voltage was reconnected, and the ground potential connected to the C- terminal. This setup allowed the battery to charge as before, but with the BMS managing the process. Once all the indicators lit up, the charging process was complete.
With the creation of the DIY e-bike Li-ion battery pack complete, the only remaining question was whether it was cheaper than buying one. According to market prices, it was indeed slightly cheaper, but the savings were minimal. Factoring in labor costs and the cost of equipment like a battery spot welder, building your own pack becomes cost-effective only if planning to create more than one pack or if specific customization is needed.
FAQs for Custom Li-ion 18650 Battery Pack
Q1: Why choose Li-ion 18650 cells for an e-bike battery pack?
A1: Li-ion 18650 cells offer a great balance of energy density and current delivery capabilities, making them ideal for e-bike battery packs. They provide excellent volumetric and gravimetric energy density, meaning they store a lot of energy relative to their size and weight, and can deliver the high currents required by e-bike motors.
Q2: Is it necessary to use a Battery Management System (BMS)?
A2: Yes, a BMS is crucial for maintaining the health and safety of a Li-ion battery pack. It ensures all cells are balanced, prevents overcharging and over-discharging, and protects against short circuits, thereby prolonging the battery’s lifespan and ensuring safe operation.
Q3: Can I charge the battery pack without a BMS?
A3: While it’s technically possible to charge a battery pack without a BMS, it’s not recommended. Without a BMS, cells can become unbalanced over time, leading to decreased performance, potential damage, or safety hazards. A BMS actively manages cell voltages, ensuring safe and efficient charging cycles.