Building and Testing a 16kWh Tewaycell DIY Home Battery

Steps

1. 1

   Step 1: Unboxing the Tewaycell battery case

   I start by unboxing the light-grey case which comes with side handles, a front LCD screen, terminals, and a built-in JK BMS. The JK BMS is great because it supports various communication protocols to work with setups like Victron. I notice the lid is slightly smaller so multiple units can be stacked together.

   Inside the box, I find insulation sheets with foam backing, a pre-installed 200-amp circuit breaker switch, and top circuit boards for the balance cables.

   What I used

   Item	Amount	Notes
   Tewaycell case	1	Includes light-grey box, handles, LCD screen, terminals, 200A switch, and JK BMS

2. 2

   Step 2: Unpacking and checking the Dejin cells

   I unpack the Dejin battery cells and check their quality using a straightedge. The cells are 100 percent straight and not bulging, meaning they are A-grade cells rather than B-grade. I measure their initial voltages and find they all sit at 3.28 volts.

   Because the cells are already at the same level, we do not need to balance the pack as heavily later.

   What I used

   Item	Amount	Notes
   Dejin cells	16	314Ah cells ordered via Alibaba
   Meetlat	1	Used to check if cells are straight

3. 3

   Step 3: Installing insulation sheets in the case

   I place the thermal and electronic insulation sheets inside the steel enclosure to prevent the cells from short-circuiting against the metal body. I place the long sheets on the long sides and the short sheets on the short sides, keeping the foam on the outside and the yellow side inward.

   What I used

   Item	Amount	Notes
   Insulation sheets	1 set	Thermal and electronic insulation for the case

4. 4

   Step 4: Assembling the cells into the case

   I place the first cell with its positive terminal facing the correct side. I add a foam sheet between each cell and also include the resin sheets for double insulation. I rotate each consecutive cell 180 degrees so the terminals alternate between positive and negative.

   I have to push hard on the last cell because the foam sheets between the cells create natural compression.

   What I used

   Item	Amount	Notes
   Foam sheets	1 set	Placed between cells for pressure and insulation
   Resin sheets	1 set	Acryl insulation sheets

5. 5

   Step 5: Mounting brackets and connecting busbars

   I mount the plastic PCB brackets labeled A and B over the cells and screw them down. I connect the vernickled copper busbars to put the 3.2V cells in series to reach a total of about 51 volts. I use nuts supplied in the bag to secure the busbars and balance wires to the cell terminals, being very careful not to cause a short circuit.

   Warning

   Never make a short circuit with the busbars because a lithium battery is extremely powerful.

   What I used

   Item	Amount	Notes
   Busbars	1 set	Vernickled copper plates
   PCB brackets	2	Labeled A and B for right and left sides
   Nuts	1 bag	For securing busbars

6. 6

   Step 6: Connecting the balance wires

   I connect the balance wires B0 through B8 to the busbars across the pack. These thin wires carry a maximum of 2 amperes, similar to a phone charger, because they are only used by the BMS to measure individual cell voltages and bleed energy from full cells so others can catch up. I use a torque wrench set to a maximum of 6 Nm to tighten all the nuts firmly without breaking the screws.

   What I used

   Item	Amount	Notes
   Moment sleutel	1	Torque wrench set to max 6 Nm

7. 7

   Step 7: Connecting wire harnesses to the printed circuit boards

   I plug the wire harnesses into the top circuit boards, putting the large connector on the left and the small connector on the right for each side. Finally, I connect the BMS power cable to the front terminal. I measure the total voltage across the pack with a meter, and it reads 52.5 volts.

   What I used

   Item	Amount	Notes
   Meter	1	Multimeter for voltage checks

8. 8

   Step 8: Powering on and configuring the BMS

   I turn on the side switch and hold the front button to boot up the BMS display. The screen starts in Chinese, so I tap the Chinese characters and select English. I check the display information, such as total voltage, cell voltages, and temperatures.

   Using the JK BMS app via Bluetooth, I can configure settings such as cell count and cell capacity.

   What I used

   Item	Amount	Notes
   JK BMS app	1	Used via Bluetooth for configuration

9. 9

   Step 9: Closing the battery housing

   I attach the top insulating plate with four small screws. Finally, I close the housing with the main lid. The cells cost 80 dollars each and the housing cost 510 dollars, bringing the total to about 1550 euros for a 16 kWh home battery.

10. 10

    Step 10: Setting up the test environment

    I disconnect my old battery pack and connect the new Tewaycell battery to my 3 times 5 kVA Victron MultiPlus setup. Because I want to inspect the internals with a thermal camera, I remove the battery lid and top plate. I also set the MultiPlus cooling fans to 100 percent to ensure maximum cooling during the test.

    What I used

    Item	Amount	Notes
    Victron MultiPlus setup	3	3 x 5 kVA inverter/charger
    Thermische camera	1	Used for thermal imaging

11. 11

    Step 11: Charging the battery and monitoring temperatures

    I start charging the battery at 1600 watts and then increase it to 3.5 kilowatts. After 2 hours of charging, I check the temperatures with the thermal camera and see that the cell center is at 28 degrees Celsius. After 3 hours of charging, with the ambient temperature at 27 degrees due to the sun, the cell center is only 30 degrees Celsius, which is very cool.

12. 12

    Step 12: Discharging the battery under high load

    Once fully charged, I discharge the battery at 5 to 6 kilowatts, pushing about 100 to 120 amps through 35 mm2 cables. The thermal camera shows that the battery handles 5 kW and 6 kW without any hotspots, and the BMS and busbars do not overheat. The cable lugs and cables remain cool despite being slightly overloaded beyond their standard 80 to 100 amp rating.

13. 13

    Step 13: Takeaways

    • Building a DIY home battery with Tewaycell and Dejin cells costs around 1550 euros for 16 kWh.

    • The JK BMS offers great versatility and communicates with various inverter brands like Victron.

    • Balance wires only carry up to 2 amps and do not need to be thick.

    • Thermal testing showed that charging and discharging up to 6 kW keeps the battery and cables remarkably cool.