The build
Robbert assembles a 314 Ah, 16 kWh battery pack from scratch in a JK ESS enclosure. The pack uses sixteen EVE MB31 cells in series (16S), a JK BMS, and the components shipped with the case. Chinese enclosures rarely include a manual. This walkthrough is the manual.
Why a JK enclosure and EVE cells
JK BMS is in many commercial batteries even when it is not branded on the outside. Build quality is consistent and the BMS is reliable. For an enclosure built around that BMS, Robbert chose the matching JK ESS case.
EVE LiFePO4 cells have been the default in Robbert's projects for years. Out of the box the cells are typically balanced to within 0.002V of each other, which means almost no balancing time on first start-up.
Why DIY instead of a commercial pack
- Pick the BMS and the cells yourself
- Repair and replace components when something fails
- Not dependent on a supplier that may not exist in a few years
- Many battery brands have appeared and disappeared, the same shakeout is likely for home batteries
- Full control over the actual build quality
Step 1: assemble the chassis
The case ships well packed. Plates are screwed together for transport. The front panel and any polished surfaces come wrapped in plastic film. Components are boxed separately rather than dumped loose.
Build order:
- Mount one side panel onto the base plate. The center brace sits closer to one edge, that edge is the bottom.
- Slot in the inner divider plate with the screw holes upward and the opening to the front. Screw it down.
- Mount the other side panel and the second inner divider.
- Place the glass-fiber insulation plates against the long and short inside walls. These prevent shorts between the cells and the case.
Step 2: install the cells
Robbert uses sixteen EVE MB31 314 Ah cells. Each cell is placed with attention to polarity. The pattern alternates: a cell with plus-minus, then a cell rotated 180 degrees with minus-plus, and so on, so that adjacent terminals are always one plus and one minus.
A thin plexiglass sheet goes between every cell. When the first row is full, the sequence reads plus-minus-plus-minus all the way across, ending on a plus. The second row begins with a minus against that plus, and the alternation continues.
The back wall slides in last. Hook the lip through the frame, rotate it up, and screw it from the outside.
Step 3: install the top bus carrier
Lay an isolator across the cell tops over the vents. Drop the top bus carrier (a metal strip with vent holes) over the isolator. The vents stay open: if a cell ever needs to release pressure, it can. Screw the carrier down with the metal screws supplied.
Even though there is a metal strip running across the cell tops, every terminal stays isolated. The carrier exists to prevent foreign objects from shorting across the terminals, not to conduct.
Step 4: connect the busbars (the dangerous step)
This is the step where shorts happen if you are not paying attention. The pack is live the entire time you build it. You cannot disconnect a battery.
For each pair of adjacent cells:
- Plus of cell N to minus of cell N+1
- Bolt the busbar onto both terminals
Single stud cells have one bolt per terminal. Double stud cells have two. Robbert uses double stud here. For the build video a regular screwdriver is fine, but every screw needs to be redone with a torque wrench at the end.
At the row turn, a normal busbar cannot reach across. A longer busbar links the last cell of one row to the first cell of the next. Same plus-to-minus rule.
Step 5: mount the BMS and its sense leads
The BMS goes on the back wall with the four long thin screws from the parts box. It can only mount one way up.
The pack ships with four red sense cable bundles, color coded. Each bundle plugs into one BMS port.
What they do: the BMS reads the voltage of every individual cell through these wires. That allows it to prevent over-discharge, over-charge, and cell imbalance, and to balance cells against each other.
Each bundle starts with two thermistor wires (a temperature sensor). The next wire is the ground (minus), which connects to the very first minus of the pack. After that comes B1 (battery one) to the first plus, B2 to the second plus, B3 to the third, B4 to the fourth. The next bundle covers B5 through B8, and so on through B16 on the last plus of the pack.
Crimp lugs on each lead and fasten under the terminal screws.
The four thermistors give the BMS four temperature reading points spread across the pack. If anything overheats, the BMS can shut down.
Step 6: front panel
Install the handles on the front panel first. They protect the panel during the rest of the assembly and make it easier to handle.
Mount the four terminal posts (two positive, two negative) using the small screws with the rubber gaskets that keep them from rattling. Mount the BMS interface PCB from behind. The LCD screen sits in front, with its ribbon cable feeding through. The power button slots in with its nut tightened from the back. The breaker mounts upside down with two yellow zinc screws into the front panel.
The pre-cut cables for the interface only fit one way. Ribbon cables have a notch. Plug the flat cable into the black BMS port, the two red connectors into the red BMS ports, the display cable into the dedicated port, and the on/off switch into the rightmost port.
Step 7: the high current path
Cut the short power cable and bolt it to one breaker terminal. The longer power cable bolts to the other side. Mount the breaker into the front panel.
For the P+ (positive output): aluminum busbar across both terminal posts, with the breaker-cable lug bolted between them. Splitting current across two posts lowers resistance and lets two cables exit if you want.
For the B- / P-: a small busbar bridges the BMS's two minus ports. The BMS exposes a separate battery-minus and power-minus so you can run two thinner cables instead of one thick one. With the bridge in place either way works.
Mount the cable from the BMS minus to the case minus connector on the front. Close the case enough to reach.
A build flaw worth knowing about
The case ships with an aluminum plate intended to bridge the BMS ground to the cell pack ground. With the EVE MB31 314 Ah double-stud cells, the screw hole on this plate does not line up with the cell. Even flipping the cell orientation does not align it.
Robbert's workaround: run two 25 mm² cables in place of the single intended 35 mm² link. He had spare 25 mm² on hand, so the substitution was free.
Worth checking before you order: confirm the case is matched to the cell variant you have.
First power-on
Bolt in the last busbar and the pack should read a real voltage on the meter. In this build, 52.1V.
Hold the power button to start the BMS. It shows 52.09V on its display. Multimeter reads 52.1V. Match.
Out of the box the BMS shows battery capacity 40 Ah, which is wrong for 314 Ah cells. Cell-to-cell voltage spread is 0.002V, which is the signature of EVE cells: balanced from the factory.
Configuration
Open the JK BMS app on a phone. Scan, find the pack, pair. The pack beeps three times to confirm pairing. The dashboard in the app mirrors the screen but unlocks settings.
Settings are password-protected. The default password is 1-2-3-4-5-6. Enter it, then:
- Battery capacity: change 40 Ah to 314 Ah
- Cell count: 16
- Continue with full pack calibration (covered in the earlier wooden-case build video)
Once the values are correct, the pack display updates immediately.
Finishing
The two extra brackets in the parts box are server-rack mounts for the front. The remaining six small screws hold the top lid in place. Lower the lid carefully: dropping it onto live cell terminals would be a bad day.
Close the sides. The pack is functional and ready to be added to the system, contributing to a total of 48 kWh of storage in this build series.
Takeaways
- Read the polarity twice before bolting any busbar. The pack is live during the whole build.
- BMS sense leads in strict order: GND on the first minus, B1 to B16 on each successive plus.
- Cells out of the EVE box are already balanced. The BMS will not have much work on day one.
- The JK ESS enclosure quality is high relative to its price. The missing manual is the only real complaint, and this video fills that gap.
- Verify the supplied internal ground plate matches your specific cell variant before you start the build.
