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How to set up & wire a JK BMS (settings + wiring)
If you have just picked up a JK BMS and are not sure where to start, you are in the right place. This guide covers the correct wiring sequence and the key settings in the app, explained in plain language. It is based on a 16S / 48V build using 100Ah cells, but the same steps apply whether you are running 4S, 8S, or 24S. Just update the cell count and capacity in the app and everything else carries across.
Please note: The settings and steps in this guide are provided as a general reference for most JK BMS B-Series builds. Values may vary depending on your specific cell model, capacity, and system setup. For more detail, review our setup video or contact our team directly on 1300 375 257 or at Support@LiFePO4oz.com. We are happy to help you configure your specific setup correctly.
1. What Is a JK BMS?
A Battery Management System (BMS) is the brain of your LiFePO4 battery pack. It monitors each cell's voltage, actively balances the cells, manages how much current goes in and out, and protects the pack from over-voltage, under-voltage, short circuits, and dangerous temperatures.
The JK BMS B-Series comes with a built-in 2A active balancer and a smartphone app for iOS and Android that gives you full control over every protection setting.
2. Before You Start
Before you connect anything, sort these out first:
- Verify polarity with a multimeter. Check positive and negative on every cell before connecting anything. Do not rely on stickers or terminal colour alone. Always confirm with a multimeter reading.
- Download the JK BMS app before you start so it is ready the moment the BMS powers on. Search "JK BMS" in the Apple App Store or Google Play Store, or find the download link in the Documents tab on any of our product pages.
3. JK BMS Wiring Diagram — Step by Step
The order you wire a JK BMS matters. Follow this sequence to avoid sparking, BMS damage, and faults on first power on.
| Step | Action |
|---|---|
| Step 1 | Check polarity and install bus bars |
| Step 2 | Connect B- cables to the main negative (always your first electrical connection) |
| Step 3 | Connect balance leads to each cell positive in sequence, working from negative to positive |
| Step 4 | Plug in balance connectors, temperature sensor, and start button |
| Step 5 | Connect load/charger negative terminal on the BMS to your inverter or charger negative |
| Positive | Main positive connects directly to inverter/charger positive |
Step 1: Check Polarity and Install Bus Bars
Use a multimeter to confirm the polarity of every cell before touching a single wire. Do not rely on stickers, as they are applied by hand and mistakes do happen. Once polarity is confirmed, install your bus bars with the yellow insulation sheets between cells.
Step 2: Connect the B- Cables
The JK BMS comes with two wiring looms. The first loom has a black wire and several red wires. Connect that black wire to the main negative terminal of your battery pack first. The B- cables from the BMS also connect to this same main negative terminal.
Step 3: Connect the Balance Leads
The balance wires tell the BMS the voltage of every individual cell. The first red wire goes to the positive of cell 1, the next to cell 2, and so on up the pack.
For a 16S build you will use both looms. The second loom picks up where the first leaves off. Watch our wiring video for the exact wire positions on the second loom for your cell count.
Before plugging in the balance connector: use your multimeter to probe each pin from B- upward. Each pin should read one cell voltage higher than the last. If any pin reads zero or skips, stop and find the issue before going further.
Most common mistake: forgetting to connect the final wire to the main positive. Without it the BMS will not work correctly.
The unused wires in the middle of the second loom should not be cut. Roll them up and cable tie them. You may need them if you change cell count later.
Step 4: Plug In the Connectors
Plug in the balance connectors, temperature sensor, and start button. All cables are unique so there is no way to plug the wrong one into the wrong port.
Step 5: Connect to Your Inverter or Charger
The main positive of your battery bank connects directly to the inverter or charger positive. The load/charger negative terminal on the BMS connects to your inverter or charger negative, typically via a circuit breaker. The BMS sits in that negative line and controls everything going in and out of the pack.
4. Activation and First Power On
Once everything is wired and connected, press the start button. You should hear a beep and see the LED start flashing. That means it is on and working.
If your unit does not have a start button, connect your charger to the pack. The BMS powers on automatically once it detects a sufficient charge voltage.
5. Connecting via Bluetooth
- Turn on Bluetooth on your phone and open the JK BMS app.
- Tap the scan icon and select your device from the list.
- Enter the default Bluetooth password: 1234
- The app remembers the password after the first connection.
Can't connect? Go into your phone's Bluetooth settings, forget the device, and reconnect from scratch.
Once connected, the home screen shows total pack voltage, current, state of charge, and individual cell voltages. The red highlighted cell is the lowest in the pack and blue is the highest. These are just visual indicators, not fault warnings.
6. JK BMS Settings Guide
Tap Settings in the app. Enter the advanced settings password (default is 123456) to unlock the parameter screen.
Tap LiFePO4 at the top to load the factory defaults for your battery type. This is your starting point. Then set two things to match your build:
- Cell Count: change from the default of 24 to match your actual cell count (e.g. 16 for a 48V pack)
- Battery Capacity: enter your cell amp-hour rating (e.g. 100Ah, 280Ah)
Note: The values below are example starting points for a standard LiFePO4 build, shown per cell. Your app may label some settings slightly differently depending on your firmware version, and your ideal values may differ depending on your cells. If you are unsure, watch our Video or contact us before making changes.
Key Voltage Settings
| Setting | What It Does | Example Value |
|---|---|---|
| Cell OVP | Max voltage per cell before charging stops | 3.62V |
| Cell OVPR | Voltage cells must drop to before charging resumes after OVP trips | 3.55V |
| Cell UVP | Minimum per-cell voltage before discharge stops | 2.9V |
| Cell UVPR | Voltage cells must recover to before discharge resumes after UVP trips | 3.1V |
| Power Off Voltage | If cells drop this low while idle, the BMS shuts itself off | 2.5V |
| Balance Trigger Voltage | Voltage difference between cells that starts active balancing | 0.01V |
| Start Balance Voltage | Minimum per-cell voltage before balancing begins | 3.2V |
Important: Each protection value must sit in the correct order. For example, your overvoltage recovery must be below your overvoltage protection. If you try to set two values equal or out of sequence, the BMS returns a sending failure and will not save the change. Keep at least a 0.10V gap between your OVP and OVPR values so the BMS does not trip on and off near full charge.
For the full settings walkthrough including current, temperature, and timing settings, watch our JK BMS Settings Video.
7. Controls Page
The Controls page is where you turn the core BMS functions on or off. For normal use:
| Toggle | Recommendation |
|---|---|
| Charge | ON |
| Discharge | ON |
| Balance | ON |
| Emergency | OFF, this bypasses all protection settings and should only be used if absolutely necessary |
Turn Charge and Discharge off while you are connecting wires so nothing is flowing during setup, then turn them back on when you are ready.
Frequently Asked Questions
Q1. What is the default password for the JK BMS?
A. There are two. The Bluetooth pairing password used on first connection defaults to 1234. The advanced settings password to unlock and edit settings in the app defaults to 123456. Both can be changed once you are in.
Q2. Does this guide apply to 4S, 8S, or 24S builds?
A. Yes. The same wiring sequence and app settings apply regardless of what cell count or voltage you are running. Just update Cell Count and Battery Capacity in the app to match your build. All the voltage thresholds are per-cell values so they stay the same.
Q3. What do the red and blue colours on the cell voltage screen mean?
A. Red is the lowest voltage cell in the pack and blue is the highest. They are just visual indicators to help you spot the spread at a glance. They are not fault warnings and will move around during active balancing, which is completely normal.
Q4. Do I need to calibrate the voltage and current?
A. Generally no. Most setups work fine without calibration. If you measure with a multimeter or clamp meter and find a meaningful difference from what the BMS is showing, then go ahead and calibrate. Otherwise the BMS readings are accurate enough to trust as your reference.
Q5. Can I run multiple JK BMS units together?
A. Yes. If you are using multiple BMS units communicating with one inverter, assign each a unique Device Address in the settings (1, 2, 3 and so on) so they can all be identified individually.
Q6. What voltage should I charge my LiFePO4 cells to?
A. 3.65V is the commonly cited maximum for LiFePO4 cells, but always check your specific cell manufacturer's datasheet, as this varies by brand. At the very top of the charge profile the cells gain very little extra capacity for a big jump in voltage, so charging all the way to the maximum is not really necessary. A cell overvoltage protection setting around 3.62V is a practical starting point with a small safety margin built in.
