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 everything from the correct wiring sequence to every single setting in the app, all 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.

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. It is one of the more popular options for DIY solar and off-grid battery builds precisely because of how much you can customise through the app.

2. Before You Start

Before you connect anything, there are a few things worth sorting out first.

First, check your cell voltages and verify the polarity of every terminal using a multimeter. Do not just rely on the stickers on top of the cells. These stickers are applied by hand and there have been cases where a sticker has been placed the wrong way around by mistake. Always double check with a multimeter before you start connecting anything.

One thing that catches a lot of people out with LiFePO4 cells  the positive terminal is actually black. That is the opposite of what most people expect. The negative terminal has a beige or light coloured base. Take a moment to confirm which is which on your cells before touching a single wire.

Second, get the JK BMS app downloaded on your phone before you start so it is ready to go the moment the BMS powers on. Instructions for downloading it are in Section 5.

3. JK BMS Wiring Diagram — Step by Step

The order you wire a JK BMS matters. Follow the sequence below and you avoid sparking, BMS damage, and protection faults on first power on.

JK BMS Wiring Order

STEP 1  -->  Check polarity and install bus bars

STEP 2  -->  Connect B- cables to main negative  // Always first

STEP 3  -->  Connect balance leads to each cell positive in sequence

STEP 4  -->  Plug in balance connectors, temperature sensor, start button

STEP 5  -->  P- becomes new main negative  // Connect to inverter/charger

POSITIVE  -->  Main positive connects directly to inverter/charger positive

NEGATIVE  -->  P- connects to inverter/charger negative (your new main negative)

Step 1: Check Polarity and Install Bus Bars

Before connecting anything, go through every cell and confirm the polarity. Use a multimeter and do not rely on the stickers alone. On LiFePO4 cells the positive terminal is actually black, which is the opposite of what most people expect. The negative terminal has a beige or lighter coloured base. Getting this wrong would be a serious problem, so take the time to confirm each terminal before you start.

Once you have confirmed polarity, go through and install your bus bars between the cells with the yellow insulation sheets between them. Get all of this sorted before you touch the BMS cables.

Step 2: Connect the B- Cables to the Main Negative

The JK BMS comes with two wiring looms. The first loom has a black wire and several red wires. Start by connecting that black wire to the main negative terminal of your battery pack. This is always your first electrical connection. The two blue and two black B- cables from the BMS also go to this same main negative terminal. You can place the balance wire on top so you know it has a solid connection.

Step 3: Connect the Balance Leads

The balance wires tell the BMS the voltage of every individual cell. Each red wire on the first loom connects to each cell positive terminal in sequence. The black wire (already connected in Step 2) goes to the main negative. The first red wire goes to the positive of cell 1, the next red wire to cell 2, and so on up the pack.

For a 16S system you will use both wiring looms. The second loom picks up where the first one leaves off. On the second loom you only need three wires for a 16S build - the first two wires from the right side and the very last wire. Both of those last wires from the second loom go to the main positive terminal of the pack.

Most common mistake: Forgetting to connect that very last wire to the main positive. Without it the BMS does not recognise the system as complete and will not work correctly. Double check this one every time.

The remaining wires in the middle of the second loom that you do not need - do not cut them. Roll them up and cable tie them. The BMS can be used up to 24S so you may need those wires in the future.

Before plugging in the balance connector: Use your multimeter to probe each pin starting from B0 and working 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. A reversed or missing tap can damage the BMS.

Step 4: Plug In the Connectors

Once the balance wires are all connected and tested, plug in the balance connectors, the temperature sensor, and the start button into the BMS ports. All the cables are unique so there is no way to plug the wrong cable into the wrong port. The temperature sensor goes into the port on the right, the main balance loom next to it, and the second balance loom in the middle. The start button plugs into the port at the bottom.

Step 5: Connect to Your Inverter or Charger

The main positive of your battery bank connects directly to the positive of your inverter or charger. The P- on the BMS becomes your new main negative this is what connects to the negative of your inverter or charger, typically via a circuit breaker. The BMS sits in that negative line and manages everything going in and out of the pack.

If your kit includes a terminal block on the P- side, you can screw the two P- cables into that block and run a single cable from there to your inverter. This makes the connection much cleaner and easier to manage.

If you are using the 300A BMS or the ESS inverter BMS, the only difference in wiring is an extra B+ cable that connects to the main positive of the bank. Everything else wires up exactly the same.

Once the inverter or charger is connected, it is worth temporarily turning off Charge and Discharge in the app Controls page before making those final connections. This stops any current flowing while you tighten everything up. Re-enable once you are ready to go.

4. Activation and First Power On

Once everything is wired up and all connectors are plugged in, press the start button. You should hear a beep and see the light on the BMS start flashing. That means it is on and working. The BMS will now appear in the JK BMS app and you can connect to it via Bluetooth to start configuring your settings.

If your unit does not have a start button, you can activate it by applying a brief charge voltage to the B+ terminal - connect your charger for a few seconds and it will power on.

5. Downloading the JK BMS App

All JK BMS settings are configured through the smartphone app, so you need this before you can change anything. There are two ways to get it. You can visit our website, open any battery kit product page, scroll halfway down to the Documents and Videos section, and you will find the download link for both iOS and Android. Or you can search "JK BMS" directly in the Apple App Store or Google Play Store.

6. Bluetooth Pairing and Home Screen

Open the app with your BMS powered on and it will automatically scan for any nearby JK BMS units. The very first time you connect, you need to pair your phone's Bluetooth with the BMS. The default Bluetooth pairing password is 1234. Once paired, your phone remembers it and you will not need to enter the password again for future connections.

If you have more than one pack, you can rename each BMS to keep things organised. Tap the Bluetooth icon in the top left of the home screen and then tap the pencil icon next to the device name to edit it.

What the Home Screen Shows You

Once connected, the home screen gives you a live snapshot of your pack. You will see the total pack voltage, amps flowing in or out, battery power and remaining capacity, and the individual voltage of every cell in the bank. On the cell voltage display, the red highlighted cell is the lowest in the pack and blue is the highest. These are just visual indicators, not fault warnings. They will jump around during active balancing, which is completely normal. At the bottom you will also see cell wire resistance values for each balance wire, which you can calibrate in settings if you ever need to.

7. Full JK BMS Settings Guide

Tap Settings at the bottom of the app. Before you can edit anything, you need to verify your advanced settings password. The default is 123456 (six digits). You can change this inside settings once you are logged in.

At the top of the settings page you will see different battery chemistry options. Tapping LiFePO4 resets everything back to factory defaults. That is handy for starting fresh, but be careful because it will overwrite any values you have already configured.

Basic Configuration

The first two settings to get right are Cell Count and Battery Capacity. Cell Count defaults to 24, so change it to match your build for example, 16 for a 48V pack. Battery Capacity is your total cell amp-hour rating, so enter whatever your cells are rated at, whether that is 100Ah, 280Ah, or 314Ah.

Next is Balance Trigger Voltage, which is the voltage difference between your highest and lowest cell that triggers active balancing to kick in. Leave this at 0.01V. Below that is Start Balance Voltage, the minimum per-cell voltage all cells need to reach before balancing begins. Set this to 3.2V. Some people go to 3.4V but that is too late in the charge profile. Max Balance Current can stay at the full 2A since the B-Series has a built-in 2A active balancer and there is no good reason to run it lower.

Voltage Protection Settings

These settings define the safe operating voltage window for each cell. Getting these right is important because they control exactly when the BMS steps in to stop charging or discharging.

The SOC 0% and UVP values above are set to target roughly 80% depth of discharge. It is a common approach for LiFePO4 packs that balances usable capacity with long-term cell health. You can go lower if you need more capacity out of your pack, but these are solid starting values.

Current Protection Settings

Temperature Protection Settings

Timing Settings

Communication, Calibration and Other Settings

The JK BMS can communicate directly with inverters and chargers via RS485 (UART1 / UART2) or CAN bus. You only need to configure these if you are connecting the BMS to an inverter via cable rather than just using Bluetooth from your phone. Both UART ports offer the same protocol options -select the one that matches your inverter brand. For CAN, a number of inverter brands are already pre-programmed into the BMS. If you are running 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 be identified individually.

For calibration, the BMS has voltage and current calibration options. If you measure your pack with a multimeter and find it reads differently to what the BMS shows, you can enter the actual reading and the BMS adjusts from there. The same applies for current using a clamp meter. Most setups honestly do not need this -the BMS readings are accurate enough to use as your reference  but the option is there if you need it.

You can also calibrate the cell wire resistance for each balance wire at the bottom of the settings page. Again, most packs work perfectly fine without touching this. If you have an external buzzer or LCD screen connected, the alarm settings let you choose which conditions trigger an alert. If you do not have anything connected, just leave the alarm setting at off (00).

8. Control Page Settings

The Controls page is where you toggle the core BMS functions on or off. Here is a quick rundown of each one.

One thing worth highlighting on Emergency mode. When it is active, the BMS ignores every safety parameter you have configured — over-voltage, over-current, and temperature protections. It automatically turns off after the Emergency Time (set to 5 minutes), but the recommendation is not to use it unless you genuinely have no other option.

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. Why is the positive terminal on my LiFePO4 cell black?

A. This is one of the most common things that confuses people when building a LiFePO4 pack for the first time. On LiFePO4 cells, the positive terminal is black and the negative terminal has a beige or lighter coloured base. This is the opposite of what most people expect. Always confirm with a multimeter before connecting anything and do not rely on the stickers on top of the cells alone — they are applied by hand and mistakes do happen.

Q3. 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.

Q4. Why won't my JK BMS save the RCV setting?

A. RCV must always be set below Cell OVP. If OVP is at 3.62V and you try to set RCV to 3.62V, the BMS returns a sending failure and will not save. Set RCV at least 0.01V below your OVP. For example, OVP at 3.62V and RCV at 3.61V.

Q5. 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.

Q6. 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.

Q7. 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.

Q8. What voltage should I charge my LiFePO4 cells to?

A. 3.65V is the absolute maximum for LiFePO4 cells. At the very top of the charge profile the cells move a lot of voltage for very little extra capacity, so going all the way to 3.65V is not really necessary. Setting OVP to 3.62V, RCV to 3.61V, and SOC 100% to 3.60V is a practical starting point with a small safety margin built in.