How we test and replace a BMS on a battery.

[Sebz]

This thread will not cover the opening of the pack it covers all the possibilities you may face when replacing a BMS. It's just to explain how we do it. Playing with exposed cells can be dangerous so it's recommended to work in a open work space (like a garage) where you can react quickly in case you short out something. So be careful, read on li-ion safety have an exit plan incase something happens.

When I work from home, I personally wear safety glasses, next to a large empty ammo box (with vent holes) I keep a large silicon flexible mat next to me (they are cheap on aliexpress) that I can use to cover the pack and grab the pack and drop it in the box, lock it and get it out the door within seconds. I also have a large fire extinguisher. Safety first folks. There are great articles on battery safety here on the Battery university website . And of course this is all at your own risks..

But if I can reassure you, problems occurs generally during charging or discharging, not while standing still after a long rest (room temperature battery). Standing still is the safest condition for a battery so the real risks are mis-handling it while testing it, physical shot/impact and such. If you have doubts leave this to the expert at your local battery repair shop.

IMPORTANT: Before we go ahead and unsolder/cut wires, we make sure you unplug the white balance harness for a few seconds and plug it back in. This is called a hard reset and can possibly fix a BMS issue when sometimes a soft reset (jumpstart with a charger) failed!!!

ALSO we to test all the BMS sense wires, make sure they are all equal voltage (or close). Changing the BMS will not help a pack with a low voltage cell group.

Please watch this video :

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How to open a soft pack click here
Hard case batteries only have a couple screws to access it, no help needed!

We will cover 3 different situations over the next few posts below:
1) Your BMS is still made and it's plug and play (with bullet connectors on the 3 power wires) and the white sense plug with 10,13,14,15,16,20....wires and up is also the same.
2) Your BMS is still made but the power wires are soldered directly.
3) Your BMS is obsolete and the white sense plug does not fit....

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Situation Home A




As you can see the replacement BMS came with connectors on the blue (P-) yellow (charger -) and black (B-)

Start by removing the white sense wire plug (balance lead), do not pull on the wires but use a screw driver like this and try to move it a little and go to the order side and repeat. Pull on the connector with your other hand, not the wires.

Next unplug the blue yellow and black wire (your may differ in colour).




Remove the BMS and put the new one in, plug the power wires first :B- and P- then CH- and at last the white sense plug.

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Situation 1B

Your new BMS has the same connector same amount of wires but the polarity is wrong. Your Starts with the positive (B14 or B13) and goes to negative (B0) and the new one is the opposite.

Solution number 1, you do a complete rewiring (see step General below)

Solution number to try to modify the connectors and flip the BMS

See this youtube video for more details:

[Sebz]

#2

Your pack is a few months old and it does not have the quick connects.

You have 2 options here, either you unsolder and resolder the wires on the BMS directly or you cut the wires and use a good quality butt splice and good quality crimper.

PS: I would not recommend soldering if you have no experience solder directly on PCBs (printed circuit board) it's not that difficult but it's time consuming if you don't have the right soldering iron tip or temperature. need solder pump, flux, tip cleaner... Karl already covered that part here, and did a really good job!


So we will cover the crimping part only!

First get some heat shrink butt connectors, you will need 10-12AWG for the P- and B- wires and the charge port wire is 18AWG so you will need 18-22AWG butt connectors (some may vary but most are 18)

10-12AWG
18-22AWG
Or a kit that covers 10-22awg
If your pack has a switch on it (like a dolphin, killer whale, slimline, jumbo shark) you will need to crimp theses 2 wires, they can be 22awg, 24awg.
Quality crimper
You will also need a heat source : lighter, torch, heat gun....
Wire cutter and stripper.

Also most BMS uses the same nomenclature,
B- is the wire soldered directly to the first cell on the negative (ground part)
P- is the negative wire going to the discharge port (case - terminal or black XT-90....)
C-, CH-, CHG- is the charger wire going to the XT-60 or the barrel plug, XLR or whatever you have!
Some have 2 little wires that does to a switch,
There is a temp sensor on all Luna BMS, so no need to cut or crimp just put it back were is was on the cells.

First step is to unplug the big white sense plug then cut the wires one by one, tape over them so you won't create a short any were, remove the old BMS.

Make sure you cut at the right place because the butt connector will be bigger and not flexible, so do not cut on a corner, only cut were you have enough place to put a crimp. Like this photo, plenty of space around here.



Next part is strip and crimp and heat shrink, so many please to look on how to do this : click here OR here

Crimp the power wires first; B- and P- then the CH- and at last the white sense plug.

Once done plug back the white sense plug and test the voltage out of the discharge and charge ports to see if you have the right voltage. Also you could plug in your charger to test it before closing everything up.

You are done!

[Sebz]

#3

Ok, you are out of luck, you can't find a direct replacement, the size is fine but the white sense plug (balance connector) is too big/small and/or has a additional wire or 1 wire less.



Let me explain first why it's possible that your 13S (48V) can have 13 wires or 14 wires and your 14S (52V) can have 14 or 15 wires.

The BMS balance harness always start with bank one positive on one side and the last bank (bank 13 or 14) on the opposite side.

The discharge and charge positive out of your pack always comes off directly from the last bank positive (not passing through the BMS) you'll also see a sense wire coming off that same positive.
--Meaning the main positive discharge wire and main positive charge wire are welded on the last bank positive (these 2 wires are not wired to the BMS.) at that same spot (weld) there will be a sense wire (B14 or B13) going to the BMS

That will be the last bank sense wire so the opposite side of the harness will be bank one positive (Called B1) But it is possible that there is an extra wire (can be black), that first sense wire opposite of the last bank sense wire will be bank one negative (B0).
--Meaning if you have a 48V batt (13s) there will be a B1 wire going to the positive of the first cell. That wire can be the first on the harness, but it can also be the second one. There are some BMS with a B0 or B-. That sense wire is linked to the B- (ground)

So that's it some BMS have a balancing lead to the bank one ground (B0) to measure and balance, some will just use the common ground attached to the BMS.



Diagram of a BMS with a B0 wire, you can wee it on the bottom right corner:



Here is a BMS (16S) without a B0 wire, it uses the battery ground wire instead.



How do I know what type I have:

• 48V: 13 wires = no B0, 14 wires = B0
• 52V: 14 wires = no B0, 15 wires = B0

Lets throw a curve ball....

Some 52V BMS (14s) can have 16 or 17 wires??? Well that's because some 16s BMS (60V) can be used as a 14S BMS! Normally they are larger and may not fit, please consult your supplier to be sure they can be used as 14S.

[Sebz]

#3 continued:

So you found a BMS but the white plug does not fit, you have 2 options in front of you:

• You replace the full balance harness, meaning you will have to fully unwrap the cells and unsolder the shite wires and resolder the new ones.
• You cut the wires on your pack, cut the plug on the new BMS and use tiny 26awg butt connectors to merge the 2.

Part A : Soldering:

You will need a good soldering iron (40w to 60W), tip cleaner and solder (with flux core if possible). Kapton tape (highly recommended, electrical tape is not good for that). and a multimeter.


You will have to do one wire at a time, just to make sure you don't mix up the wires... start by the last one (B10,B13, B14 or whatever you have), Look at you new BMS it should be written on it, mark the last wire with a black marker if necessary. Take you current pack and look on the BMS to locate the B14 (or else) test with a multimeter between the battery common ground (B-) and the B14 to see if you have the full voltage of the pack.

Take you iron and heat up the connection and quickly remove the old B14 wire and put the new one in the melted solder. If you need you can add a little more solder. Do not heat it too much, cells don't like heat!!!



You can move to the next one...and on till you reach the B1 or B0. Do not plug the balance harness in the new BMS yet.

Once the balance harness is swapped you will have to change the 3 power wires, CH-, B- and P- using the same procedure

Solder the power wires first; B- and P- then the CH- and at last the white sense plug.

B- is the wire soldered directly to the first cell on the negative (ground part)
P- is the negative wire going to the discharge port (case - terminal or black XT-90....)
C-, CH-, CHG- is the charger wire going to the XT-60 or the barrel plug, XLR or whatever you have!

Check that you have all the wires in the right place, test all the banks with your multimeter:
Put the negative multimeter lead on the common ground on the cell (it's were the big gauge (normally black) wire if soldered to and use the red positive lead to measure the DC voltage starting from B1 connection you just soldered, you should have between 3-4.2V (depending on the state of charge). Leave the negative tester lead to the ground and move the positive to B2 solder joint, you should have exactly double the amount so if you were at 4.0V, you are now at 8V on B2. Test all the other B's till the end if the voltage goes down from the previous wire it's because you messed up the wiring...

Once ready plug the balance harness on the pack and test the voltage out of the discharge connector, the charge connectors and do a quick charger test to see if all is working fine. but the pack back togeter and you are done.

[Sebz]

Part B

You Have a new BMS with a balance harness but you don't want to solder on the nickel strips and solder directly to the BMS board?

Note: If your new BMS has a B0 wire and the previous one had none you will have to solder it to the B0 battery nickel strip, there is no work around on that. How to find the B0 nickel is pretty simple, follow the B1 wires and look on the opposite side of the pack, you should see the ground 12awg wire soldered there, you can solder the B0 wire not too far from it.

No problem, you can use regular heat shrink crimping butt connectors like we showed you before but you could also use crimp less heat shrink solder butt connectors... all you need is a kit like this, Click here . ON this tutorial I will only cover the "Crimpless" method.

• You will need a heat gun or a hobby torch or a simple dollar store torch lighter.
• You will need a wire striper.
• Some Butt connectors, wires gauges are 12awg, 16awg and 22-24awg for balance and switch (if applicable).

Next to is learn how to do it, take a few wires and try it a few time before trying to do this procedure. Take a look at the video, it will show you how easy it is to do it.

Your results may vary, but this video was my first try (actual 3rd, because I did the 2 other right before)

Here you have:

12AWG
18AWG
26AWG (or 28AWG can't see that small)

It's really solid and waterproof too once done.

Next the battery pack:

You will have to do one wire at a time, just to make sure you don't mix up the wires... start by the last one (B10,B13, B14 or whatever you have), Look at you new BMS it should be written on it, mark the last wire with a black marker if necessary. Take you current pack and look on the BMS to locate the B14 (or else) test with a multimeter between the battery common ground (B-) and the B14 to see if you have the full voltage of the pack.

Take your wire cutter and cut the Last B wire, cut were you will have enough space to have a crimp, do the splice and make sure it's perfect and no wire strands actually pierced the insulation.

Repeat with the other B wire till you reach the last one.

Check that you have all the wires in the right place, test all the banks with your multimeter:
Put the negative multimeter lead on the common ground on the cell (it's were the big gauge (normally black) wire if soldered to and use the red positive lead to measure the DC voltage starting from B1 connection you just soldered, you should have between 3-4.2V (depending on the state of charge). Leave the negative tester lead to the ground and move the positive to B2 solder joint, you should have exactly double the amount so if you were at 4.0V, you are now at 8V on B2. Test all the other B's till the end if the voltage goes down from the previous wire it's because you messed up the wiring...

Once ready plug the balance harness on the pack and test the voltage out of the discharge connector, the charge connectors and do a quick charger test to see if all is working fine. but the pack back togeter and you are done.

[Sebz]

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