JK BMS Wiring Instruction Confusion -- Help needed


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TheHappyNomads

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Quick wiring help question. I've removed all the temp sensor leads and pin connector from the old BMS so that all I have are the soldered leads waiting to be spliced into the JK BMS leads.

Each battery pack will consist of three of these modules (2 pos \ 1 neg). I've highlighted where each cell lead is in yellow, with the final cell highlighted in blue (although there are two leads spliced into each of the blue soldered joints?). The straps must've been where each POS pack was connected in series to the NEG pack, yeah?

I find the JK instructions difficult to wrap my head around tbh. Where's the best place to start? Should I begin with one POS pack? How do I know which lead is the main battery +\- with these? Do I just pick one of the + \ - terminals and make it the main and wire from there?

Thanks!
 

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It looks like you've marked a main + and main - in the pic above and if these are LifePo4 then you have 16 in series.... What's you're hesitation in doing this?
I was just marking them for reference in the picture. There will be a third unit and so my hesitation was if there was a right\wrong main +\- of if I just pick one for use as a main +\-

My other hesitation was that I don't fully understand the wiring instructions that came with the JK BMS. Just wanting to do my best to do it right since this is my first battery build is all.
 
I was just marking them for reference in the picture. There will be a third unit and so my hesitation was if there was a right\wrong main +\- of if I just pick one for use as a main +\-
Assuming these are LifePo4 and you're doing 16s for 48v nominal, then the original pic with 2 x units would make 'a battery' and I would start at the - and then hookup 16 additional wires... you can measure them as you hook them up or after you've hooked up the wiring harness but before plugging into the BMS. You should see the voltages increment on each wire. Let's say the battery is at 3.3v/group-of-parallel-cells, then from the negative wire you should see 3.3v, 6.6v, 9.9v, ..... 52.8v on the 17th overall wire.

IF these are 3.0 - 4.2v INR type chemistry then it's the same thing but only 14s and you'll have 2 x groups-of-parallel-cells left over. You can skip these or use them in the next 2 x units and you'll have 4 x groups-of-parallel-cells left over etc....


My other hesitation was that I don't fully understand the wiring instructions that came with the JK BMS. Just wanting to do my best to do it right since this is my first battery build is all.
I'm sorry I'm not familiar with JK BMS.

You mention 3 units but the way I'm imagining this, you would only use 2 x units (8s each = 16s or use 8s from 1st and 6s from the 2nd = 14s) to make a battery. So you need the units in pairs either way.
 
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Assuming these are LifePo4 and you're doing 16s for 48v nominal, then the original pic with 2 x units would make 'a battery' and I would start at the - and then hookup 16 additional wires... you can measure them as you hook them up or after you've hooked up the wiring harness but before plugging into the BMS. You should see the voltages increment on each wire.
I'm assuming I will also need to connect the modules together in series in addition to all the BMS leads, right? I was thinking running a cable from each of the terminal straps to the next. Should I use SS nuts+bolts or is there a better material for this?

Let's say the battery is at 3.3v/group-of-parallel-cells, then from the negative wire you should see 3.3v, 6.6v, 9.9v, ..... 52.8v on the 17th overall wire.
You are correct, this is how it's currently wired.

IF these are 3.0 - 4.2v INR type chemistry then it's the same thing but only 14s and you'll have 2 x groups-of-parallel-cells left over. You can skip these or use them in the next 2 x units and you'll have 4 x groups-of-parallel-cells left over etc....
Correct, these are LifeP04 and yes my aim is 16/18s 48v/54v nominal

If these are 6s on each module wouldn't I need 3 packs together to get the 18s (leads) I would need to hit the 54v?
 
I'm assuming I will also need to connect the modules together in series in addition to all the BMS leads, right? I was thinking running a cable from each of the terminal straps to the next. Should I use SS nuts+bolts or is there a better material for this?
Personally, I use nuts+bolts in my powerwall to connect things and they work well.

Correct, these are LifeP04 and yes my aim is 16/18s 48v/54v nominal

If these are 6s on each module wouldn't I need 3 packs together to get the 18s (leads) I would need to hit the 54v?
Yes (my bad - I counted wrong in the pic) you'll need 3 packs to get 16s or 18s.

16s is the standard LifePo4 configuration to accommodate common LifePo4 BMSs and the general range of 48v nominal equipment. 18s can be done if the BMS and equipment tolerate the higher operating voltage range but can severely limit you're choices in equipment.

There's no 'right answer' except to make sure that if you do 18s the equipment you purchase matches up to the higher 18s voltage range.

FYI - It's also OK to go 16s and leave 2s unused.... for the sake of being 'more normal' in terms of equipment. I know that sounds bad but then I'm in my 7th year and my perspective has matured to take into account overall operations and equipment replacement and just wanted to confirm it's OK if that helps you. :)

For example, if you go 18s it will tend to 'lock you in' and you won't be able to parallel in standard 16s LifePo4 batteries in the future where the price is getting pretty good these days. For example, I have 20% extra panels / wasted PV in summer so that I have more in winter. These are examples of things I'm thinking of when I say it's OK to 'waste capacity' (2s in this case) a bit to maintain a better overall, long term system design/operation. Later on, when you have a working system, maybe you can re-visit the 2s 'unused' packs and figure something out for them.
 
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There's no 'right answer' except to make sure that if you do 18s the equipment you purchase matches up to the higher 18s voltage range.
Or, you only charge the cells to max working voltage of the equipment. This may be lower than the cells datasheet voltage, but that's fine. This will help increase the longevity of the cells of cycling and years of run time.
if you go 18s it will tend to 'lock you in' and you won't be able to parallel in standard 16s LifePo4 batteries in the future where the price is getting pretty good these days.
However, this is the big point here. You don't want to lock yourself out of future upgrades, or drastically limit your options.
 
FYI - It's also OK to go 16s and leave 2s unused.... for the sake of being 'more normal' in terms of equipment. I know that sounds bad but then I'm in my 7th year and my perspective has matured to take into account overall operations and equipment replacement and just wanted to confirm it's OK if that helps you. :)

For example, if you go 18s it will tend to 'lock you in' and you won't be able to parallel in standard 16s LifePo4 batteries in the future where the price is getting pretty good these days. For example, I have 20% extra panels / wasted PV in summer so that I have more in winter. These are examples of things I'm thinking of when I say it's OK to 'waste capacity' (2s in this case) a bit to maintain a better overall, long term system design/operation. Later on, when you have a working system, maybe you can re-visit the 2s 'unused' packs and figure something out for them.
Insightful for sure -- thanks! Does it matter which 2s I leave unused or just keep them uniform for each pack?
 
I'd keep them uniform in each pack and the last 2s would be the spares or for future plans. If you put those in the middle somewhere, there could possibly be caused issues of some sort. Especially if you decide to use them and need to run wires across the other packs. Would make a mess and increase chances of possible accidental shorting out (well, I suppose it could be purposefully shorting out, but that's probably not a good idea :p )
 
For sure, I hear you on that. Is the "split cell" on either end the 2s you would recommend leaving out of the 16s scheme for these?
 
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Using this image as an example, and the Neg terminal in the upper right, I would leave out the bottom right 2s (which would mean to split that middle pack.
This is because I would suggest having the Neg and Pos external tab on opposite sides of the whole battery. So with the above orientation, the Neg most packs would be on the right side of the whole battery, and the Pos tab on the left. This would put the connect tabs at a distance from each other and you can do some interesting things in your cabinet build. Make one side the Pos bus bar and the other the Neg bus bar, to connect other batteries to, expanding your storage very easily

Or ultimately take the packs completely apart out of the housing and then rearrange them in the correct way to make things easier. For instance, just have a whole pack that's shorter than the image, removing 2 parallel packs from the groupings
 
A couple of interesting updates and a couple new questions as I finally had a few minutes to start back up on re-wiring these for my system.

#1 The terminal straps and/or buss bars blocked some very helpful +/- markings on these packs :LOL:

#2 Speaking of terminal straps and/or buss bars -- I tried taking one of them off and I cannot for the life of me get them to budge. Factory install must've used some permanent thread lock to keep them from moving. So, now I've got to figure out how to connect these together either with the new leads via the straps or something else.

#3 I had been wondering why they had, what seemed to be random, kapton tape on some of the binding straps but then as I looked closer they used the tape as another set of indicators for polarity of each unit o_O as you can see in the pictures (the tape on the terminal straps is mine)

#4 I've learned that battery cable lengths being uniform is important. Is there an ideal length to stay within when I connect three of these in series to create a single 48v pack?
 

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