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How to do an 84V to 100.8V battery pack modification?


Planemo

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[Discussion split from here.

The question is: what is the right way to modify a 84V battery pack with an extra 16.8V battery pack to get a 100.8V battery pack?]

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This is interesting.

So it looks like 84v packs have been used and then 16.8v packs serially added to make 100.8. I appreciate that this doesnt sound like a good idea, but can someone enlighten me on why please. I have an idea about batteries/packs but I am tying myself in knots trying to work out why connecting external packs in series is any different to connecting them internally in 1 pack.

And no, I am not considering this in any way simply because it would be awesome to get a 100v board and upgrade my 84v... :whistling:

Edited by meepmeepmayer
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8 minutes ago, Planemo said:

I appreciate that this doesnt sound like a good idea, but can someone enlighten me on why please.

Disclaimer: I'm certainly no expert. From what I have been told...

These serially connected packs will not auto-balance and have the same voltage (unlike parallelly connected parts). So even if there is an extra BMS in the extra mini pack (I wouldn't bet on it), you'd also need an overall BMS between the 2 packs.

I guess you can very often charge to 100% and keep the charger in to get all cells to the same voltage (or hope so). But it is not professional (at least) to build it like this. There's no check on what the battery is doing.

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Cheers meep. It does sound 'wrong' but I still cant see why. Part of me is concerned that you would be using a 100v charger, but the existing BMS' in the big packs would still be 84v. How that works with the extra packs I have no idea, even if each extra pack had its own BMS although I dont know how that would work either as they are 16.8v strings...

But then is the 'voltage' of a BMS only relevant to how many outputs it has? I mean, is it really interested what the input/output voltage is, or instead is it simply looking to hit 4.2v on each string irrespective of the total charge?

Dunno, it certainly is unusal in the normal sense of how packs are put together, but on the face of it it seems all they are doing is adding extra cells in series just as you would in one big pack.

Clearly I am missing something fundamental, and I agree with you in that it sounds like its in the area of charging.

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Not sure if the BMS knows the external voltage or just supervises the cell voltage.

As I understand it, the problem is there's nothing to make sure both packs have the same voltage per cell (same internally for the second small pack which presumably has no BMS of its own). So their voltages can drift apart. The charging to 100% + balance may just be a way to prevent that (my guess).

14 minutes ago, Planemo said:

Dunno, it certainly is unusal in the normal sense of how packs are put together, but on the face of it it seems all they are doing is adding extra cells in series just as you would in one big pack.

Extra cells in series that aren't connected to the BMS. Unlike in a "real" battery pack.

17 minutes ago, Planemo said:

But then is the 'voltage' of a BMS only relevant to how many outputs it has? I mean, is it really interested what the input/output voltage is, or instead is it simply looking to hit 4.2v on each string irrespective of the total charge?

Maybe @Chriull knows more.

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38 minutes ago, meepmeepmayer said:

Extra cells in series that aren't connected to the BMS. Unlike in a "real" battery pack.

And that I think is the crux of the matter. It would seem madness not to use a BMS though, especially when the wheel appears to have a Gotway connection (according to the guy in the video).

Still very interested in how they done this.

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3 hours ago, meepmeepmayer said:

Maybe @Chriull knows more.

"Classic" BMS have per cell two fixed voltage comperators. One activates the bleeding resistor for balancing. The second cuts off the charge input (cell overvoltage protection).

So these circuits need to know nothing about the other cells and are easily cascadable.

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So are we saying that if one were to create a 4s6p pack, with its own bms system (even gotway bms' can be purchased, although I guess only 6 taps would be used) and added it serially to the existing 2x 800wh 84v packs, everything would be ok?

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1 minute ago, Planemo said:

So are we saying that if one were to create a 4s6p pack, with its own bms system (even gotway bms' can be purchased, although I guess only 6 taps would be used) and added it serially to the existing 2x 800wh 84v packs, everything would be ok?

No, you would also need a "BMS" that keeps the voltage between both packs the same, too. So three altogether: two (one in each pack), and an overall one.

Professional picture:;)

image.png

The cells in the 84V pack are kept balanced by their BMS, the cells in the 16.8V pack are balanced by their BMS, and the outer BMS makes sure the packs (cells in each pack) have the same voltage, too. Otherwise one pack could be 4V per cell and the other 3.5V or something like this.

That's how I understand it.

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I can split this discussion into a 84V->100V battery upgrade thread if you want and that's your plan. Then more knowledgable people can tell you how you can do it right. Just say so.

I think this mod may be mainly motivated because 100V Gotway BMSes weren't available then? You could also build your own 100V pack from your individual cells (I assume you can do that - spot welding etc. - if you can build your own 16.8V pack).

But maybe there's an easy modification to this 84V+16V approach that makes it "good". A simple BMS-BMS or a communication channel between the two inner BMSes. I wouldn't know.

(Probably it's easier (and cheaper?) to sell the 84V and buy a new 100V:P)

Edited by meepmeepmayer
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31 minutes ago, Planemo said:

So are we saying that if one were to create a 4s6p pack, with its own bms system (even gotway bms' can be purchased, although I guess only 6 taps would be used) and added it serially to the existing 2x 800wh 84v packs, everything would be ok?

 

17 minutes ago, meepmeepmayer said:

The cells in the 84V pack are kept balanced by their BMS, the cells in the 16.8V pack are balanced by their BMS, and the outer BMS makes sure the packs (cells in each pack) have the same voltage, too. Otherwise one pack could be 4V per cell and the other 3.5V or something like this.

That's how I understand it.

The "outer" BMS is not needed. There is no need for balancing sync/knowledge.

As it was implemented in one BMS one comperator switches the bleeding resistor parallel to the corresponding cell once it reaches 4.2V. So it gets less charging current and the other cells can catch up.

Once the cell reaches 4.28V the second comperator gets active and cuts the charge input of the whole pack.

So this is absolutely foolproof to cascade - with just one point to consider.

The cell overvoltage protection mosfet which is designed to operate and withstand 84V. Theoretically the 100.8V should distrubute between both BMS and not endanger one, but that's not a really nice/safe design. Imho.

Same for the voltage supply/circuitry of the whole rest of the BMS which is designed for 84V. No idea how and if this could pose a problem.

 

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4 minutes ago, Chriull said:

The "outer" BMS is not needed. There is no need for balancing sync/knowledge.

Ok, then I've been told wrong. Thanks!

So all you need is an extra 16.8V pack with its own BMS and a bit of luck that the BMSes still work, and you're good?

Edited by meepmeepmayer
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Cheers meep

I like your diagram and I get what you are saying. However I am struggling to understand why the '3rd' bms is needed.

The 84v pack and 16.8v pack would be serially connected to make 100.8v. The bms in each pack would not know this, nor would they need to, because as I understand it all they are doing is monitoring each string to try and charge it to 4.2v.

Charging at 100.8v into the complete setup would show 100.8v at each bms, and each bms would keep charging each string until 4.2v was seen.

Whether the 84v pack or 16.8v pack were fully charged first would make no difference? Lets say the 16.8v completed charging first (all strings at 4.2v and bms cuts off), the total voltage would still be under 100.8v so the charger will still be running, supplying the bms in the 84v pack until that also reached 4.2v on all strings. Only when at 100.8v would the charger switch off.

Not sure if I am making sense, I just dont see the need for the 3rd bms?!

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Just now, meepmeepmayer said:

Ok, then I've been told wrong. Thanks!

So all you need is an extra 16.8V pack with its own BMS and you're good?

Don't know if GW BMS have some extra logic (sync wires,..) - if so one has to adapt their logic.

Taking a 16.8V pack with a 16.8V BMS could get more dangerous for the protection Mosfet and the internal power supply?

But i'd assume the battery cells are good enough as safe voltage divider!

Ps.:

Why not take one new 100.8V BMS and distribute sense wires to the small pack!?

Maybe one could get some of this new "super duper cell voltage reporting to the phone" bms!

Its just some waste of space since the pcb is layed out to fix the cells directly - no idea if this fits inside at all....

Maybe the BMS PCB has nicely distributed components so it can be easily subdivided in 20 and 20 and 4 cells with external wiring?

4 minutes ago, Planemo said:

Whether the 84v pack or 16.8v pack were fully charged first would make no difference? Lets say the 16.8v completed charging first (all strings at 4.2v and bms cuts off), the total voltage would still be under 100.8v so the charger will still be running, supplying the bms in the 84v pack until that also reached 4.2v on all strings. Only when at 100.8v would the charger switch off.

No. The first pack to be full (one cell reached the overvoltage threshold) will stop the whole charging process of both packs!

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15 minutes ago, Chriull said:

No. The first pack to be full (one cell reached the overvoltage threshold) will stop the whole charging process of both packs!

Ok so now Im lost again! You said that the 3rd bms would not be needed but the above suggests that it does?

Why would charging stop for both packs when 1 pack is fully charged?

I know this all sounds a bit crazy and yes the simple answer would be to sell my 84v 1859 and buy a 100v 1850 but Im just thinking of options. My wheel is virtually new, it would cost me more to sell it and buy a new 100v even taking into account buying the cells/board/charger to do mine and at the end I could recoup a little more money by selling my 259wh pack to someone with a 1600wh msx.

Plus of course I would be without a wheel for 2 months if I sold mine to buy a 100v. A sobering thought if ever there was one! No wheel for 8 weeks!

 

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1 minute ago, Planemo said:

Ok so now Im lost again! You said that the 3rd bms would not be needed but the above suggests that it does?

Why would charging stop for both packs when 1 pack is fully charged?

OK - we need to go into details of how and when charging stops:

Lets go with the figures of one BMS with two fixed voltage comparators and not output overcurrent protection - 4.2V for switching the bleeding resistors for balancing, 4.28V cell overvoltage threshold.

- If the cells are misbalanced the charging will stop once the first cell will reach this overvoltage threshold of 4.28V. This stopping is achieved by a Mosfet cutting of the charge input. Since both packs are in series and one cuts the connection the other one will be cut off, too...

- if the cells are balanced enough that none reaches the 4.28V (just some reach the 4.2V and the bleeding resistors is connected in parallel to them to decrease their "personal" charging current) they get charged up to the max charger output voltage (100.8V).

In this case the charger shut cut off once the current goes under some threshold (batteryuniversity.com recommends imho some (low) x% of C?).

So one should watch out that the two packs are always in about the same charging state! Especiall the small pack has not much more charge than the big one! (But that's quite exactly the same problem one has with every pack that gets misbalanced....)

For example:

If the 84V pack is almost empty (3.3*20=66V) and the 16.8V pack is full (4.2V*4=16.8V) the charger sees 66V+16.8V=82.8V - this would be the CC phase/1 charging stage and the charger tries to deliver its max constant current (1.5-2.5A) by increasing the voltage.

So there is current forced through the already full 16.8V pack and all 4 cells will get pulled up to the cell overvoltage threshold of 4.28V and the BMS will cut off. The 20 cell pack can never be charged in this example....

 

1 minute ago, Planemo said:

I know this all sounds a bit crazy and yes the simple answer would be to sell my 84v 1859 and buy a 100v 1850 but Im just thinking of options. My wheel is virtually new, it would cost me more to sell it and buy a new 100v even taking into account buying the cells/board/charger to do mine and at the end I could recoup a little more money by selling my 259wh pack to someone with a 1600wh msx.

Plus of course I would be without a wheel for 2 months if I sold mine to buy a 100v. A sobering thought if ever there was one! No wheel for 8 weeks!

 

 

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I am not sure how they wire the bms inputs tbh. If both bms' are wired to the charge port, it wouldnt matter if one bms shut down due to all cells reaching 4.28v would it? The other bms would keep going until it also reached 4.28v on all strings?

That said, could you even feed a 16.8v bms with 100v?! Does it care? I guess its only goal is to make 4.28v on each output. Does it matter what the input is (within reason!)

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32 minutes ago, Planemo said:

That said, could you even feed a 16.8v bms with 100v?!

No! Never!

If the BMS circuitry survives short 100V peaks and the charger can reduce his voltage to this ~16V it could work for charging - but i would not bet on any of this two points...

 

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To be safe you'd probably need a 100V bms. Gotway sells these.

Personally I'd look for a 100V machine and sell the 84V after it arrives.

It's too much of a hassle otherwise. 100V control board, battery, bms, charger, motor?, lots of time lost modding. And in the end it's a pile of DIY you own which will be a difficult sell when the next upgrade comes.

Edited by alcatraz
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All points taken. It is interesting though how all of the 100v 1860 versions I have seen use long top packs...

It seems odd that Gotway wouldnt just add packs to the existing short 1230wh packs

I would not be surprised if it turned out that there was more than a few 84v+16.8v conversions out there...

Unless you hacked up the packs or measured the voltage from each pack you wouldn't know...

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Further to this, using US69's quote from the other thread 'a proper mod would be to add a nice custom 24s2p pack with 615wh....adding it to a 100V 1230wh version'...

...and seeing that on the 1860wh from ewheels (I use theirs as an example only, many others look the same) there are 2x extra packs but neither of them are big enough to be 24 cells...which would suggest either:

1. Each pack is 12 cells and they are linked across the wheel to make a single 24S1P

Or:

2. Each pack is 12 cells and they are indeed 16.8v each, in a 4S3P configuation.

Unless someone can clarify either of the above I am on the fence as to what is actually being used and indeed whether one is inferior to the other. It would certainly seem odd for whoever makes these conversions to use (in the case of 1. above) 100v top packs which are that length, as it would mean a somewhat odd configuation because although the 1230wh packs are much shorter I am not sure there is enough room to add a minimum of 24 cells to each pack to keep them 100v.

As I say, interesting.

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  • 2 weeks later...

OK, so I have now completed this conversion and it works very well indeed 😊

I used 2 ‘daughter’ packs, each 16.8v (so 4s3p) and each has their own BMS. They must be wired serially to the existing 84v packs, so I fitted a 16.8v pack to each side of the wheel to keep wiring simple and retain wheel balance.

The positive charge wires, which originally went to the XT30 plugs of the original packs, must be detached (battery side), new charge wires soldered to the plugs and then T’eed into the positive outlets of the 16.8v packs. The negatives on the original packs can remain, so I kept and reconnected the XT30’s for that purpose.

So the 100.8v charge is now sent through the entire setup. I presume the 16.8v packs actually run 100.8v capable BMS’ for this reason. I also presume that the input voltage is largely irrelevant to the BMS’ (taking aside the actual voltage limits of the components) and more to do with the fact that all the BMS is looking to do is charge the cells to 4.2v. I don’t think it’s too fussed about how many are actually connected. Either that, or they are very special BMS’. I don’t know for sure as the packs were made for me by an EUC specialist. I can’t say who as the deal was only done on the basis that I didn’t name them, as they don’t want to get involved with supplying these to the general public. There’s an awful lot that can go wrong if you aren’t 100% clear on how to hook these up, and they don’t want problems as a result. For sure, I checked and double checked everything 3 times before connecting/soldering anything. It’s a fairly nervous process but if you know how it works in your head it’s relatively straightforward.

Obviously I had to fit a 100v mainboard, and although I didn’t really need to, I fitted a 5pin GX16 just to keep it all as it should be had it been factory supplied. Whilst I was in there, I also fitted an XT90 main board plug (original was XT60) and ran 14AWG silicone (same as on the motherboard) back to where the two battery halves link up. Oddly, the GW wiring is 16AWG throughout, even after the 2 sides join. It’s only after the main plug that it switches to 14AWG. So GW run 100.8v from all packs along 16AWG wire right up until the main board plug at which point they run 14AWG. It seems a bit of a bottleneck the way GW do it, but I presume they know what they are doing. I only uprated the wiring because I had everything out and it never hurts to run beefier wire.

Throughout my research on doing this conversion, I gathered as much information and pics as I could on 100v 1860wh MSX's. At this time, for reference, I am 99% sure that any 100v 1860wh MSX sold is done in the same manner (serially). The only way to be 100% would be to measure the output voltage on a disconnected daughter pack but I am quietly confident it would indeed be 16.8v.

 

Cheers

PS pic below was taken during test process, prior to final wire securing. I didn't take a pic of the other side, but the packs are identical.

16_8v.thumb.jpg.b4b573f1adc04d9aa62c03edd2a4f9dd.jpg
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