Paralleling packs/size


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Rasmusm88

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Mar 2, 2022
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Hi all

Just a question I can't wrap my head around.
I currently have 14s200p, 400ah packs.
What happens when I wanna add another 14 packs to the system. I figured I was gonna do it like the drawing here (yes only 4s, too lazy :) )

The next 14 packs I'm gonna make is with much better cells and higher Ah cells, from e bikes.
I believe my next 14s200p will be around 480-500ah maybe.
Can these be paralleled with no issue, or will I have to take out cells from the packs to match the first 400ah packs I've build?

Thanks
 

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Oleksii

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Mar 18, 2020
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My personal decision - interconnection between each cell in string (4 strings, 14S), using the same single BMS for all them all.
So, sort of P is increasing, while S is staying the same.
 

Korishan

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You can connect up the way you show in your diagram. That is perfectly fine. The biggest issue is the BMS connections. Depending on which kind of BMS you go with will depend on how you wire up the extra connections.
If you have Batrium, for example, then each pack will have its own longmon and any cross connections will not be needed. This would be the same for diyBMS.
But if you have a unit that has a bunch of wires running from the main BMS unit that is to be connected between each pack, then you might want to consider one of two options: 1) Buy a 2nd BMS to handle the 2nd string, or 2) interconnect each adjacent parallel group with the neighbor in the other string. So String1Pack1 would have a wire going to String2Pack1, String1Pack2 to String1Pack2, etc. Then one BMS can handle both string
However, this has a very real limit, that each parallel string added to the system decreases the ability of the single BMS to effectively balance directly by dividing the number of strings connected.
So if it can do a 5A bleeding/balancing, then 2 strings connected means it can only 2.5A per string. Still 5A total, but it's spread out further. In a 200p, 5A would mean 0.025A per cell. In a 400p (two strings connected) it would be 0.0125 per cell. You see the issue here

For starters to learn the system, go with 1 BMS and connect up. And monitor everything closely. Then add a second BMS at a later time. Unless of course you already have a BMS that can be expanded upon like Batrium or diyBMS or similar.
 

Rasmusm88

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Mar 2, 2022
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Hi all
Thanks :) yes I am running batrium with the new cellmate k9, so I will order a new k9 unit for the second strings.
The main problem I couldnt figure was if having 14packs at 400ah and another 14packs at 500ah would make them go out of balance, or drain the first 14 at 400ah faster than the other 14 at 500ah, but I guess not when both strings share the same voltage
 

Korishan

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The main problem I couldnt figure was if having 14packs at 400ah and another 14packs at 500ah would make them go out of balance, or drain the first 14 at 400ah faster than the other 14 at 500ah, but I guess not when both strings share the same voltage
Being the same voltage they will deplete at about the same rate. The only time you'll notice a difference is under extremely high discharge, but even this would be small difference.

I am running batrium with the new cellmate k9
Wasn't aware of the cellmate-k9. Looked it up. Dayem!!!!!! Them things are freakin expensive!! 😲😲😲😲😲 (topic for another thread)
 

Oleksii

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Hi all
Thanks :) yes I am running batrium with the new cellmate k9, so I will order a new k9 unit for the second strings.
The main problem I couldnt figure was if having 14packs at 400ah and another 14packs at 500ah would make them go out of balance, or drain the first 14 at 400ah faster than the other 14 at 500ah, but I guess not when both strings share the same voltage
You are advised to follow these threads to understand what happens if 2 cells are connected in parallel:

Logic is pretty the same - 2 single cells or 2 packs (multiple P).
 

Wolf

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Sep 25, 2018
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In my case I have (4) 14s80p batteries. All 4 have individual circuit breakers. They are then tied to the same bus.
This allows me to take a single 14s80p Battery out of service for maintenance if it is ever required.
In my case Batrium is monitoring every pack so I have 56 longmons.

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|Additionally my packs & batteries are fairly well balanced with only the first battery build (Frankenstein) being ≈ 20Ah lower in capacity than the rest. In the whole scheme of things my max Amp discharge is ≈50A so I am drawing at max 0.156mA from each individual cell That small difference really doesn't affect the whole battery.

I was myself initially also considering 100p 150p and I think my first prototype was 204p but after considering the bulk and what ifs I decided to go with 80p. I commend you on your 200p packs I on the other hand am getting too old to lift such heavy packs.:oops:
So yes you can put the batteries in parallel as long as the Ah of each battery is not considerably different. Of course the proper answer is they should be the same or very close to it. However in the real world there are no such things esp. with used cells.

I personally would not tie each new pack to the corresponding older packs i.e. Battery1,pack1 + Battery2,pack1 Battery1, pack2 + Battery2, pack2 and so on as I am not sure how well your Batrium K9 would handle the potential balancing current that may be required.
For that reason I went with the Watchmon4 as the balancing is done by each individual longmon.
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Wolf
 
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floydR

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Aug 23, 2017
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Lets face it the Batrium is exspensive but The Wmcore + 2 K9s is only $38.09 USD more than a Wmcore and 14 longmons when bought in a bundle. The k9 cell monitor is the least expensive cell monitor that Batrium makes.
$521.32 USD Wmcore +14 longmons vs $559.41 USD Wmcore + 2 k9
each additional 14 longmons 281.39USD or K9 $166.16USD when bought separately
later floyd
 

Korishan

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Lets face it the Batrium is exspensive but....
Topic for another thread ;) Keeping this thread on topic, which is discussing layout and connecting options. Not pricing of the BMS's
 

OffGridInTheCity

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In my case - I vary the cells per pack so they all have 260ah (+/- 2ah). Some packs have 120cells and some have 88cells - but they are all 260ah worth of cells. I'm up to 8 batteries of 14s in parallel = 112 Batrium Longmons - and all plays very nicely.

My point is - if you have higher capacity cells - it's OK to continue with the original pack size and just use less cells - leave empty slots. Just add up the ah of each cell till you get to the ah of the original packs. +/- a few ah doesn't matter much at low stress.

The other comments above are great as well!

I'm running a .05v max difference at the moment - perfectly normal for my operation. I vary between 40mv and 70mv max difference within the 3.5v -> 4.0v / pack operating range.
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Korishan

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ome packs have 120cells and some have 88cells
Yeah, but this would cause large imbalance during heavy loads. Pull 100A from the battery, and the 120 cell packs will only see 0.83A/cell, but the 88 cell packs would see a whopping 1.14A/cell. How would you cover this situation?? Or do you just make sure that battery pack doesn't ever see high current loads?
 

OffGridInTheCity

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Yeah, but this would cause large imbalance during heavy loads. Pull 100A from the battery, and the 120 cell packs will only see 0.83A/cell, but the 88 cell packs would see a whopping 1.14A/cell. How would you cover this situation?? Or do you just make sure that battery pack doesn't ever see high current loads?
It's a good point but that's why I said low stress above. Pulling >1a from laptop cells is not low stress from my view. My powerwall's average charge/discharge is <150ma / cell and powerwalls in general are more oriented to low stress (larger capacity). I would say up to 500-600ma/cell would be OK for laptop level cells.
 

Rasmusm88

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Yea that was why I'm curious on this. If I leave out cells / empty slots that would also be okay I think, since they will all be higher discharge rated cells from ebikes. But ye, my max draw atm is 0.5A pr celle, adding another 14 packs would be 250ma hmmm

I could try make a small test some day when I get around to it using 2 x 4s5p for example and make a discharge test with my iCharger at my max discharge rate. Maybe that would indicate something
 
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Rasmusm88

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Hi all
So to continue this topic, I made a small 4s pack to test the concept of different sizepacks (ah) per string.

Front row is 2150mah ebike sanyos, next row is 2900mah Panasonic 3rd row is 3450mah LG cells. So decent spread in capacity, each series is about equal around 50mah difference in capacity.

Matched all the cells voltage to 3.70v before making the pack. Both charge and discharge at 1.5a / 3 = 0.5a draw from each string, that is my teoretical max draw per cell on the big packs.

Charged to 4v, difference was within 0.005v then discharged to 3.5v still within 0.008 iirc.
That could indicate that different size strings is ok, as long as they share the same voltage. Atleast in this test :) nothing the batrium won't be able to handle I believe

What do you think?
 

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