15s configuration help. Big 5s1p Packs.

I have some EV car batteriesthat come in modules that are 5 in series, 1 in parallel.

I decided to use them 3 modules in series to give me 15s configuration, but the complexity is getting it wired up efficiently to use a BMS across the bank, vs a separate BMS per each individual 15s string.

I am going to spend money on something like a Batrium I dont think it would be cost effective to try and monitor each parallel string separately, even though it would give me the most data as each series string is electrically disconnected from the next pack.

That said, it appears it would not be too difficult to wire each series cell to one another across the parallel banks.

My concerns are more to do with:
-Balance lead wire size between cells
-connecting balance cable cell to cell using a y cable vs each cell going directly to a bus bar.
-wire lengths (should I keep them all the same
-fusing off each cell tab at the battery for balance leads

I wanted to share some ideas and get the community opinion on how best to attack this configuration as I cannot easily (physically) group all the parallel cells together in one large pack due to them being in module format.

Considering its just a balance cable, perhaps 14 AWG to 12 AWG size wire would be more than sufficient


Here are the connections at each balance lead location, very easy to tie in!


[size=small]A better view of the module connections.[/size]
[size=small][/size]


Sketch 1 Wire each balance head from Cell 1 to Cell 1 using Y-cable at the battery (not sure if this should be fused at the point of connection) A break in the connection on one cell would leave it stranded from the rest of the pack.





Sketch 2 All series cells are wired to a bus bar and then from the bus bar to the BMS.




Option 3 Do not wire any series cells in parallel and opt for discrete BMS or monitoring per parallel battery pack.

CrimpDaddy said:

My concerns are more to do with:
-Balance lead wire size between cells
-connecting balance cable cell to cell using a y cable vs each cell going directly to a bus bar.
-wire lengths (should I keep them all the same
-fusing off each cell tab at the battery for balance leads

Click to expand...

* - Balance leads can be fairly small as they aren't carrying much current. You could use a 20swg without issues.
* - Not sure what you mean here ????
* - It would be recommended all lengths be the same to the balance boards and to the bms. This will give you accurate voltage readings
* - Not sure if fusing would be needed for the balance leads as they won't have much of a load and are not connected directly (or even indirectly) to any loads other than the balancing circuitry.

*Understood, but I was thinking of using 14 awg to give me better voltage drops across a distance.
* See pictures below (y-cable or some sort of accessory to make wiring cleaner/easier)
*Trying to see if Cell to Cell to Cell, vs Cell to Bu Bar with equal length wires for all would be idea. See Pictures
*I was more or less worried about a wiring issue during maintenance which may cause a short.









vs using a Bus Bar with each cell connecting to something like this. Each cell has its own connection independent of the other. Longmonsor similar leads would attach to the bus bar instead of the battery as I can only monitor one cell from each module anyways.

Each string (packs connected in series to get system voltage) should have it's own bms. The only way that would really be feasible and worth the effort would be where you connect all the packs that are in series together in parallel. But this would only ever really yield 1 string with variable parallel connections internally. However, the major problem with this is, if you take a pack out for any reason, then each section of parallel will not be capacity balanced with the others in the other sections.

Overall, best thing to do, is have a BMS for each string. Less work load on the BMS and you get more detailed information.

If I use everything I have, I would have 11 parallel packs in this system... 40kWh of modules in total.

That would mean I need 165 longmons from Batrium to do a 15s system if each parallel pack has its own BMS.

I wasn't sure if that was really feasible from a setup or cost standpoint.

Someone recommended I parallel all the series cells and create one big pack which are electrically connected with the cells residing in different modules. That way I could use 15 longmons to monitor the system.

That is what I was hoping to demonstrate with my illustration, but unfortunately it isnt really present the scale of this project. I literally have a pickup truck bed full of these batteries.

If I wired each cell to a bus bar, I would have 15 bus bars with 11 connections to each which would be my 11p. 15s11p wired bank.

I am really trying to get a better handle on pack design for something like this. Luckily for me, all the cells seem to be in healthy condition, and came from Samsung / Bosch so its not like I am working with unknown quality items.

Well, with that many connections, 1 longmon per cell (or section or whatever), that would really tax the longmon. I suppose you could use 2 or 3 per section. As long as they don't drift too much, you'll be fine, I suppose.

But yeah, I see what you mean cost wise overall.

I never thought about meeting it somewhere in the middle... so I guess another idea is to only connect 2 to 3 series cells in parallel at a time.

I guess my next question is how many longmons can a Batrium system handle before the cost becomes out of control? I am still new to the BMS side of things, so I need one Watchmon? What is the maximum amount of longmons on one board?

That said, I intend on connecting EVERYTHING in parallel for a couple weeks, allowing everything to balance / equalize to each other so perhaps one per 11 cells would be ok? Or is 700 amp hours too much for one large parallel group of cells? Most people with a 100P - 140P 18650 pack use 1 longmon, which is about 300 amp hours.

These cells have been sitting for a while, they hold their voltage, and the cells dont really drift much at all. Everything right now has a delta is no more than 10mV but I haven't checked them all.

I guess my last question is can you attach more than 1 longmon to monitor the SAME parallel pack as they would electrically be connected to the same battery, or would that cause a conflict?

Really my goal here is to figure out the best way to arrange these modules, but it appears the BMS part of the conversation needs to happen now too.

I think you can have over 200 longmons per Watchmon. Daromer knows the theoretical number and I'm sure he'll pipe in here soon with that info

Since everything is working on the same overall voltage, you can use one Watchmon and hook up as many longmons as you need to keep everything balanced. Since each string is connected in parallel, then they must all be the same voltage. So no issues there. You're dealing with one chemistry, so no problem there either. You're dealing with all series with the same capacity, so no problem there as well.

You really got a gold mine there overall