[brian_]

Every production EV pack layout that I have seen connects the required number of cells in parallel first (at the lowest level), then connects those groups in series. This ensures that each group gets the same current (which wouldn't happen with your Option 1), and means that the BMS only needs to monitor one set of cell groups.

Unfortunately, you are working with modules which already connect cells in series. No manufacturer puts modules like this in parallel in production EVs, because they size the modules to suit the vehicle so the modules only need to be connected in parallel.

I have no suggestion about what to do if you cannot internally reconfigure the modules to be suitable for your desired total capacity.

 

[brian_]

Unfortunately, you are working with modules which already connect cells in series.
...
I have no suggestion about what to do if you cannot internally reconfigure the modules to be suitable for your desired total capacity.

Well, it took a while, but the light bulb finally went on...

The eGolf cells are metal-cased prismatics with threaded holes (for bolts) in the terminals. Unlike pouch cells with welded tabs, it is reasonable to re-configure these cells by building new interconnecting cables (or straps). So you can - if you want - connect all 240 of the cells so that they are first connected in parallel (groups of 6 cells), then 40 of those groups are connected in series.

I don't know what the BMS plan is, but at least some of the original VW BMS may be reusable (with wiring work), although it would be handling 150 Ah groups of 6 cells rather than 75 Ah groups of three cells. It would be seeing less than half a pack, but with double the Ah capacity which it expects.

You should even be able to re-use the module cases, still containing the same 12 cells each, but connected as 2s6p rather than 4s3p.

 

[brian_]

Unfortunately not, welded busbars inside the modules.

only (some?) VW E-Up modules had threaded studs and can be changed.

That's unfortunate. All of the images that I have of the cells in this battery show threaded terminals, but they might just be showing a generic sample of the cell series rather than the actual cell variant used. I would want to open a module and have a look, if practical, just in case.

 

[brian_]

... always use these cells under compression, they will swell when not confined (if someone does re-arrange them)

That's one reason to keep the original module cases, even if electrically reconfiguring them.

 

[brian_]

I would connect each pack in parallel with another, and then connect those twin packs in series.

So, not just the 2 ends of the 10s string, the 2 ends and the 9 pairs of internal wires too.

So would this be Option 1?

I have the pos and neg ends of the 10s string, got that, that is the end of the pack. Sorry I don't understand, what would I do with the other 9 pairs?

This was illustrated in another discussion:

I've reconfigured a Volt pack into two parallel strings of 3 45V modules and 1 22V module (2 separate 42S3P strings).

Is there any advantage or disadvantage to making parallel connections between the two strings at the module terminals?

See attachment.

The cell counts in the modules are different, but the idea is the same.

 

[brian_]

Except swoozle has a 6S module (22.5v) there... Which will basically get destroyed if it's paralleled at module level with the 12S (45V). (option2)

As swoozle already noted in the thread where the diagram was posted, each module is in parallel with an identical module, so there is no problem (no 6s in parallel with 12s). Of course in terrorr's situation, all modules are identical (all 4s), anyway.

 

[brian_]

So Duncan are you saying Option 1 with a batt bridge between the two packs?

The Batt-Bridge monitor:
Lee Hart's Batt-Bridge Battery Balance Alarm
Batt-Bridge "Battery Pack Balance Monitor" (wiring diagram & instructions)
[EVDL] Lee's Batt-Bridge

I believe that Duncan is suggesting Option 1 with two Batt-Bridges, one for each of the two packs. The "Pack+" and "Pack-" tap points for the two Batt-Bridges would be the same, but each Batt-Bridge would connect to a "Pack Center-tap" in a different string of modules (between the fifth and sixth module of each string).

You can't compare the state of charge of two parallel strings with a Batt-Bridge, since it only looks at voltage and two things in parallel inherently have the same voltage across them.

 

[brian_]

Sorry guys, I swear I saw a 6S in parallel with a 12S. But there's no such thing, so I should probably stop posting before I have my coffee in the morning.

Been there, done that!

 

[brian_]

A thread about reconfiguring Volt packs and this one about reconfiguring a Volkswagen e-Golf pack are getting mixed up; blame me for that, as I borrowed swoozle's wiring illustration from the Volt thread for use here.

Although the e-Golf pack has many more modules, and the modules are consistent (unlike the Volt), the rest of the considerations are similar.