[terrorr]

Series first then parallel? Or vice versa?

Ok I am designing my battery pack. - 144 volt / 150 ah

20 modules - 14.4 volt/75 ah each module

I could wire the pack as follows:

Option 1: 10 modules in series giving me 144 volts plus another 10 modules in series giving me 144 volts

Then connect the 2 144v packs in parallel for 150ah

So I think that would be written 10s2p ?

Option 2

Wire the modules in parallel in sets of 2 giving me 14.4 v/150 ah each set of 2
Connect the 10 sets of 2 in series to give me my 144 v/150ah pack.

2p10s?

Is there any advantage or preference of one way or the other? Is one more efficient?

Cheers!

 

[terrorr]

Nobody?

I want to do it right the first time so appreciate any expertise.

 

[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.

 

[MattsAwesomeStuff]

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.

 

[Duncan]

I will give a contrary view

When I first wired up my headways I did the parallel first and then series arrangement
The problem with this was that when a cell died (and some did) it killed it's neighbours so I had four dead cells

After that I used four separate strings with a "Batt Bridge" on each string

When a cell died I could immediately see it and disconnect that string

How reliable are your modules? - modules from a production car are probably reliable enough not to worry but "new cells" from one of Chinese companies seem to have a failure rate of about 5%

 

[terrorr]

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?

 

[terrorr]

After that I used four separate strings with a "Batt Bridge" on each string

When a cell died I could immediately see it and disconnect that string

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

 

[terrorr]

The modules are almost brand new. They came from a wrecked VW Egolf with less than 8000 km.

I kind of like the idea of 2 separate 144v packs that are connected in parallel. It might be easier for maintenance . Also if a problem occurs in one pack I can disconnect it and at least keep the truck running with the other pack temporarily.

However from my research it seems like lithium batts should have parallel connections made first then series.

I got this from Battery University:

"With Li-ion, the parallel strings are always made first; the completed parallel units are then placed in series. Li-ion is a voltage based system that lends itself well for parallel formation. Combining several cells into a parallel and then adding the units serially reduces complexity in terms of voltages control for pack protection."

And apparently charging the packs will be uneven with Option 1?

What complicates things in my mind is the modules are already a mix of series parallel (Each module contain 12 x 25AH cells in a 4S3P configuration) so does it matter what I do after that?

 

[terrorr]

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.

Just reread your post. Just to clarify I think you are saying each module in parallel then in series. In other words Option 2, correct?

 

[MattsAwesomeStuff]

Just reread your post. Just to clarify I think you are saying each module in parallel then in series. In other words Option 2, correct?

A hybrid between the two.

Electrically it will look more like a ladder, with a positive rail, a negative rail, and pairs of packs paralleled and then connected in series.

Also BatteryUniversity is a poor source, it's been banned for use on Wikipedia, and has many falsehoods. It's like, 90% right so it's just enough to give you confidence and then it'll say other bizarre and backwards stuff.

 

[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.

 

[boekel]

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.

Unfortunately not, welded busbars inside the modules.

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

I'd just use as many strings as needed, the modules come with bms (usable from 8-12s) so a lot is possible. give every string it's own fuse, and preferable even it's own contacter...

 

[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.

 

[boekel]

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.

It might even only have been pre-production models that had threaded terminals...

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

 

[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.

 

[BogdanT]

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)

Bogdan

 

[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.

 

[BogdanT]

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.