Adding 2nd set of 7 module

[DaveTM]

I think I'm attempting to say the same thing. All I'm saying is before an install, you want the "old pak" and the "new pak" to have as close to the same voltage (preferably each cell) as possible so the BMS does not get over heated or over worked when balancing.

But...maybe that's my non-engineer brain that's telling me this.

[Sergio]

Setting up all cells to the same random intermediate voltage is not balancing.

A 100AH battery and a 60AH battery will have the exact same voltage at their 50% capacity.

The issue is that when you wire and charge then back in series, you are adding the exact same amount of AH to each battery.

So after 30AH are returned the batteries, the 60AH would be full, but the 100AH is only at 80AH capacity, so your BMS will work overtime trying to correct the imbalance by continuously bleeding the lower capacity cell.

That is why I said:

Since there is a good possibility the new pack will not be an exact match in capacity, the top voltage is the best method to reduce the load on the BMS and get the entire new pack in balance.

You can only really balance a pack with slight different capacity cells at the top or at the bottom since those are terminal states that can be reproduced.

Attempting to equalize the voltage at a middle point will have the BMS bleed the voltage from the higher capacity cells during discharge and then bleed the voltage from the lower capacity cells again during a charge cycle, so basically the BMS can not get out of its own way.

Amazingly some folks recommend that method for their BMS setups.

[DaveTM]

Thanks Sergio. Now it makes sense....more sense (better sense?) to me anyway.

[cricketo]

Originally Posted by Sergio

Attempting to equalize the voltage at a middle point will have the BMS bleed the voltage from the higher capacity cells during discharge and then bleed the voltage from the lower capacity cells again during a charge cycle, so basically the BMS can not get out of its own way.

Amazingly some folks recommend that method for their BMS setups.

Can't quite recall - did I have "an argument" with you about internal cell resistance, or was it somebody else ? Either way, I yield to your wisdom :)

[bronsonj]

How so? A 60AH and a 100AH in parallel won't charge well in parallel?

If I start at 54.6v (3.90vpc). Both packs are wired in parallel. I put the charger on for an hour and now it is at 56.0v (4.00vpc). Both packs are still wired in parallel so both packs will be at 56.0v... The 60AH pack will not be at 58v and the 100ah at 55v specifically because they are wired together in parallel.

I'm not talking about individual cell imbalance, that can happen on a single pack as well as a parallel pack. But I am seriously confused that you state a 60ah pack can be overcharged when wired parallel with a 100ah pack (even though at the exact same pack V)...

We commonly use V as a way to gauge SOC. If you are using a much more intelligent charger that measures internal cell resistance or other factors than V, ok then I understand. But if your charger and your BMS are all based on V then how can two parallel packs being at the same V be also at different SOC?

[Sergio]

Who said parallel?

[bronsonj]

AH, OK. I re-read your post and you specifically said series. I missed that.

I thought the OP was talking about adding another 7 cells in parallel making two 7S packs and running those parallel to each other. I didn't realize the conversation had changed.

[Sergio]

The context of that reply was preparing both old and new 7S1P packs to be reconfigured as 7S2P.

Wiring two 7S1P packs in parallel would require two separate BMS and ideally a master controller.

It would also introduce a lot of possible dangers in case a cell in one pack has an issue or the due to the imbalanced current between the two packs during discharge and should have a means of isolating the two packs when not being used.

Here is some good information on this subject from a different source:

https://www.orionbms.com/manuals/pdf...ngs.pdf#page=6

[cmysstailights]

Originally Posted by bronsonj

AH, OK. I re-read your post and you specifically said series. I missed that.

I thought the OP was talking about adding another 7 cells in parallel making two 7S packs and running those parallel to each other. I didn't realize the conversation had changed.

I did ask about 2 7 sets in parallel.

Sent from my SM-G960U using Tapatalk

[noquams]

I purchased and will be adding another 7 modules as well. I initially thought it would be good to have two sets in parallel however I can not figure out a way to add the second set to my Zeva BMS. Seems in my case the only option will be to re configure the packs so its one and wire like others have kindly shared on this site.
It anyone has any suggestions I would welcome them.