2p8s or 8s2p

[Steve_S]

If Im not mistake I see that you are using the bussbars to parallel connect the two battery pack instead of paralleing them fist then runnning the wires to the busbars. I was wondering about why? Thank you

It is all about balancing the packs, having same wire lengths etc.. it is all explained in the Victron Doc I posted.

 

[Pneuma]

It is all about balancing the packs, having same wire lengths etc.. it is all explained in the Victron Doc I posted.

Ah I totally missed it! Thank you!

 

[BiduleOhm]

NB: I would put the fuses near the batteries terminals as the closer the fuse to the battery the safer it is.

 

[Jan Gils]

I'm too starting a project with 280Ah cells for a 24V battery bank. At the moment I want to make 8s3p to have a total capacity of 840Ah on 24V. When using 3 BMS's for 8s, I think this would be the best setup? Each cell is monitored in the bank, and hopefully by doing a top charge the 8 cells will match.

The batteries can have a max discharge current of 1C = 280A, and the 8S BMS can handle about 100 A. So in theory the BMS is limiting, and I can draw 300A out of the total battery bank. Is this correct?

update: the downside of this system will be that each of the 8s 24V battery will be limited to the weakest cell. So what's the best configuration for the 24V 840Ah with 280Ah cells?

 

[toms]

At first I though well If I paraller 2 and then put them in series those two should balance each other.

That is correct.

Keep in mind that no matter which way you go, the entire setup is limited by the weakest cell.

If you parallel cells first the performance overall will be better, because in a series connection the pack is instantly limited to the performance of the weakest cell, in a parallel first connection the weakest cell will be supported by its pair until it is dragged down.

In any case, if you want expandability it makes more sense to use multiple inverters. If you parallel a new pack to a 3 year old pack you are still limited to the performance of the 3 year old pack.

 

[Steve_S]

BMS' vary a lot. Depending on type of BMS whether it uses relays / contactors or is MosFet based etc.

LFP does NOT suffer the same issues as FLA (Lead Acid).
LFP packs are independent, even if on a Bus. The Lowest Cell denominator is a hang over from Lead.

 

[MadMax03]

Not a valid question. Series up until you get to your target voltage then parallel away until your heart's content. There is no other way.

 

[Dzl]

I suggest reading this short article for a very basic high level overview. I would caution that anyone presenting one or the other option as absolutely superior either doesn't fully understand the topic, are too narrowly focused on their own use case or priorities, or is not accurately representing the trade-offs for one reason or another.

There are many advantages to multiple packs in parallel ("series first"), however with any design choice there are trade-offs. If series first was the only valid option, you wouldn't see reputable EV, powerwall, and energy storage companies using parallel first in many situations.

I don't consider myself knowledgeable enough to have a definitive opinion, or a full understanding of all the trade-offs. I am still learning. And in fact the more I learn, the more I feel that its a question not of what option is better, but a question of trade-offs and what approach best fits your design goals and constraints and philosophy, and what fits best with the rest of your system.

 

[fafrd]

I suggest reading this short article for a very basic high level overview. I would caution that anyone presenting one or the other option as absolutely superior either doesn't fully understand the topic, are too narrowly focused on their own use case or priorities, or is not accurately representing the trade-offs for one reason or another.

There are many advantages to multiple packs in parallel ("series first"), however with any design choice there are trade-offs. If series first was the only valid option, you wouldn't see reputable EV, powerwall, and energy storage companies using parallel first in many situations.

I don't consider myself knowledgeable enough to have a definitive opinion, or a full understanding of all the trade-offs. I am still learning. And in fact the more I learn, the more I feel that its a question not of what option is better, but a question of trade-offs and what approach best fits your design goals and constraints and philosophy, and what fits best with the rest of your system.

Good article - thanks.

I was about to start a new thread on exactly this question but was led to this one instead.

I’ve got 16 280Ah cells and am debating between 2 8S batteries (8S2P) or a single 2P8S battery and from reading that article, the primary advantage of 8S2P seems to be fault-tolerance and especially redundancy and safety in case of a shorted cell (a shorted cell will not destroy a second cell).

Aside from the reduced cost of only a single BMS and the advantage of having capacity limited by the lowest pair of cells (so lowest average of weak cell + strong cell) rather than the weakest cell and 7th weakest cell, another advantage of 8S2P is that it allows shorter wire length.

If you want a battery configuration that results in + and - terminals next to each other (ideally near the center of the length), that’s difficult to achieve with 2P8S. About the best you can do is to have the + and - terminals is each 8S string abut (so 1- 1+ 2+ 2-) and even that design requires a few longer internal wires (or all internal wires to be l

With 8S2P it’s possible to design a 2-row battery so that the + and - terminals end up centered and all internal wire length are uniform and relatively short.

Not the most significant advantage in the world, but I’m thinking that the combination of reduced cost plus marginally greater battery capacity with easier and shorter wiring outweighs better ability to deal with a shorted cell for my (stationary) application.

Are there any other significant disadvantages to an 8S2P configuration other than better ability to deal with a shorted or failing cell?

 

[MezzBatt]

With 8S2P it’s possible to design a 2-row battery so that the + and - terminals end up centered and all internal wire length are uniform and relatively short.

Ok, I'm slowly picking my way through endless threads on here to get to a comfortable conclusion of how I need to proceed with my system design and build. This thread has helped but also added confusion. Can 'fafrd' show me how you do the above please....???

My goal is to go 24v/ 8s2p batt config. I want to use (x2) 8s BMS modules but I'm still in doubt as to whether this is possible and how/what the hard wired schematic even looks like..???

I see this diagram below on this thread but it seems vague showing the pos(+) wires going through the BMS module.

Is running (x2) 8s BMS modules on a 24v 8s2p battery a possibility...???

 

[Steve_S]

Ok, I'm slowly picking my way through endless threads on here to get to a comfortable conclusion of how I need to proceed with my system design and build. This thread has helped but also added confusion. Can 'fafrd' show me how you do the above please....???

My goal is to go 24v/ 8s2p batt config. I want to use (x2) 8s BMS modules but I'm still in doubt as to whether this is possible and how/what the hard wired schematic even looks like..???

I see this diagram below on this thread but it seems vague showing the pos(+) wires going through the BMS module.

Is running (x2) 8s BMS modules on a 24v 8s2p battery a possibility...???

That drawing is a logical diagram not a technical one. It was used to show where shunts & fuses should go.
Yes, many of us run with more than one battery pack within a battery bank.

Here is the diagram from my current system, (from "About my System" page)

 

[SnpR.SttR]

Guys I jump in. 2p8s or 8s2p kind of paradox there are ups and downs on each side, however I need to mention two cells (or more in parallel) completely fine it will balance out in time BUT if one of them goes bad, bms cannot detect since it monitor them together not individually. Therefore, I lean toward that set up. We are basically trying to save on bms. It is up to you one or two, how well you want to protect and monitor your bank. Any bank if well balanced and not pushed to the limits should be fine, self-failure of a cell can happen however what are the chances?

Edit: In case you want to save on the second bms the manual fix is check each cell voltage time-to-time, if there is a runaway and/or dead cell just replace it from the money you saved on the second bms.