New here, wiring question

[6yolithium]

Hi all, new to the forum. We've been off grid for 20 years.
I built a 432ah 24v lifepo4 battery bank 6 years ago. I want to split that into two 12v to use elsewhere, and have
purchased 8 EVE280k grade A from 18650 and had planned on a 2p8s 560amp 24v battery.

However, reading here has brought up questions. Some here say 2 batteries in parallel is better than one larger and I can see where it would be nice to be able to take one out if work was needed on it and still have power, but the whole point to me of lithium is to NOT need to "work on it".
The second thing I read is that 2p8s needs a 16 connection bms, and I had almost ordered an 8s, since I assumed the paralleled cells only needed one connection? So I was wrong there? Good thing I did a search on here!

One reason I wanted the 2p8s was to have both + and - at one end. Our current battery is in a large cooler in an unheated power shed, the cooler has a false bottom with a ceramic terrarium heater (18 watts? I think) hooked up to an automatic temperature sensor. Plus I have a wireless thermometer so I can keep an eye on the temps from the house. So the batteries stay comfy even in - temps.
The connections to the inverter from battery are at one end and the +and- are the same length.

I wanted to do the same layout with the new batteries.

So, number one: I guess I need a 16 connection bms? Or buy 2 bms if i parallel two 8s batts?

Is there a definite reason two 8s batteries run in parallel is better than one battery 2p8s, other than having power "if needed" ?
Which is really better for the longest life of the cells?? That's what is most important.

Thanks!

 

[sunshine_eggo]

However, reading here has brought up questions. Some here say 2 batteries in parallel is better than one larger and I can see where it would be nice to be able to take one out if work was needed on it and still have power, but the whole point to me of lithium is to NOT need to "work on it".

Count me in. Redundancy + BMS are also current limited. 2x batteries add the BMS current ratings, so you can pull more current from 2 BMS than one.

The second thing I read is that 2p8s needs a 16 connection bms, and I had almost ordered an 8s, since I assumed the paralleled cells only needed one connection? So I was wrong there? Good thing I did a search on here!

This is the way.

So, number one: I guess I need a 16 connection bms? Or buy 2 bms if i parallel two 8s batts?

Not really a thing except for 48V LFP. If you have a 8S battery for 24V, you need an 8S BMS. There are BMS like the Batrium that uses a module to monitor individual 8S batteries but treats them as a single battery with regards to cell balance and protection rules.

Is there a definite reason two 8s batteries run in parallel is better than one battery 2p8s, other than having power "if needed" ?

8S2P:
Redundancy. Spare battery + spare BMS.
Single cell failure: take one whole battery offline and work on single battery - back online quickly.
BMS failure: 1) combine into 2P8S and run on single BMS, or 2) take one battery offline and operate on a single battery - back online quickly.

2P8S:
no redundancy of any kind.
Single cell failure: Disassemble battery and determine WHICH of two cells failed - may require extended sitting to sort it out. Rebuild single 8S battery to continue operation.
BMS failure: No option but to run without BMS. Great way to destroy cells.

Which is really better for the longest life of the cells??

Neither. That is governed by conditions and usage (DoD, current, charge voltage, temperature, etc.)

IMHO, given that you're off grid, redundancy and maintaining suitable backups is paramount.

 

[Rednecktek]

?Everything he just said! ?

Beat me to it, but add in that 8 cells is easier to carry around/work on/lift than 16 cells. I don't know about you but MY back isn't getting any younger!

 

[6yolithium]

Thanks for your answers. I bought grade A so less worry about a bad cell or two...
The batteries are put together right in the cooler at waist height, I would never have to carry more than one cell!

I still have my 6yo lithium battery for redundancy. I was going to split into two 12 volt for power in a greenhouse and a barn later after some time spent with the new cells. Problems should crop up pretty fast...

So my takeaway is that I definitely need a bms with 16 leads, not 8 if I build 2p8s...

 

[sunshine_eggo]

Thanks for your answers. I bought grade A so less worry about a bad cell or two...

"Grade A" is a made up term for grey market cells. There is EV grade and non-EV grade.

MANY people have bought "grade A" and been displeased with cells that don't meet capacity or fail in a year or two. You're buying EV cells that didn't meet the EV spec for some reason. Buyer beware.

So my takeaway is that I definitely need a bms with 16 leads, not 8 if I build 2p8s...

It's like you didn't read anything I said.

Again, no such thing for parallel cells. I indicated above that your initial impression of needing an 8S BMS was correct. Whatever you have read to the contrary is wrong.

When you parallel two cells, they are a single cell. BMS leads work based on the differential voltage from the last lead, i.e., for 8S:

3.2 3.2 3.2 3.2 3.2 3.2 3.2 3.2

16S on an 8S battery: 3.2 0.0 3.2 0.0 3.2 0.0 3.2 0.0 3.2 0.0 3.2 0.0 3.2 0.0 3.2

8 of 16 cells will register 0.0V, and you will be in continuous discharge protection.

 

[Rednecktek]

So my takeaway is that I definitely need a bms with 16 leads, not 8 if I build 2p8s...

Got that backwards. 8s batteries need an 8s BMS. If you're pairing cells the BMS treats each pair as a single cell which is still 8s.

There is a good diagram of that in Another Thread On Here.

 

[6yolithium]

"Grade A" is a made up term for grey market cells. There is EV grade and non-EV grade.

MANY people have bought "grade A" and been displeased with cells that don't meet capacity or fail in a year or two. You're buying EV cells that didn't meet the EV spec for some reason. Buyer beware.



It's like you didn't read anything I said.

Again, no such thing for parallel cells. I indicated above that your initial impression of needing an 8S BMS was correct. Whatever you have read to the contrary is wrong.

When you parallel two cells, they are a single cell. BMS leads work based on the differential voltage from the last lead, i.e., for 8S:

3.2 3.2 3.2 3.2 3.2 3.2 3.2 3.2

16S on an 8S battery: 3.2 0.0 3.2 0.0 3.2 0.0 3.2 0.0 3.2 0.0 3.2 0.0 3.2 0.0 3.2

8 of 16 cells will register 0.0V, and you will be in continuous discharge protection.

Actually, you said "this is the way" which to me was ambiguous. You did highlight where I said one connection per pair but that's not definitive, so that's why I reiterated 16 connections, to see what your response really meant.

Sorry I should have just said "what do you mean" by this is the way.

 

[6yolithium]

Got that backwards. 8s batteries need an 8s BMS. If you're pairing cells the BMS treats each pair as a single cell which is still 8s.

There is a good diagram of that in Another Thread On Here.

Thanks

 

[sunshine_eggo]

Actually, you said "this is the way" which to me was ambiguous. You did highlight where I said one connection per pair but that's not definitive, so that's why I reiterated 16 connections, to see what your response really meant.

I was responding to the bolded portion of your quoted text.

 

[6yolithium]

Yeah sorry it wasn't clear to me, ok?