BMS Needs?

[LivingTheDream]

Hello Everyone,
Noob alert. I am building a system for my RV and the last part I’m confused on is the BMS. Here are the details…
- 4*405w panels (top 2 in series, bottom 2 in series, then parallel)
- 16 lifepo4 280a cells
- 24v 2400w hybrid controller
I want to achieve a 24v 560a system. Any suggestions on how to wire this up as well as what BMS I should use. Please keep it simple. This isn’t my day job. Lol!

 

[LivingTheDream]

Ok, that didn't get any responses. I'll order an OverKill and just wing it.

 

[wiseacre]

Well, I'm not one to give advise. I can give my thoughts I'm fairly certain of

with 16 cells you can build (2) 24v batteries or a single (with pairs of cells in parallel) to get the 560Ah.
You'll need 2 BMS for a pair of batteries or just one with the single configuration

Will has a video explaining how to build a single 12v battery with parallel pairs that would apply to a 24v

If it was me I would build 2 batteries and parallel them in a bank

 

[LivingTheDream]

Well, I'm not one to give advise. I can give my thoughts I'm fairly certain of

with 16 cells you can build (2) 24v batteries or a single (with pairs of cells in parallel) to get the 560Ah.
You'll need 2 BMS for a pair of batteries or just one with the single configuration

Will has a video explaining how to build a single 12v battery with parallel pairs that would apply to a 24v

If it was me I would build 2 batteries and parallel them in a bank

So in that configuration you mentioned, I would need two 4s BMS's? If I did the single bank, one 8s BMS with each lead going to two cells, correct? I ask because I already ordered an 8s BMS from OverKill.

 

[wiseacre]

So in that configuration you mentioned, I would need two 4s BMS's? If I did the single bank, one 8s BMS with each lead going to two cells, correct? I ask because I already ordered an 8s BMS from OverKill.

You ordered correctly. You need 8s BMS

Using the parallel pair method you would only need a single BMS - attach the BMS leads to the proper "pairs"
building 2 batteries - you need 2 BMS (one for each battery)

I would go 2 batteries. I like redundancy in case one fails (not likely but Murphy's Law always applies)
and the amp draw would be split between the batteries
-I'm on shaky ground now-
I think the BMS is limited to a 100a discharge, I'm not sure what happens if more is attempted.
with 2 batteries the load is split, that gives you the ability to go to 200a draw (not recommended to go max on a constant draw)

For this reason I plan on building multiple batteries to be wired in parallel.
The Ah increases and so does the bank's ability to supply a large amp load.

with a single (parallel paired) battery you get the Ah but limit the load it can supply unless you go with a higher amp discharge BMS

 

[LivingTheDream]

Looks like I ordered the 100a BMS. Here is what my system looks like, tell me if I'm on the right track...
- 4 * 405w panels = 1620w
- 16 cells (lets say 280ah, still debating over these or the 304's) wired 8 in series * 2 and then the two in parallel for a total of 560ah in 24v

Is this correct? I don't know why this is making my head hurt so much. LOL.

 

[wiseacre]

Looks like I ordered the 100a BMS. Here is what my system looks like, tell me if I'm on the right track...
- 4 * 405w panels = 1620w
- 16 cells (lets say 280ah, still debating over these or the 304's) wired 8 in series * 2 and then the two in parallel for a total of 560ah in 24v

Is this correct? I don't know why this is making my head hurt so much. LOL.

Me too
Nurse! Nurse! My brain hurts

With my limited knowledge I'd say you're correct.
8s battery x 2, wired in parallel, each battery pack with it's own BMS

I'm planning to do the same thing only with 100Ah cells. (can't afford a pair of 304s)
I was going to build only one battery with the 304s but realized the limitations of a single after constant reading of the forum.

 

[LivingTheDream]

Me too
Nurse! Nurse! My brain hurts

With my limited knowledge I'd say you're correct.
8s battery x 2, wired in parallel, each battery pack with it's own BMS

I'm planning to do the same thing only with 100Ah cells. (can't afford a pair of 304s)
I was going to build only one battery with the 304s but realized the limitations of a single after constant reading of the forum.

So I'll need another BMS for this setup? I only have one right now.

 

[MisterSandals]

Wiring up 16 big cells into a 2P8S will work just fine. You will have better results if you can match the cell pairs as well as possible. So that each pair is equal to the other pairs that is. I just saw another thread on this that did this scientifically but the idea is to rank all 16 cells in terms of capacity (better ones will charge slower and discharge slower and be less likely to run away at the high and low extremes). Pair best (1) with worst (16). Then pair 2 with 15, 3 with 14...

There are a lot of ways to do this, some like the scientific way did a capacity test on each cell with is a LOT of time and effort. I built 4S batteries and put them thru charge/discharge cycles (i was powering a greenhouse so a good stress test). I rearranged the runners into one pack and best into the other. From there i could judge the best and worst from each 4S pack after a few cycles. There are many ways to do this i suspect.

And, my 2P4S battery works great. Its a big battery... too big for a normal human to move easily, even in a wood carrier.
I cannot imagine doing much at all with a 2P8S battery - it would be huge and heavy.

 

[LivingTheDream]

Wiring up 16 big cells into a 2P8S will work just fine. You will have better results if you can match the cell pairs as well as possible. So that each pair is equal to the other pairs that is. I just saw another thread on this that did this scientifically but the idea is to rank all 16 cells in terms of capacity (better ones will charge slower and discharge slower and be less likely to run away at the high and low extremes). Pair best (1) with worst (16). Then pair 2 with 15, 3 with 14...

There are a lot of ways to do this, some like the scientific way did a capacity test on each cell with is a LOT of time and effort. I built 4S batteries and put them thru charge/discharge cycles (i was powering a greenhouse so a good stress test). I rearranged the runners into one pack and best into the other. From there i could judge the best and worst from each 4S pack after a few cycles. There are many ways to do this i suspect.

And, my 2P4S battery works great. Its a big battery... too big for a normal human to move easily, even in a wood carrier.
I cannot imagine doing much at all with a 2P8S battery - it would be huge and heavy.

So...should I be looking at making four batteries, 4 cells each. Then, put two of them in series, the other two in series, then finally the two sets into parallel? If so, how does the BMS come into play being that I have an 8s 100a from OverKill? Do I need to buy another one?

 

[wiseacre]

Yes, you need to buy another BMS - Build (2) 24v batteries

I'd prefer to build 24v.
keep in mind that each separate battery pack requires it's own BMS
Building 4s packs you'd need to buy 4 BMS (the 8s BMS wouldn't work on a 4s battery)
with 12v batteries you'd need to wire them in series then parallel (2s2p). I'd prefer to skip that and just wire my bank in parallel.

 

[MisterSandals]

You need to figure out what battery voltage you want. It "sounds" like you're set on 24V.

You have a few options:
4x 12.8V batteries in 4S, wired 2P2S (same as 2S2P) results in 25.6V battery bank. Requires 4x 4S BMSs
2x 25.6V batteries in 8S, batteries connected in parallel, 8S2P. Requires 2x 8S BMSs.
1x 25.6V battery, wired 2P8S. Requires 1x 8S BMS.

 

[LivingTheDream]

You need to figure out what battery voltage you want. It "sounds" like you're set on 24V.

You have a few options:
4x 12.8V batteries in 4S, wired 2P2S (same as 2S2P) results in 25.6V battery bank. Requires 4x 4S BMSs
2x 25.6V batteries in 8S, batteries connected in parallel, 8S2P. Requires 2x 8S BMSs.
1x 25.6V battery, wired 2P8S. Requires 1x 8S BMS.

I think I was wanting to go for the last option, the 2P8S setup. Would this yield a 560ah system if I used 280ah cells?

 

[wiseacre]

I think I was wanting to go for the last option, the 2P8S setup. Would this yield a 560ah system if I used 280ah cells?

Yes, connecting pairs of 280s in parallel will make a 560ah pack.
You'll have 8 pairs of 560s - when wired in series you'll get 29.2v (series adds volts - parallel adds Ah)
Then wire the pairs like they were a single cell in series and wire the BMS accordingly - each pair as a single/separate cell.
You will only need the one BMS you already have

 

[MisterSandals]

Here is what a 2P4S looks like. A 2P8S would have more "4 post bus bars" in the middle, otherwise pretty much the same. Not the 2 leftmost cells are negative to top of picture. Next 2 cells negative to bottom. Next 2 negative top...
There are other ways to arrange the cells instead of a long pack if the form does not work (it will be pretty long).

 

[MisterSandals]

Here is a good guide:
Cell Configurations for 12V 24V and 48V LiFePo4 Batteries

 

[LivingTheDream]

Great! I will review this when my head is clear. LOL! Thank you both!

 

[Just John]

When using a 100 amp BMS at 24v, don't plan on an inverter larger than 2000 watts.
If using two 24v batteries, each with a 100 amp BMS, then you can go up to 4000 watts.

Depending on your needs, likely two batteries each with a 100 amp BMS is likely what you want, since most people want more than 2000 watts (3000 watts is a common size).

 

[Gadgetnerd]

Hello Everyone,
Noob alert. I am building a system for my RV and the last part I’m confused on is the BMS. Here are the details…
- 4*405w panels (top 2 in series, bottom 2 in series, then parallel)
- 16 lifepo4 280a cells
- 24v 2400w hybrid controller
I want to achieve a 24v 560a system. Any suggestions on how to wire this up as well as what BMS I should use. Please keep it simple. This isn’t my day job.

When using a 100 amp BMS at 24v, don't plan on an inverter larger than 2000 watts.
If using two 24v batteries, each with a 100 amp BMS, then you can go up to 4000 watts.

Depending on your needs, likely two batteries each with a 100 amp BMS is likely what you want, since most people want more than 2000 watts (3000 watts is a common size).

If you have 16 cells why not get a 48v charger/inverter - that would be my first question. for 24V you would want to get 2 8S 24V BMS's for your two parallel banks of 8 cells each. The 24V system will somewhat limit the inverter output, thats why 48V systems makes way more sense.

 

[Gadgetnerd]

If you have 16 cells why not get a 48v charger/inverter - that would be my first question. for 24V you would want to get 2 8S 24V BMS's for your two parallel banks of 8 cells each. The 24V system will somewhat limit the inverter output, thats why 48V systems makes way more sense.