LifePO4 Parallel Cells and Packs.

[TotallyShocked]

Need advice before I blow myself up!

I am planning on building an OFF GRID Powerwall that would consist of:

Ten 24V Battery packs of 64 32650 6ah cells each in an 8s8p configuration.

Each pack of 64 cells would have a DALY 8s SMART BMS (common port) handling up to ~100a (2c for a 6ah cells).

The cells in each pack would have screws and connect to 26 gauge copper plates in 8s8p. Each pack configuration would have a solid copper bus bar connection to a larger (¼ x 2in) copper bus bar that connects all 10 battery packs in parallel. These 10 packs should give me 12.25kw at 1C or 24.5kw at 2C (~ 900amps).

I was considering connecting this battery bank to (2 to 3) 24v Growatt 3Kva split phase SPF300TL LVM hybrid Inverters in parallel, (will also add 400watts PV to each Growatt unit).

Questions:

Is an 8s8p configuration too many cells for a DALY 8s BMS to manage reliably?

Or Should I configure 8s5p or 8s4p as packs, the idea here is longevity of cells, monitoring, and ability to replace cells easily. Specifically what is good parallel cell ratio for LFP 3p, 4p, 5p, 8p?

Looking for 7-10 years system lifespan.

Can 10 battery packs (with BMS’s) be put in parallel without special equipment other than fuses?

 

[snoobler]

You likely mean 8P8S, i.e., 8 groups of 8 cells in parallel with the 8 groups in series. 8S8P implies 8 separate 8S batteries in parallel.

8P doesn't matter. The bus bars joining the cells in parallel are managing the 8 cells with respect to each other. 48Ah for a cell is fine, but this assumes you've tested and matched each parallel group to be the same capacity. I've seen no guidance on parallel cell count.

This seems like a very expensive battery with 10 BMS. Aren't smart Daly over $100?... $1000 in BMS?

You could make 2X 8S 280Ah batteries and parallel them. Cells and BMS would run about $2K.

 

[TotallyShocked]

Your right 8p8s it is.
I know it would be expensive, but I don't want a large (280ah) to go bad. I'd rather monitor and pull/replace smaller units.
i think storing replacements, and or logistics charges/time on replacements would be less in long run (7-10 yrs)?
And I want to put it (< 250lbs) on a grounded wagon/cart (portable) for small cabin or RV placement.
My thoughts were i might want to keep 3 packs and 1 inverter for mobile, and 7 packs and 2 Inverters static (Powerwall).

All cells would be Grade A (i hope), and should be close in specs.
I don't know if smaller cells are more reliable than the larger prismatic cells.

The Growatt units have a charger on them, can I charge with 2 chargers at the same time on the DC Bus?

 

[DThames]

I am not sure why you want to built 64 cell units but I am sure you have good reason. One thing you might consider is to bring balance leads (maybe 18gauge) from each series connection point, from each pack. Then connect the balance points on a bus. The main current would still be from the positive and negative of each 64 cell unit, with those cables carrying the current for charge and discharge. This would effectively look like a 80p8s configuration. or you could do 2 x 40p8s, or 4 x 20p8s. There would be less need for so many BMS units. However, your more modular design might let you spot weak areas as the system degrades over the years. I have salvaged some parallels and if you have 4p and one low cell, you will end up with 4 low cells as the one will drag the others down with it. Rarely can you then separate them and the other 3 be okay/typical.

David Poz (Youtube) took 10 (as I recall) large 8s batteries and made 10p8s out of them, with a balance lead network. Maybe it was a 5p16s 48v, I am not sure but he connected all of the cells in parallel. I have two of those same batteries and did the same thing, using one BMS.

Edit, I was writing this before your last post. Yes, smaller units have some advantage.

 

[snoobler]

Your right 8p8s it is.
I know it would be expensive, but I don't want a large (280ah) to go bad. I'd rather monitor and pull/replace smaller units.
i think storing replacements, and or logistics charges/time on replacements would be less in long run (7-10 yrs)?
And I want to put it (< 250lbs) on a grounded wagon/cart (portable) for small cabin or RV placement.
My thoughts were i might want to keep 3 packs and 1 inverter for mobile, and 7 packs and 2 Inverters static (Powerwall).

All cells would be Grade A (i hope), and should be close in specs.
I don't know if smaller cells are more reliable than the larger prismatic cells.

The Growatt units have a charger on them, can I charge with 2 chargers at the same time on the DC Bus?

Dig into the specifics of the Growatt unit you select. Not sure if it applies to all models, but their AC charger is atypical. It basically just floats batteries to a pre-set voltage you can't change. Their solar charge controllers are with programmable battery types.

 

[TotallyShocked]

Thanks All! Thoughtful and appropriate responses with wisdom!