18 cell "48"v bank on a 5kva Victron Multiplus II

[The Terrible Triplett]

Listening to one of Will's vid's on cells lasting maybe 20+ years, keeping away from 3.65v at all cost after having seen what a BMS that disconnects can do, keep the cells "in the flat of the curve", I have connected and run 18 cells at 3.45v (62.1v) on my 5kva Victron MPII.

The key is to have the cells A grade, Batch matched, and properly top balanced.

Does anyone have an opinion on why that is not a good idea to continue with this setup?

 

[Samsonite801]

This is one of my favorite diagrams to post here (not sure why I like it so much)...

Lithium Iron Phosphave Charge Range

Sheet5 Lithium Iron Phosphate Charge Voltage Range (LiFePO4) What to charge your cells/batteries/packs to. (SOC),Never More than,3.650,Shown is the "FLAT CURVE" of LiFePO4 cells/batteries/packs. What to not discharge past. (DOC),Nominal,3.200,The steap parts on the left and the right are the "Le...

docs.google.com

But generally, they say if you stay in the green, the cells will last longest.

Some people even go 20%-80% (instead of 20%-90% or 10%-90% even)...

I guess it's a tradeoff, DoD for longevity. There is also a compromise of this 20 year expectancy in technology and innovation. Maybe your battery will last 20+ years, but maybe in 10 years a new battery technology will come along that is 1/4 of the price for twice the kW/h and make all the savings not as important by then.

The only issue I'd have with 18 cells though, if your batteries accidentally got charged to 100%, will this bring your inverter beyond it's max voltage? Will it shut itself off or will it fry it or anything? As long as the potential maximums and minimums of the battery bank won't put any of the equipment at risk for damage, then theoretically it should be ok.

Assuming you will use an 18s BMS (aka 16s-20s BMS)

Why not just use 16s and run in the desired SoC range (lower average voltage)? What is the supported input voltage range on your Victron?

 

[The Terrible Triplett]

This is one of my favorite diagrams to post here (not sure why I like it so much)...

Lithium Iron Phosphave Charge Range

Sheet5 Lithium Iron Phosphate Charge Voltage Range (LiFePO4) What to charge your cells/batteries/packs to. (SOC),Never More than,3.650,Shown is the "FLAT CURVE" of LiFePO4 cells/batteries/packs. What to not discharge past. (DOC),Nominal,3.200,The steap parts on the left and the right are the "Le...

docs.google.com

But generally, they say if you stay in the green, the cells will last longest.

Some people even go 20%-80% (instead of 20%-90% or 10%-90% even)...

I guess it's a tradeoff, DoD for longevity. There is also a compromise of this 20 year expectancy in technology and innovation. Maybe your battery will last 20+ years, but maybe in 10 years a new battery technology will come along that is 1/4 of the price for twice the kW/h and make all the savings not as important by then.

The only issue I'd have with 18 cells though, if your batteries accidentally got charged to 100%, will this bring your inverter beyond it's max voltage? Will it shut itself off or will it fry it or anything? As long as the potential maximums and minimums of the battery bank won't put any of the equipment at risk for damage, then theoretically it should be ok.

Assuming you will use an 18s BMS (aka 16s-20s BMS)

Why not just use 16s and run in the desired SoC range (lower average voltage)? What is the supported input voltage range on your Victron?

Thanks, Samsonite.

That diagram is priceless.

Victron is maxing at 66v and yes, if the cells go out of balance, and exceed 66v, I'm sure it will cause drama.

Hence one sets the MPPT, Venus and inverter to steer well clear of that.

Yes, JBD 8s/20s BMS with Bluetooth and RS485 port add.
RS485 port connects to the Venus via a driver to further mitigate risks. his one: https://github.com/Louisvdw/dbus-serialbattery/wiki
Bluetooth port I use the App on my iPad to see the cells continuously.

Why not 16 cells?
I rather have more ah using more cells, yet "lightly" used, by setting the equipment to accommodate such a move. Have it, use it, no?

Also don't like multiple BMS'es, more wires, and more things to monitor.
Worst case I can go 18s x 2p if I really have to, on one BMS.

 

[Wibla]

I'd probably be more comfortable with 17s, but as long as everything can handle 18s, there's not a lot of downsides. Using 300 Ah cells as an example, you gain about 1 kWh per additional cell.

 

[The Terrible Triplett]

I'd probably be more comfortable with 17s, but as long as everything can handle 18s, there's not a lot of downsides. Using 300 Ah cells as an example, you gain about 1 kWh per additional cell.

Thanks, Wibla,

Why would you prefer 17? Because of the max volts?

I must stress this point for newbies: The cells must be A Grade Batch Matched or at least the same resistance, and they all have to be top balanced all the time.

Same resistance ... as a test, whilst waiting for our latest order to arrive from China, I used 18 cells, but with a caveat.
1) Wanted to walk the talk on mixing and matching different AH Lifepo4 cells.
2) Wanted to see if one can start with 18 cells, able to remove 3 cells down to 15 in the event one has bought dud cells whilst waiting for warranty replacements.

So I took:
Cells 1-13 are 2nd Life 100ah cells
Cells 14-18 are A Grade Batch MAtched brand new 280ah cells.
Set as a 100ah bank, daily cycled.

This is definitely NOT a long-term solution, but it can be done in a pinch.