Replacing Lead Acid with LiFePO4 Sizing questions

[241_pizza]

Hi all.

I've got an off-grid system with 4.4kw of pv production, a Magnum 4448 PAE inverter, a OutBack flexmax 80 charge controller, and 16 Rolls s-530 FLA batteries (10 years old), and a 15kw back-up generator. I'm looking to replace the batteries with SOK LiFePO4 server rack batteries. I am trying to size them correctly, I believe 20kwh of storage should suffice, but I can get a better KWH price at 25kwh. My concern is that I will have a hard time getting the higher storage capacity battery bank charged, especially with loads running. I have no immediate plans to increase the rest of the system for a few years. Does it make more sense to go with the smaller number of batteries first? Are there any factors I'm missing in my consideration? Any recommendations are welcome. Thanks!

 

[sunshine_eggo]

Hi all.

I've got an off-grid system with 4.4kw of pv production, a Magnum 4448 PAE inverter, a OutBack flexmax 80 charge controller, and 16 Rolls s-530 FLA batteries (10 years old), and a 15kw back-up generator. I'm looking to replace the batteries with SOK LiFePO4 server rack batteries. I am trying to size them correctly, I believe 20kwh of storage should suffice, but I can get a better KWH price at 25kwh. My concern is that I will have a hard time getting the higher storage capacity battery bank charged, especially with loads running. I have no immediate plans to increase the rest of the system for a few years. Does it make more sense to go with the smaller number of batteries first? Are there any factors I'm missing in my consideration? Any recommendations are welcome. Thanks!

I'm seeing a logic flaw.

First, LFP is HIGHLY efficient in charging - near 100% vs. 85% for FLA (love them Rolls have 4 S-605 myself), so you're already going to get more usable out of your PV and generator.

Second, you have a 16 * 6V * 400Ah = 38400Wh of FLA.

How could you possibly struggle to charge 25kWh @98% efficiency if you can charge 38.4kWh @85% efficiency, and those damn Rolls LOVE to be charged to high voltage, and are really finicky about being fully charged. By other manufacturer's standards, they require a little over-charge every day for optimal health.

So 20kWh vs 25kwh is a complete non-issue.

You recharge based on production vs. consumption regardless of battery capacity. If you use 10kWh/day, it doesn't matter if your battery is 20kWh or 100kWh, you're simply replacing the 10kWh/day. Either you have enough production or you don't.

So, yeah! Go 25kWh!!!!

 

[yodamota]

I found This https://diysolarforum.com/resources/system-energy-audit-and-sizing-spread-sheet.12/
helpful in determining what I would need to charge batteries.
with lead acid, I needed something like 16kwh of storage
with lithium I need 9 kwh of storage