LiFePo4 batteries (like Jakiper) and long life charging settings

[rvdbijl]

I'm new to solar storage (been running grid-tied for years, but no battery storage). But I have been driving EV's for a few years now. For those, they recommend charging to 90% or lower to extend battery life, and never discharge below 10-20% (the car's "BMS" manages that). I was thinking I'd do the same with the solar battery bank, giving up some kWh's by not topping up to 100% every day, but limiting to 80 or 90%. But I'm reading so many articles about the cells needing to be balanced, and that seems to happen at the top end of the charging range (i.e. 100%).

My battery bank is going to be used for an additional set of panels (5kW or so) I'm putting in myself, running the Sol-Ark inverter in net-0 mode (no energy into the grid, but balance the house load w. panels and/or batteries). That way I avoid complex agreement changes with the utility, especially since I'm already close to the upper limit for grid-tied systems before transformer upgrades are needed (~20kW). The battery would also function as a "UPS" protecting electronic equipment from power outages (I currently have a "whole house" 3 KVA UPS that I'm looking to decommission).

So in my mind, depending on the season, I don't see the batteries (I'm buying 20kW) discharging below 40-50% and spending most of their life at a higher charge level. That's why I figured I'd let charging stop at 80-90% and let them sit there (extending longevity hopefully). But in reading this forum, it's unclear to me if that's unhealthy for the battery, and I should let it charge & sit at 100%.

Would like to hear from others on the forum how I should proceed with this setup and battery settings!

 

[sunshine_eggo]

I'm new to solar storage (been running grid-tied for years, but no battery storage). But I have been driving EV's for a few years now. For those, they recommend charging to 90% or lower to extend battery life, and never discharge below 10-20% (the car's "BMS" manages that). I was thinking I'd do the same with the solar battery bank, giving up some kWh's by not topping up to 100% every day, but limiting to 80 or 90%. But I'm reading so many articles about the cells needing to be balanced, and that seems to happen at the top end of the charging range (i.e. 100%).

You can't target peak SoC based on voltage with any reliability. The most successful approach is choosing a lower voltage resulting in lower current and longer charge times.

Charge to 3.45V/cell - allow 2-3 hours for absorption.
Float at 3.375V/cell

So in my mind, depending on the season, I don't see the batteries (I'm buying 20kW) discharging below 40-50% and spending most of their life at a higher charge level. That's why I figured I'd let charging stop at 80-90% and let them sit there (extending longevity hopefully). But in reading this forum, it's unclear to me if that's unhealthy for the battery, and I should let it charge & sit at 100%.

You must have missed it, but letting LFP sit at 100% for extended periods is not healthy for it. If it's in a controlled human-comfortable environment, then the issue is minor.

Most LFP manufacturers recommend storage at somewhere between 30-70% SoC.

 

[rvdbijl]

You can't target peak SoC based on voltage with any reliability. The most successful approach is choosing a lower voltage resulting in lower current and longer charge times.

Even if the BMS communicates with the inverter?

Charge to 3.45V/cell - allow 2-3 hours for absorption.
Float at 3.375V/cell

Got it! I'll give that a go when I receive all the pieces of this kit and get it installed.

You must have missed it, but letting LFP sit at 100% for extended periods is not healthy for it. If it's in a controlled human-comfortable environment, then the issue is minor.

Most LFP manufacturers recommend storage at somewhere between 30-70% SoC.

That's what I figured. My sentence was constructed awkwardly. I meant to say, based on what I read in this forum, I understand it's unhealthy to let the battery sit at 100%.
Now when I say "sit", it would likely charge in the morning for some time, and reach peak charge in 3-4 hours (provided the sun is out), and then sit for another 4-8 hours at that charge level until the sun goes down and the battery starts supplying loads in the house. Is that considered bad if I were to charge to 100%? Or is the concern really for multiple days sitting at 100%?
The batteries will be in a conditioned (cool) environment, typically between 60 and 68F depending on the season -- 60F in winter, 68F in summer.

 

[sunshine_eggo]

Even if the BMS communicates with the inverter?

that's an exception; however, SoC accuracy drifts over time, and you'll need to run it to true 100% at least a couple times a month.

That's what I figured. My sentence was constructed awkwardly. I meant to say, based on what I read in this forum, I understand it's unhealthy to let the battery sit at 100%.
Now when I say "sit", it would likely charge in the morning for some time, and reach peak charge in 3-4 hours (provided the sun is out), and then sit for another 4-8 hours at that charge level until the sun goes down and the battery starts supplying loads in the house. Is that considered bad if I were to charge to 100%? Or is the concern really for multiple days sitting at 100%?
The batteries will be in a conditioned (cool) environment, typically between 60 and 68F depending on the season -- 60F in winter, 68F in summer.

You're cycling them in that scenario, so no concerns with daily runs to 100%.

What you don't want is to charge them to 3.65V/cell and then put them in a hot garage for a few weeks or months.

 

[rvdbijl]

that's an exception; however, SoC accuracy drifts over time, and you'll need to run it to true 100% at least a couple times a month.



You're cycling them in that scenario, so no concerns with daily runs to 100%.

Great! I was afraid of that when I started reading here.

What you don't want is to charge them to 3.65V/cell and then put them in a hot garage for a few weeks or months.

Got it. My basement (where they're going) stays at a nice 68-70 even on the hottest days in the summer here in NH.

Thanks!