LIFEPO4 @ 100% SOC long-term?

Mrgulabull

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I have an oversized LIFEPO4 battery bank and oversized panel array. In typical use I'm only depleting ~10% of the bank's power overnight and am fully charged by just 10-11am. This means the batteries are sitting near 100% SOC for the majority of the day. Should I be concerned about potential degradation of the cells in this scenario?

I've manually changed the high voltage cut off to be about 95% SOC, thinking that it's probably not best to sit at 100%. Should I consider something lower, like say 80% SOC high voltage cut off to get the longest life span out of these batteries? From what I've read I can expect the most cycles if I stay within the 20-80% SOC range. It's a shame to not take advantage of the full battery capacity, but if that means I can get more cycles, it seems a worthwhile tradeoff.

System specs:
12x Lion Energy UT1300 wired series parallel for 48v
18x 250watt panels
 

snoobler

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It's not sitting. It's cycling. "Sitting" is literally that... days, weeks, months, BUT since you're clearly not needing much of the bank, might as well take additional steps to preserve it.

How are you assessing SoC? Are you using a battery monitor or using voltage?
 

Mrgulabull

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Jul 9, 2020
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Good distinction regarding sitting vs. cycling. So even though I'm cycling near 100% would it be better to keep that cycling in the 20-80% range for longevity?

I'm using a Victron smart shunt to assess SOC, but also cross checking on the batteries which have a basic built in diode that reads in 20% increments.
 
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fafrd

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Good distinction regarding sitting vs. cycling. So even though I'm cycling near 100% would it be better to keep that cycling in the 20-80% range for longevity?

I'm using a Victron smart shunt to assess SOC, but also cross checking on the batteries which have a basic built in diode that reads in 20% increments.
If I was only using ~10% SOC, I’d plan to cycle between 35% to 45% (recommended SOC for storage is 30% ~50%).

Only downside to that is if you occassionally need more than 10% in which case I’d plan for ‘empty’ to be 15% and ‘full’ to be 15% + worst-case discharge.

I’ll be using a full 85% over summer months but less than half of that over winter months, so I plan to change limits over winter to ~20% empty and 60% full for exactly this reason (extend cycle life).
 

Forbisher

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I have an oversized LIFEPO4 battery bank and oversized panel array. In typical use I'm only depleting ~10% of the bank's power overnight and am fully charged by just 10-11am. This means the batteries are sitting near 100% SOC for the majority of the day. Should I be concerned about potential degradation of the cells in this scenario?

I've manually changed the high voltage cut off to be about 95% SOC, thinking that it's probably not best to sit at 100%. Should I consider something lower, like say 80% SOC high voltage cut off to get the longest life span out of these batteries? From what I've read I can expect the most cycles if I stay within the 20-80% SOC range. It's a shame to not take advantage of the full battery capacity, but if that means I can get more cycles, it seems a worthwhile tradeoff.

System specs:
12x Lion Energy UT1300 wired series parallel for 48v
18x 250watt panels
Do you ever use more than 10% and how often?
Are you going to increase your useage in the future?
Why did you oversize your battery at 12 expensive Lion Energy batteries?
 

Maast

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Do you ever use more than 10% and how often?
Are you going to increase your useage in the future?
Why did you oversize your battery at 12 expensive Lion Energy batteries?
It's pretty difficult to 'oversize' a lifepo4 bank as there is no downsides to sitting at partial states of charge and a pretty good reason (as noted above) to stay in the 20-80% state of charge window.

I can't speak to the OP but I've got a 65kwh lifepo4 battery bank w/ 9500w of panels, my personal reasoning is in summer the system is horribly 'oversized' for the loads of the house and I'll bet it never drops more than 5% SOC in a day, however in WINTER here in the Pacific Northwest that 9500w array becomes a 2000w array and I'll be lucky to keep my SOC above 30% without having to revert to grid power.
I wanted 3 days of autonomy in winter and 65kwh is the best I could do.

You don't size a system for the best conditions, you size it for the worst and try to get as close to 100% coverage as you can.
 

Mrgulabull

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Do you ever use more than 10% and how often?
Are you going to increase your useage in the future?
Why did you oversize your battery at 12 expensive Lion Energy batteries?
Yes I’ll likely use more than 10% in the future with a large home theater system, additional lights and computers, and a large freezer. I’m completely off grid and so I sized the bank to be both future proof and something to get me through multiple cloudy / rainy days. I’m on the wet side of Hawaii island and we get ~200 days of rain each year.
 

Forbisher

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Watch how much this guy struggles to get a few wh out of his gutters before you go too far down that path! :LOL:
I was joking and have already seen that video.
200 days of rain so at least his solar panels will be very clean.
 

Mrgulabull

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Watch how much this guy struggles to get a few wh out of his gutters before you go too far down that path! :LOL:
youtube
Haha, yep, I went deep down the youtube rabbit hole on water generators. I saw a guy essentially diverting an entire river worth of water for a measly 500 watts. They look like fun to build, but decided they would only really make sense if you lived in a cave with no access to the sun.

 

copec

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I have 20kw/h effective of NMC Leaf batteries, and I keep them between 75-85%, reserving a certain amount in case grid goes down (which it does quite a bit living an hour outside of Vegas in the desert). I wish hydrogen storage and electrolyzers with fuel cells were effectively practical.

I use the somewhat standard inverter programming to only use battery mode when solar is available and use grid at night. That way I offset a good portion of my bill and still have seemless power for grid outages.
 
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