Lowest safe voltage for long-term storage? (LiFePO 280Ah)

[kolek]

What's the lowest voltage you would want to store 280Ah LiFePO batteries long term without doing damage to the cells? If it matters cell is EVE LF280.
(leaving aside the damage caused by calendar aging). For reference I received my cells from Luyuan at 3.29V, that's my only data point, but I'd like to go lower.

 

[kolek]

Asking because when demand exceeds production, battery is almost perpetually drained down to the low voltage disconnect level. This low SoC can last for long stretches depending on the season. Low voltage disconnect currently set at 51V, 3.1875V per cell.

 

[kolek]

According to Luyuan, recommended storage SoC is 30-50% which I believe translates to per cell voltage of 3.22-3.26V. So maybe having the battery below 3.22V for long periods is not so great.

 

[time2roll]

I doubt there is significant issues from 3.220 to 3.188 or even 3.000

I would focus on reducing the load at some point to reduce time spent at the bottom. Or find a new source of power to restore the charge.

 

[kolek]

@time2roll
Thanks for the good advice.

 

[time2roll]

Depending on use.... could increase the fault release voltage so the battery stays off until significant charge is restored. Reduce the short cycling to the bottom.

 

[kolek]

Interesting idea, I hadn't thought of that. I think that's called "restart" in the Deye settings and I have it currently set to 51.2V to minimize grid use. Electricity is expensive here in Japan.

 

[caddnima]

Some suggested that if there is a BMS connected that you keep it a little higher. BMS consumes power too, and you want to avoid depleting the power completely.

 

[hwy17]

When usage exceeds production I don't think setting a lower floor really results in less grid consumption.

I have my grid backup come on at 50% SOC, and this leaves 50% room to store daytime excess to be used that night, which on 9/10 days is enough to store all of our excess without hitting 100%.

But I understand everyone's system has different options and there can be configuration difficulties in recreating what I'm describing above.

On a typical day solar will take over from the grid completely at 11-12, and leave us with 70-90% at 6:30pm which will last anywhere from 9pm-9am before it gets back to 50% and grid kicks on.

 

[time2roll]

Any chance the utility has TOU rates to charge at lower cost and use the battery during high rates?

 

[kolek]

Any chance the utility has TOU rates to charge at lower cost and use the battery during high rates?

I'm familiar with that but unfortunately it's not an option for us in our area.

 

[kolek]

When usage exceeds production I don't think setting a lower floor really results in less grid consumption.

It's probably not great for the batteries but my thinking was:

1. The difference may be small but it's more efficient (in terms of using less electricity overall) to draw from the utility at night than charge the battery and then draw from battery at night.

2. If you could have squeaked by without charging on a given day, then charging the battery to 50% is energy you paid for that you didn't need to buy.

No?

 

[hwy17]

2. If you could have squeaked by without charging on a given day, then charging the battery to 50% is energy you paid for that you didn't need to buy.

No?

Only if the solar can make it up again.

I could usually go a whole day without grid input, but it would take me down to 30% the first day, 10% the next day, and then I would be stuck at 10-60% or 0-50%. So it would be a one time withdrawal of that energy from the battery and one time savings, before going back to my same 50% range.

But if your solar is insufficient for overnight and it can charge your battery from 0-100 some days, then it could be true that every bit not discharged is wasted production. My solar excess is always only around 20-60% of my batter capacity and not more.

 

[kolek]

@hwy17 I'm going to sleep on your comment, there's a few parts I don't understand but I may get it when I'm more awake.

 

[kolek]

I have my grid backup come on at 50% SOC

Following up on this again.
So you never allow your batteries to fall below 50% SoC, regardless of the time of day. Is that correct? just trying to understand.

 

[hwy17]

Following up on this again.
So you never allow your batteries to fall below 50% SoC, regardless of the time of day. Is that correct? just trying to understand.

Correct. For context, I have 20kWh of battery, generate about 24kWh per day, and consume about 30kWh per day.

Most of our usage happens during production, so when the battery starts the day off at 50% the 10kWh of available room is enough to store all the day's excess on most days. I'll try to get a screenshot of the battery graph that will illustrate it.

Another way to look at it, is solar power is only lost when you hit 100% during solar production and solar gets curtailed. So as long as your SOC floor is leaving enough room to avoid hitting 100% then you are getting all your solar power, and further lowering the floor won't get you any more.

But operating in mid SOC range will cause balance problems over time of course, so about every two weeks I will manually drive it up to 100% with grid power to balance it and reset the SOC.

 

[hwy17]

Here's a week of usage. My SOC bounces on the 50% floor because it's a double conversion system with a charger that kicks on at 50 and off at 55. I'm guessing with a hybrid inverter you would get a flat line in those spots as it just switches to grid passthrough.

So you can sort of see that if I were to drain down below 50% that would get me those kWh out of that battery, for one day, but then the pattern would just repeat lower down after that. I'd end up in a 0-50% cycle instead of a 50-100% cycle. And my grid usage would be the same, aside from that one day or two where I saved 10kWH during the 50-0 drain.

 

[kolek]

I'd end up in a 0-50% cycle instead of a 50-100% cycle.

Much appreciate the graphic view. Previously did you test and find yourself in a 0-50% cycle, and setting the 50% floor was your solution?

 

[gotbeans]

I bought 64 batteries that were sitting in a barn for multiple years (2 or 3), they all perform good and I have tested all of them to be 273-284 amp hours (they're rated at 280)
the voltages below do not correlate at all with their capacity or where they fall to after being fully charged and sitting for 24 hours.
So I'm just giving you their voltages they were sitting at for years:

Spoiler: battery voltages

3.303
3.295
3.289
3.289
3.295
3.289
3.289
3.259
3.296
3.284
3.295
3.295
3.295
3.295
3.295
3.295
3.296
3.295
3.295
3.295
3.295
3.295
3.288
3.288
3.2955
3.2955
3.295
3.295
3.295
3.295
3.295
3.291
3.295
3.295
3.295
3.295
3.295
3.295
3.295
3.295
3.295
3.295
3.294
3.294
3.296
3.296
3.295
3.295
3.295
3.295
3.295
3.282
3.295
3.295
3.21
3.295
3.295
3.295
3.295
3.295
3.295
3.295
3.295
3.295

anyway from this I conclude it probably doesn't matter as long as voltage is like 20-80% it's plenty fine

 

[hwy17]

Much appreciate the graphic view. Previously did you test and find yourself in a 0-50% cycle, and setting the 50% floor was your solution?

Yes. Not 0 but I think I started with 25% and then 75% was too much and finally 50%.

Now air conditioning season is starting and I will have less excess soon, so I will move to 60%. This is my first year so I don't know yet but in winter I will probably be way up at 80 or 90 with almost no excess solar.

 

[kolek]

Yes. Not 0 but I think I started with 25% and then 75% was too much and finally 50%.

Obviously if you could operate like this, you'd save money by removing the 50% floor:





But instead you were in a cycle like this, but maybe the lowest levels were too low for comfort? How low were you getting before you set the floor?

 

[hwy17]

So you were in a cycle like this, but maybe the lowest levels were too low for comfort? How low were you getting before you set the floor?

I knew that I wanted to be as high as possible, because my system being double conversion means that a battery shutdown would mean whole house power shutdown, so I saw on the first day I went live that it was going to sit in the lower range and adjusted it that night.

I would consider anything under 20% in my setup like an emergency reserve, not to be intentionally used.

 

[hwy17]

Obviously if you could operate like this, you'd save money by removing the 50% floor:

Yes if I had more solar to charge up 75% of the range in a day I would set a 25% floor to allow it to do that.

But instead you were in a cycle like this, but maybe the lowest levels were too low for comfort? How low were you getting before you set the floor?

My setup always needed a floor because of the double conversion, charger has to come on somewhere. First day I got down to 40% and solar charged to 80% and that's when I realized I was going to lose another 20% the next day and end up in 20-60 range. So I set a 75% floor and realized a day after that 75% was too high, hitting 100% on solar mid afternoon.

I knew beforehand that I wanted to set the range as high as possible, but it was a learning experience.