How many of you set your battery max discharge under 20% ?

[sunshine_eggo]

So if that's true and you should only use the middle 60% of your lithiums

Then to me it almost seems like it's not worth paying out to go LiFePo4? You may aswell save a boat load of money and stick with lead acid at 50% usable capacity

As a big fan of FLA and AGM, I disagree.

50% DoD cycle life of quality AGM is about 1200
50% DoD cycle life of quality FLA maintained diligently is about 2000 cycles.
60-80% DoD cycle life of LFP is 6000+ cycles

How is lead advantageous with LFP delivering 4X the total energy over its life? QUALITY lead acid is going to run you > $150/kWh.

 

[Will Prowse]

Ive covered this in a lot more detail over multiple videos but I'll try to summarize the main points. the 80/20 studies are for NMC. Not LiFePO4. I posted these studies before I deleted my Tesla solar battery videos.

Next, look at the studies. Look at the rate of degradation from calendar aging. Especially at various soc. With a quick Google search I found this one: https://www.mdpi.com/1996-1073/14/6/1732

And I have posted many more in my videos.

Next, the cycling tests that sok, Catl, byd and others have performed to get the 6000 cycles is at 1C rate. Never will a solar battery experience this rate of charge or discharge. Usually .2C for an undersize battery without multiple days of autonomy. If you size your system properly, it will be very low. Look at the data for degradation at these rates. This is a major contributor to cycling degradation and for solar application, not even close to being an issue.

Next, the ambient temperature will also determine calendar aging rates. This matters a lot. A hot shed versus an air conditioned room will change the rate significantly over 48 months.

Next, look at stability at high soc of LiFePO4 versus other chemistries. Multiple studies say they like 90% more than 50% soc to reduce degradation. This is also why Tesla recommends charging to 100% every single time for their LFP electric vehicles. But they don't recommend this on their NCA models. Very different degradation characteristics.

This is not something to debate really. Just read the studies. I posted some really good studies under multiple videos. Check them out

 

[Will Prowse]

Now if you're using LiFePO4 to do multiple cycles everyday at 1-3C rate (such as an e-bike), you could discharge to 10% and there will be a bit of benefit. Not for solar though. Again, look at the studies and the data. Its very easy to find.

Also, if your system is properly sized with multiple days of autonomy, you won't discharge below 20% anyways ? but if you do, it's a non issue.

There is a race to cycle the cells before calendar aging degrades them. And the rates of calendar aging are substantial! Cycling them hard is the best way to get the most bang for your buck when it comes to solar. If you don't, they will degrade on their own. Now when they degrade, you can still cycle them just fine. So don't worry about it. Cycle them hard.

If you want lower rate of degradation, lower the c rate. By making the battery bigger. This will lower the internal temp of cell while cycling and also reduced calendar aging rate. Also find a way to keep the cells cold. That can help more than anything else above!

 

[Mattb4]

Not being up on lithium batteries as much as you all folks so I find all of this interesting. I tend to discount number of cycles as being a good metric in off grid solar application simply because battery charge/discharge is never an uninterrupted process. What be of far more use is estimated lifetime in years under normal off grid use. In my Truck I simply schedule to replace my battery every 5 years because that is a point that it likely is still good but it could fail and being unable to operate is a pain in the rear.

So is setting 3.0v/cell LiFePO4 as transfer back to grid under normal operating parameters where you account for voltage droop a reasonable plan? Is there any consensus?

 

[EastTexCowboy]

I'm gonna steal someone else's line on the battery cycling and just drive it like I stole it. Best practices is one thing but real world is another, in virtually everything it seems. I'm going to try to set things up to my advantage if possible but at the end of the day the system needs to work under a wide variety of circumstances and I'm not going to babysit it. In the spring and fall it's going to hit 100% by 11 am and not discharge below 70% most days. This is my first summer but yesterday was 114 heat index and it will probably be worse today. Discharging to 40-50% and it takes most of the day to get back to 100%, but so far it has. Cloudy day in the summer, it's probably going to grid. Same thing in the winter. In any of those cases I just expect it to work and if it shortens the life of the battery, well I did this whole thing to use it and that's what I'm going to do.

 

[OPTIMALSOLAR]

Thx Will. I'll go with your opinion. You've never steered me wrong. Plus, never cared for frozen waffles.

 

[JossG]

Now if you're using LiFePO4 to do multiple cycles everyday at 1-3C rate (such as an e-bike), you could discharge to 10% and there will be a bit of benefit. Not for solar though. Again, look at the studies and the data. Its very easy to find.

Also, if your system is properly sized with multiple days of autonomy, you won't discharge below 20% anyways ? but if you do, it's a non issue.

There is a race to cycle the cells before calendar aging degrades them. And the rates of calendar aging are substantial! Cycling them hard is the best way to get the most bang for your buck when it comes to solar. If you don't, they will degrade on their own. Now when they degrade, you can still cycle them just fine. So don't worry about it. Cycle them hard.

If you want lower rate of degradation, lower the c rate. By making the battery bigger. This will lower the internal temp of cell while cycling and also reduced calendar aging rate. Also find a way to keep the cells cold. That can help more than anything else above!

Hi Will,
(Re. LiFePO4 for solar off-grid use)
You've warned not to discharge too low - understood. But is there an optimum SOC when it's best to recharge in order to maximise battery lifespan?
Eg. all else being equal, which will last longer?
Battery A - being discharged down to 80% each cycle and then topped up again to 100%
Battery B - being discharged down to, say, 50% each cycle and then topped up again to 100%
So Battery A would be charged 5 times in the time that Battery B would be charged twice. So A would always undergo 2.5x more cycles than B.
In theory, A would degrade faster simply because higher number of cycles... but is it actually the case?

I get everything else you've said about temperature and C-rate being much bigger factors, and that calendar ageing will probably be the bigger factor than number of cycles... but I'm still curious!
And when you say "find a way to keep the cells cold" - how cold are we talking? Cool like 15 celsius but not freezing - something like that?

Many thanks

 

[SamDeleted]

Hi Will,
(Re. LiFePO4 for solar off-grid use)
You've warned not to discharge too low - understood. But is there an optimum SOC when it's best to recharge in order to maximise battery lifespan?
Eg. all else being equal, which will last longer?
Battery A - being discharged down to 80% each cycle and then topped up again to 100%
Battery B - being discharged down to, say, 50% each cycle and then topped up again to 100%
So Battery A would be charged 5 times in the time that Battery B would be charged twice. So A would always undergo 2.5x more cycles than B.
In theory, A would degrade faster simply because higher number of cycles... but is it actually the case?

I get everything else you've said about temperature and C-rate being much bigger factors, and that calendar ageing will probably be the bigger factor than number of cycles... but I'm still curious!
And when you say "find a way to keep the cells cold" - how cold are we talking? Cool like 15 celsius but not freezing - something like that?

Many thanks

I don't mean to speak for Will sorry , but the general rule of thumb is stay out of the top & bottom 10% ,

Hold between 10%soc & 90%soc



I can't speak to how accurate that is , but that's what said . With the amount of cycles LiFePo4 supposedly can achieve I don't think it really matters too much anyways

 

[Lt.Dan]

I have heard the 10-90% and also the 20-80% rules, but in my personal experience, I always do a 5-100%. Charging fully to ensure the battery is properly balanced is far more important, and as long as the battery isn't held at a super high state of charge (3.45v+) for very long, there will be no damage done to the battery.

Remember, the batteries will die from Calendar Aging, far before they will die from Cycle Life. Meaning they will die in the same amount of time if you only cycled from 70-80%.

As for Temperature, they like to be the same temperature as you. 25*C. Any higher or lower will reduce its lifespan, lower not as much, but higher temps will kill it fast!

 

[2TrevorJ]

My setup is configured to switch to grid at an indicated 1% (0% & 2.94V/cell triggered some sort of forced grid charging - baked into the Sol-Ark code). I've yet to get that low during normal operation though...after a couple cloudy days with the summer heat and AC utilization I bottomed out at 14% yesterday morning (3.193V / cell). Finally recovered to 100% again early evening today.

Since I posted I've spent quite a bit more time down in the single digits (thanks crazy Texas summer!) and I found that my BMS could request forced grid charging as high as 4% indicated SoC (roughly 3.093-3.107v average per cell) after not top balancing for 8 days. I've since set my discharge floor to 5% (3.147v) and it's working great.

 

[JossG]

I have heard the 10-90% and also the 20-80% rules, but in my personal experience, I always do a 5-100%. Charging fully to ensure the battery is properly balanced is far more important, and as long as the battery isn't held at a super high state of charge (3.45v+) for very long, there will be no damage done to the battery.

Remember, the batteries will die from Calendar Aging, far before they will die from Cycle Life. Meaning they will die in the same amount of time if you only cycled from 70-80%.

As for Temperature, they like to be the same temperature as you. 25*C. Any higher or lower will reduce its lifespan, lower not as much, but higher temps will kill it fast!

Sorry for the ignorant question (I'm quite new to this) but what is it about charging fully that ensures the battery is properly balanced? Is that something the BMS can only do at full charge, or what?

 

[Solarcabin Channel]

I don't set my batteries for a discharge set point and the inverter setting will take care of that.

The reason is I also use direct DC connections for things like my 12/24 volt fridges and some lights and if I get a few low sunshine days I want those still able to run even if the inverter won't.

With AGM or sealed you have to be careful to not drain them too low as it can damage the batteries but that is not an issue with LFP.

In general your rate of degradation will have more to do with temperature and overcharging and heat is hard on LFP batteries.

Keep them cool and don't overcharge and they will last their expected lifecycle and probably a lot more.

JMO

 

[Sabre36]

Our property is off-grid so, I decided up-front not to care about bottom end SoC. In a marine application no, but on the homested no big deal. For example last weekend I changed the fuel filter on the tractor. I was just test driving it in the driveway and it ran fine, I got side tracked, and left every light in the barn on. the KiloVault HLX+ batteries hit LVD yet again. I refuse to baby these batteries as I consider them a tool like any other tool on the homestead..This abuse included -23F last winter. Batteries still well exceed rated capacity.

 

[SeaGal]

I don't set my batteries for a discharge set point and the inverter setting will take care of that.

The reason is I also use direct DC connections for things like my 12/24 volt fridges and some lights and if I get a few low sunshine days I want those still able to run even if the inverter won't.

With AGM or sealed you have to be careful to not drain them too low as it can damage the batteries but that is not an issue with LFP.

In general your rate of degradation will have more to do with temperature and overcharging and heat is hard on LFP batteries.

Keep them cool and don't overcharge and they will last their expected lifecycle and probably a lot more.

JMO

WARNING: Despite @Solarcabin Channel's claims of 20+ years of knowledge, the information in bold is incorrect.

LiFePO4 cells will be damaged if they are discharged below 2V and probably partially damaged if discharged to less than 2.5V.

There are others on this forum who can chip in more about the underlying chemical changes, but IMHO don't discharge LiFePO4 below 2.5V

 

[Solarcabin Channel]

WARNING: Despite @Solarcabin Channel's claims of 20+ years of knowledge, the information in bold is incorrect.

LiFePO4 cells will be damaged if they are discharged below 2V and probably partially damaged if discharged to less than 2.5V.

There are others on this forum who can chip in more about the underlying chemical changes, but IMHO don't discharge LiFePO4 below 2.5V

I see after you were put on ignore you have decided to troll my comments.

LiFePO4 BatteriesThis means that you can safely discharge these batteries to their full capacity. However, most manufacturers recommend still using a 80% DoD for these batteries to prolong their lifespan. Even if you occasionally use 100% of the battery capacity, the battery will not get harmed. https://ecotreelithium.co.uk/news/lifepo4-battery-depth-of-discharge/#:~:text=LiFePO4 Batteries,-LiFePO4 battery cells&text=This means that you can,battery will not get harmed.

Is it bad to completely drain a LiFePO4 battery?

LiFePO4 batteries can be continually discharged to 100% DOD and there is no long-term effect. However, we recommend you only discharge down to 80% to maintain battery life. https://www.relionbattery.com/blog/tech-tuesday-depth-of-discharge#:~:text=LiFePO4 batteries can be continually,80% to maintain battery life.

ADDED: It appears from Will's comment above he also agrees with what I posted.

 

[stienman]

There are too many preferred outcomes so there is really no universal rule.

What do you mean by "cell death"? If not abused, then a LiFePO4 will continue to be useful long after it drops below 80% of it's design capacity, and that 80% figure is what most companies appear to aim for in their lifetime requirements by either cycle count or time.

7kwh of my battery capacity consists of 64 severely abused thundersky cells from 2006 - they were thrown out by the electric motorcycle company I was working for, most of them from the development and testing period when sometimes the charger and BMS weren't operating correctly, or they discovered design decisions that would continue to draw the battery down past its limit. New these cells would have had a capacity of 12.6kWH, so I'm using them at 7kwh, but because their internal resistances aren't matched I only take them down to about 5%, and up to about 95% - it's enough that the BMS can handle the runners. Since part of my system runs in that range, then the other 30kWH of new 280AH cells also runs at that range, and it's costing me perhaps 2-3kwh to not run them at their ends - but that's made up for by the 6kwh that the used pack adds over and I'm gaining the benefit of running the newer, more expensive cells more gently.

So the battery won't suddenly die one day. You might get a runner and have to trade it out, or get a stronger BMS, or add some capacity parallel to that cell, but as long as you keep an eye on things a few times a year, identify these issues before they become damaging to the individual cells, then the pack will last much, much longer than the design time, and it will still be worthwhile using them.

The reduction in capacity happens whether the cell is used or not over time.

As Will points out, the real issue isn't running it low, it's using high charge and discharge current, particularly near the ends of its capacity. The cell generates gasses near its ends, and if charged/discharged slowly then the gasses are no issue - they migrate and liquify. If done quickly, though, the gasses will cause voids in the electrolyte, which in turn accelerates the generation of these gasses and less surface area is available to handle the current. This happens on a microscopic scale, and over time this accelerates the overall increase in internal resistance and decrease in capacity.

As Will points out, in a typical solar system the charge/discharge rate is well under 1C (ie, a 280AH cell won't be charging/discharging at 280A - which is 1C). In my 33kwh battery system with a 7kw solar array and 12k inverter, my max charge rate is 0.2C, and max discharge rate is 0.4C.

If you want to make a meaningful impact on your solar system battery lifetime, limit your charge and discharge rate. That will do more than limiting your charge range.

But even then - as my used cells, nearly two decades old and severely abused during their life (then left nearly empty to self discharge below 2.5v for many of them for years before I got to them) - the end of life for a cell is not the end of usefulness.

So when you describe your decision to limit your capacity usage or not, please indicate what you mean by dead cell, and what your priorities are that made you come to your conclusion.

And if you do throw away your cells when they reach 80% of their capacity, please allow me to recycle them for you, free of charge! In fact I guarantee you I'll cycle them much more than most battery recycling services do.

 

[OffGridInTheCity]

I have a 121kwh 18650 NMC powerwall and I'm running an overall (5.5yrs and counting) 37% average DOD between 49v (3.5v/cell) and 56v(4.0v/cell) with an overall average peak of 53.8v (3.84v/cell). All based on this Battery University chart.... where I'm somewhere between the purple (75-45%) and blue/green (75-25%) lines.


So far, 5.5yrs, I'm at 1,809 cycles on my oldest 14s100p battery and it has no sign of degradation.

I have 9 x DIY batteries built over the years, so cycles are not the same for every battery in the bank - here's the chart as of today:


I'm hoping to report 3,618 cycles in another 5.5yrs but only time will tell / I hope I'm able to report on this long term....

 

[Solarcabin Channel]

I have a 121kwh 18650 NMC powerwall and I'm running an overall (5.5yrs and counting) 37% average DOD between 49v (3.5v/cell) and 56v(4.0v/cell) with an overall average peak of 53.8v (3.84v/cell). All based on this Battery University chart.... where I'm somewhere between the purple (75-45%) and blue/green (75-25%) lines.
View attachment 172111

So far, 5.5yrs, I'm at 1,809 cycles on my oldest 14s100p battery has no sign of degradation. I'm hoping to report 3,618 cycles in another 5.5yrs but only time will tell / I hope I'm able to report on this long term....

Nice breakdown!

At 60 years old the new LFP I installed this year will probably out last my life cycles.

I started with an old truck battery and 45 watt HF panel in 2003. I have used just about every battery type out there and just replaced my Vmax tanks after 10 years with an LFP.

I hope to never replace another battery again!

 

[DIYPV]

I have a 121kwh 18650 NMC powerwall and I'm running an overall (5.5yrs and counting) 37% average DOD between 49v (3.5v/cell) and 56v(4.0v/cell) with an overall average peak of 53.8v (3.84v/cell). All based on this Battery University chart.... where I'm somewhere between the purple (75-45%) and blue/green (75-25%) lines.
View attachment 172111

So far, 5.5yrs, I'm at 1,809 cycles on my oldest 14s100p battery has no sign of degradation. I'm hoping to report 3,618 cycles in another 5.5yrs but only time will tell / I hope I'm able to report on this long term....

Good to know.
What is your daily charge current?

 

[OffGridInTheCity]

Good to know.
What is your daily charge current?

Interesting, I don't track that statistic. But anecdotally it routinely reaches 200a but more often in the 100a range due to loads.
However, at the power walls size, 200a is only 222ma/cell - so *low stress* (low amps / cell and mild ambient temps) is another hallmark of my operation.