Voltage settings to maximize LiFePO4 lifetime for off grid system.

[apctjb]

I would like to fine tune my charging settings to maximize the cycle life of my LiFePO4 battery bank (2p16s 280AH cells). Solar is the only charging source.

Following the recommendations on this forum I initially set bulk to 56.4V (3.53) and float to 54V (3.38). With those settings I noticed that my battery bank was maintaining 54V or greater most of the time, which I also had read on this forum is a no-no for LiFePO4. So I lowered settings to 55V (3.44) bulk and 53.8V Float. The graph below shows my battery voltage during the course of the day. While the graph varies day to day, this is representative of the voltage range for my battery.

As you can see below with bulk at 55V and float at 53.8V, my batteries are still above 54V a fair amount of time and never dropping below 52.5V.

I don't have a SOC meter but estimating SOC using the voltage chart I have seen on this forum it appears I am maintaining my battery bank between 80 and 99% SOC most of the time, and wondering if that is really ideal for maximizing cycle life?


Would love to hear from others what battery voltage range I should be targeting to maximize cycle life.








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[snoobler]

Trying to target a charge voltage to limit your upper SoC is essentially impossible especially with the charger absorption phase. With a long absorption phase (that you can't control), you can get your battery to > 95% SoC at 54.4V (3.4V/cell). If you had a SoC meter (and you should get one), you could see the voltage at which your battery hits 80-90% SoC, BUT that would only be good for that particular scenario, i.e., the net current from your array charging and your loads. If you could force the charger to terminate the moment that voltage is hit, you could get close, BUT you can't do that. Floating at 3.38V will still continue to accumulate charge slowly, and you'll likely go over your target.

A recent post detailed that charging to lower peak voltages over a longer period of time at lower current (absorption phase) indicated that very little degradation occurs over time. I don't recall the specifics, but the numbers were along the lines of 95% remaining capacity after 5 years charging to 3.45V vs. 80% capacity remaining after 5 years charging to 3.65V.

Setting your absorption at 54.4V would be 3.45V/cell. This would have the effect of stretching out your absorption phase, longer time, less current. For decent solar days, you'll likely get your battery to peak before you lose sun, BUT on cloudy days, you might not be able to get to peak charge like you would at the higher voltage.

 

[apctjb]

Thanks for the response. To summarize you suggest I try dropping bulk from 55 to 54.4V; would you keep float at 53.8 or drop that a little lower as well.?

Its a fine balance as with my Growatts if bulk and float are to close together the MPPT's stop charging (they drop out of float) and then the battery voltage has to drop 1.5 to 2V before they resume. (discussed elsewhere).

My BMS does have a SOC meter, but can't monitor it remotely the way I can system parameters with the Growatt Shine server monitoring.

 

[snoobler]

Sorry. I typod, and I completely missed that you had already dropped to 55V. I intended to recommend 55.2V.

If the system is working satisfactorily at 55/53.8, I'd say you've already found a scheme that will maximize cycle life.

I would vigorously recommend a battery monitor - even the cheap $50 budget one that Will recommends would be more accurate than estimating based on voltages.

 

[Forbisher]

You bought 32 280Ah cells but are only using 20% of that battery.
Why not just buy 16 cells and use more of the battery?
Why do you need such a big battery?
Calendar aging will kill your battery too even if you dont use it.

I would like to fine tune my charging settings to maximize the cycle life of my LiFePO4 battery bank (2p16s 280AH cells). Solar is the only charging source.

Following the recommendations on this forum I initially set bulk to 56.4V (3.53) and float to 54V (3.38). With those settings I noticed that my battery bank was maintaining 54V or greater most of the time, which I also had read on this forum is a no-no for LiFePO4. So I lowered settings to 55V (3.44) bulk and 53.8V Float. The graph below shows my battery voltage during the course of the day. While the graph varies day to day, this is representative of the voltage range for my battery.

As you can see below with bulk at 55V and float at 53.8V, my batteries are still above 54V a fair amount of time and never dropping below 52.5V.

I don't have a SOC meter but estimating SOC using the voltage chart I have seen on this forum it appears I am maintaining my battery bank between 80 and 99% SOC most of the time, and wondering if that is really ideal for maximizing cycle life?


Would love to hear from others what battery voltage range I should be targeting to maximize cycle life.






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[apctjb]

You bought 32 280Ah cells but are only using 20% of that battery.
Why not just buy 16 cells and use more of the battery?
Why do you need such a big battery?

I sized the battery to provide 3 days of storage, during the winter months when my loads are larger. In the summer my loads are lower and sunshine abundant so yes currently only using 20% of battery, but that will not be the case come winter. Regardless my understanding is cycling a battery 20% will provide a longer life than cycling it say 80%.

I also opted for no generator as buying more battery and panels was cheaper than a good generator and less hassle.

 

[toms]

Get a BMS that has more control of your charger, pay attention to charge rates with respect to temperature (ie don’t charge when outside the 0-30’C range)

That will get you long cell life.

 

[apctjb]

• Get a BMS that has more control of your charger

Okay; do you have a suggestion as to brand of BMS that can regulate/control a MPPT solar charger without having to built some sort of interface? I have not found one..

 

[toms]

I have used Batrium, REC, and Zeva. All control the MPPT Charger in my SMA (Sunny Island/Sunny Boy) system.

 

[houseofancients]

Okay; do you have a suggestion as to brand of BMS that can regulate/control a MPPT solar charger without having to built some sort of interface? I have not found one..

seplos makes nice ones, that can directly communicate with a bundle of inverters using amongst others the pylontech protocol