Power Cycles vs Depth of Discharge vs Longevity

[aaapilot]

Fine tuning the LifePo4 battery pack for optimal life.  My Rosie has AC Grid support, so I can set a specific SOC that I can drain to and/or set specific times when only the battery will be used, etc.  I also have excess PV available in the summer, so I can choose to keep batteries in a 'float' state for 8 hours a day and fully charge to 100% most days, but what is best for the batteries and what is the battery warranty REALLY based on?

Battery life is specified in Cycles (MNPowerflo are 6000 cycles), but I don't see a depth of discharge specified by most manufacturers and not from Midnite (did I missed it somewhere in the literature?)  Some sites RECOMMEND 20%, but what numbers are the MANUFACTURERS really basing these cycle life claims upon? 

I'm not totally off-grid, so I have the luxury of draining less than the **number** recommended, let's just say 20% is the number to get 6000 cycles, if I drain to only 50%, what does that do, if anything, to increase the Life Cycle count the battery will achieve? We know what it does for lead batteries, but I haven't seen any hard numbers from Lithium manufactures.

BMS's have cycle counters, so I'm going to assume that's what would be used to decide if a battery doesn't perform to the warranty. So, if I drain a battery to 20% and charge to 80-90%, will the battery count that as a cycle or no?  My experience says no, so does NOT charging a battery to 100% effectively extend the life cycle count 'life' and extend the warranty, per se. Discounting the warranty, is it better for longevity to charge a lithium to 100% every day or keep it in the 20%-80% range for a week, then charge to 100%?  My 30kw pack has been in operation since February and the 6 individual 5kw batteries have from 31-34 cycles on them.  I haven't intentionally restricted the charging, but have 2 separate PV arrays w/different controllers, so have been testing output and adding loads to the backup panel, etc. The backup panel loads are ALWAYS powered by Rosie/battery, so it can use 6-10kw from 6pm until 9am when I begin to get PV, 4 kw more in summer when the well pump runs for irrigation, more if it's cloudy.

I haven't seen much in-depth discussion on these factors from sources that I would trust, like the battery manufacturer themselves or folks that have been early adopters and have enough experience to have hard numbers.  Or White Papers. Folks on this forum are mostly in for the long haul and would appreciate some incites from what 'YOU' are doing .... or not doing!

Dave

[ClassicCrazy]

It will be interesting to see what the answer is to this.
On my lithium batteries it seems to count a cycle when the voltage and current get up to what is programmed in the bms. And below that the bms reported SOC gets all out of whack and can't trust it. My Classic reported SOC which averages all the batteries is usually always reliable. But of course that is going to depend on what Midnite has used for a bms and what it is programmed to report if those are the batteries you are wondering about.
Every other lithium battery manufacturer or bms often has different ways they do things.
I made a video on how I observed the soc fluctuate as reported by bms . Of course I have an open loop system and not sure how that would differ from closed loop ones.
https://youtu.be/eLsq2q8eHfQ?si=tttP-VllszoCqCjJ
Larry

[dapdan]

Gents,

This is indeed the question that is on the mind of most person using lithium nowadays. I have personally decided that I will use as much energy and cycles I can from the batteries because I currently subscribe to the idea that current lithium batteries will die (or degrade) from old age first before they do so due to overuse or cycle death. I arrived at this view primarily based on the view and experiments of Andy from the youtube channel " offgrid garage" and my own experience.

Typically they will say that lifep04 has a shelf life of 20 to 25years. I had installed some new CATL 280Ah cells at a client. The system has 30kwh installed using two parallel banks of 15kwh. I visited the installation last week and one of the BMS was showing a cycle count of 140 after a year of installation. The current specs on these batteries indicate 6000 cycle to 80% dod. On setting them up I have not limited maximum charge or discharge and have the inverter charging from grid at around 44v, so basically I am looking at as much discharge as possible. So for this discussion lets call it 80% discharge. At the current rate it will take 42.8yrs, at a cycle count of 140 per year, for the batteries to reach a state of health of 80%. I am not sure if I will be alive at that point (I would be around 90yrs at that stage), I will update this response if I am :>.  There by the time the battery gets to 80% SoH it will be well past its shelf life. This why I subscribe to the idea of taking as much energy as possible now be cause the batteries will die of old age first before it runs out of energy cycles as the batter still has thousands of cycles to go before it decrease to 60% SoH.

I can say that I am now defaulting to undersizing battery banks, use the grid as back up when the battery falls to it lowest possible SoC to cycle batteries deeper in an attempt to get the maximum possible lifetime energy extraction before the battery loses energy storage or cycles due to old age all the while minimising the upfront capital cost of battery. If there is to much storage where DoD is shallow then you will not be able to maximise lifetime energy extraction from the batteries.

Damani

[aaapilot]

Interesting take, Dapdan. The batteries are certainly the most expensive part of the system, so maximizing their ENERGY output for any given amount makes sense.  I selected my battery size to allow for aprox 36 hours of back-up panel operation, no more, and not running large 'utility' size loads for any period of time.  Microwave for a short period, sure.  Well pump for short periods, absolutely. My thinking was to elliminate running a generator overnight for a load of just 300-500 watts for a couple refrigerators, a fan, lights, internet, TV.  If there's no sun/PV charge for 2 days, then run the generator at full power as necessary to recharge batteries and do laundry & other load heavy devices, then back to battery for another 36 hours.
I believe/hope these lithium batteries will last as long as you describe.  What I'm a bit more concerned about is the BMS board that controls them! If the BMS fails in 11 years, I'll bet an original BMS replacement won't be available.  Then I'll have batteries that are still good, but no BMS to control them.  Technology will have moved on to the next 'better' chemistry & the electronics to support it.  This thinking supports your philosophy of undersizing the battery system and using the snot out of it NOW!

Dave

[dapdan]

aa,

I would not be to concerned about the latest and greatest battery tech. Just look at lead acid it was around for decades before lithium started to de throne it in the past 5yrs or so. Lithium were out commercial for the past 20yrs and i bet they will be king for the next 20-30yrs comfortable because of how long it really take to bring mature battery tech to the commercial market. The advancement of lithium will continue to be driven by the automotive market. It is automotive market we have to thank for the sharp drop in price of lithium that it became viable for residential and commercial use. This took about 15-20yrs to happen. If you look at the automotive market there are no new wide scale battery tech being deployed apart from Lifep04 in base model EVs. So I think we are good for a minimum of 20yrs comfortable based on the maturity period for new battery tech to trickle down to us DIYs/ off-gridders. As for the BMS, there was a time bms were non existent or very expensive and guys bottom balanced and operated without them and regulated the voltage with other devices and had fair success doing so. One can still operate lithium with external regulation from a trusted reliable inverter or CC. So the community will always find a work around. Lot of brilliant intelligent engineers out there. There is nothing to worry about here.

Damani

[ClassicCrazy]

aa,

I would not be to concerned about the latest and greatest battery tech. Just look at lead acid it was around for decades before lithium started to de throne it in the past 5yrs or so. Lithium were out commercial for the past 20yrs and i bet they will be king for the next 20-30yrs comfortable because of how long it really take to bring mature battery tech to the commercial market. The advancement of lithium will continue to be driven by the automotive market. It is automotive market we have to thank for the sharp drop in price of lithium that it became viable for residential and commercial use. This took about 15-20yrs to happen. If you look at the automotive market there are no new wide scale battery tech being deployed apart from Lifep04 in base model EVs. So I think we are good for a minimum of 20yrs comfortable based on the maturity period for new battery tech to trickle down to us DIYs/ off-gridders. As for the BMS, there was a time bms were non existent or very expensive and guys bottom balanced and operated without them and regulated the voltage with other devices and had fair success doing so. One can still operate lithium with external regulation from a trusted reliable inverter or CC. So the community will always find a work around. Lot of brilliant intelligent engineers out there. There is nothing to worry about here.

Damani

I got 48v worth of 100 ah Calb lithium batteries in 2015 to try out. At that time there were barely any bms like you say, and what was out there was very expensive and not that great. I tried to bottom balance like someone on this group said he did, and since I didn't really understand I thought I ruined my batteries because I had no low voltage cutoff and the Classic took them down too far in voltage when I was letting them just idle and not around. They just sat around for years no charge or attention and last year I was very surprised that all of the cells were in acceptable voltage range. I hooked them up and gave them a charge which they took, but I never load tested the pack. I now have a good active balancing jkbms on them but took the pack off line to make space for a newer battery. Not sure what I will do with them, but amazing they still have life in them. I was sure some of the cells were down to 2 volts years ago and that is why i neglected them. Guess they healed themselves.
Larry