I agree a cycle is not well defined, but here's what I do....
Note: I have 18650 INR type chemistry, large Powerwall so voltage change makes this easier than it would for LifePo4 = but the principles apply.
I define a cycle as a contiguous sequence of charge + discharge(loads). The natural add capacity, use capacity cycle of an off-grid system.
In my case, the low is morning. PV adds charge and loads cause discharge till the low is hit sometime after the PV is gone.
In winter it can take a few days before the charge is significant enough to turn on the inverter for discharge. Thus I have <365cycle per year. It's around 330 cycles per year.
I subtract the low SoC% from the hi SoC% in a cycle to get the SoC% change or DOD%. Over the last 6+ years, my overall average is 35.9% SoC change per cycle. Spring and summer can reach as high as 47% SoC change and in winter as low as 24% SoC change.
In my case, it's the charge + discharge that make a cycle from which I track the hi/low SoC% as a key metric.
My system forces operations in the middle range of 75%-25% SoC for longer life. In winter, the 36% average SoC change is at the lower end of this 50% SoC operational range and in spring/summer at the upper end.
After 6+ years, the oldest battery has 1,982 cycles at an average of 35.9% SoC change within the 50% middle operational range of the powerwall.
Since I don't ever shutdown the Powerwall, I'm attempting to track degradation by recording the average ah/v during discharge with no PV (night time is pure discharge) for each month to be able to do year over year comparisons - e.g. am I getting the same capacity per volt. So far, the ah/v are close enough year over year that there's no 'obvious' degradation.
Plus I track kwh of load each cycle and year to year these have remained constant enough that I can't cleary see loss of capacity. BUT - I've added new batteries here and there to expand the Powerwall, so it's a bit of a fuzzy picture.
If I get another 7 years (14 total) and reach 4000+ cycles on the original battery I'll be very satisfied. But honestly, no one knows what happens with 18650 INR chemistry at low DOD / low stress after 14years so I hope to live long enough to find out.
