I log how much power it takes to charge my Ego batteries after I use them. I’m using this as a stand-in for how much capacity they retain over time. You should know that I have a large lawn (in Florida) and cycle these batteries at least once a week (more in the summer) so these batteries are getting more of a workout than if you had a small city lot that you could mow a few times before charging the battery.
I have two 7.5 AH batteries (one bought before the other). They took 410 watt hours to recharge when new. My older battery has three years of use, and has 63% of it’s original capacity (260 w/h). The drop from year 2 to year 3 was from 75->63% [The batteries have a 3 year warranty, so the first battery is now out of warranty.]
My second battery appears to be following the same general trajectory, although it’s capacity is just slightly less than the first battery (at the same years in service mark).
For the bar chart above I averaged the capacity of all measurements in a particular year, but if you graph all measurements you can see that they are generally decreasing over time (although the batteries do sometimes go “up” in capacity, possibly due to ambient temperatures). The following graph is for my 2nd 7.5AH battery (manufacture date of 2018).