I own three 7.5AH Ego batteries. My earliest had a manufacture date of 2014 (although I purchased it in 2017 with a mower manufactured in 2016). I now have four years of operation on this battery.
As shown in the graph, the capacity dropped off over time (down to a low of 260 watt hours in year 3, which is 63% of the original 410 watt hour capacity), but in year 4 it has actually increased a bit to 290 watt hours (70% of the original capacity).
As you can see, its graph has gone down, and so far hasn’t popped back up. At the three year mark it is also down to a 260 watt hour capacity (63% of it’s new capacity).
My third 7.5AH battery has a 2019 manufacture date, but it’s still in its first year of “use”, so I don’t have a fancy graph for it yet, although I have started to include it in the comparison bar graph below:
So far, all three of my 7.5 AH batteries have followed a very similar capacity vs time curve, and you can expect one of these batteries to retain an average of 90% capacity in year 1, 72-75% capacity in year 2, and 63% capacity in year 3.
[Plus, they MAY bounce back up to 70% capacity in year 4, if the other two follow the path taken by my first battery.]