This seems to be typical behavior with the Clara after about 30 months. I can't speak to other Kobo models. See my comments in another thread:
https://www.mobileread.com/forums/sh...6&postcount=11
I have not one Clara, but two. Both were purchased 3 years ago. One was in daily use 90% of the time, the other only sporadically when I fiddled with options and patches or when unit #1 was charging. Yet BOTH now have the same battery capacity -- with the almost instant dropoff below 35% or 30%. So the one with very light use has had its battery degrade as rapidly as the one with heavy use. That is what I find odd.
Lithium Ion batteries do last a long time on a charge, but then drop rapidly at the end. No argument there. That's normal behavior. Maybe the displayed percentage Kobo gives us is not very accurate. Maybe 30% is really 5%. Dunno.
But years ago I used this same system with several Sony readers. Those batteries lasted way longer (in years) than the Kobo's. Also, a Sony reader I had basically in storage for a couple of years acted like brand new when I put it in use. It did not exhibit the deterioration of the stored Kobo.
Meanwhile, the Clara is the best reader I've ever had. So, while I think the batteries could be better, I'll just accept the reality and move on.