I notice a slight lowering of resting voltage on my lead acid deep cycle batteries after a few years. I've been doing this constant monitoring for over 20 years now with the same digital meter. The batteries are always charged equally and I make a habit of checking them after fully charging and resting long periods. The best is in the spring after a full fall charge.

They're still "good" , just not as good as a newer one. I checked voltage the other day and noticed the combination starting battery is still "good", but it's serving notice....