AGM battery questions

I guess this would be a good place to post this since it's all related.

Don't trust the "smart" battery chargers that tell you to service or replace the battery. I have a Black & Decker 10 amp charger which has worked well charging between rounds and at home for a few years now. I went to charge my race car battery like I do periodically to top it off. It told me to service or replace the battery. (It worked fine the previous weekend at a race.) I put it on my trailer battery and it said the same thing. I put the charger on a brand new battery and it was fine, and fully charged. (I got depressed at the thought of having to a new battery or two.)

Still suspicious that something wasn't right, I put it on a third used but fully charged battery and it also said to service or replace it! Since I'm obsessed with having spare parts & equipment, when possible, I had a spare new charger just like it. It read and charged fine on all of the batteries I tried before!

* I figure that there is a sensitive sensor in the used battery charger that was malfunctioning. So double & triple check your batteries, if at all possible, before believing a "smart" battery charger.

These were all AGM batteries - used Odyssey & 2 Optimas and a new Sears Die Hard Platinum.