Recommendation for amp/hour monitors?

(I searched the archive for the term "amp/hour monitor" and came up completely empty -- am I asking the wrong question?? But there are quite a few for sale out there, so this item does exist. Please forgive me if this question has been asked and answered many times, but I could not find a prior discussion on point.)

Anyway, I understand that monitoring the voltage gives me the state of charge, which can roughly tell me how many amp/hours I still have in the battery. The problem, of course, is that the voltage reading varies for a lot of reasons -- temperature, how much load there was on the battery a few minutes ago, etc. I am hoping that someone can recommend an after-market device that will indicate remaining amp/hours.

Thanks in advance for your suggestions.