What Does “20 Amp Hr Rate” Mean?
Lead acid batteries are complicated things. Take some lead and sulfuric acid, stick them in a container together and you end up with a potential chemical reaction that can produce electricity. The question is, when you need to know how much electricity is able to be created with that chemical reaction, what is the standard rating, and how is it set? We have answered this question the complicated way on the Peukert's Law Tutorial, but here's a more basic look at determining what you need to know.
Several years ago when you would go to buy a car, the little tag on the window of a coupe might have read 42 mpg. Of course everyone knew that the only way to make that car get 42 miles to the gallon was to drive at 25 miles per hour on a newly paved level track. So in 2008 when the EPA updated their standards, the ratings all dropped to much more realistic numbers that most light-footed people can achieve. Battery ratings are very similar. The faster a battery is drained, the less overall amperage is available. The battery’s AH rating goes down the faster you use it. This is not the same thing as saying you use up what is available faster, but you actually decrease the total overall capacity itself.
To ensure that ratings are given in a realistic way, lead-acid batteries have a few parameters on how they get that “AH” rating. In order to get an AH rating, the battery that is being tested has to be drained down to 0 over the course of a specified amount of time. The amount of amperage that it took to get it down to zero, over that specified amount of time constitutes the AH rating.
Because of the Peukert effect (aka, the faster a battery is drained, the less overall amperage is available), if you discharge a battery over the course of 100 hours, the AH rating looks higher than if you discharge that same battery over the course of 1 hour. So, there has to be a standard.
For deep cycle batteries the standard rating is 20 hours. So, if a battery has a rating of 100AH @ 20Hr rate, then that battery was discharged over 20 hours with a 5 amp load. Starting batteries, on the other hand, are typically rated at 10Hr rate, because they are used faster, so the 20Hr rate is not as important. So, that weird 20Hr rate that you see after the AH rating on batteries tells you that the rating in question is the realistic, common rating—rather than an over-inflated number to make the battery look better than it really is.