What Does “20 Amp Hr Rate” Mean?
Lead acid batteries can seem complicated, but they really aren't. Take some lead and some sulfuric acid, stick them in a container together, and you end up with a chemical reaction that can produce electricity.
However, the complicated part is knowing how much electricity can be created with that chemical reaction, what the standard rating is, and how it is set. Below we tried to answer the following questions:
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.
What does the AH rating mean?
To ensure that ratings are given in a realistic way, lead-acid batteries have a few parameters on how they get that “AH” rating. To get an AH rating, the battery is tested by draining the battery down to zero 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.
What does the 20 hr rating mean?
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 a 10-hour rate, because they are used for cranking and the 20Hr rate is not as important. So, that weird 20Hr rate that you see after the AH rating on batteries tells you the rating that was used to measure the battery's capacity.
Why does a battery have a higher than normal AH rating?
While most manufacturers use the common 20-hr rating, we have seen some try to inflate their amp/hr rating by publishing a 100 hr rating, thereby showing an much higher AH rating than normal.
Just remember, in most cases from manufacturer to manufacturer, the AH rating will be similar, as you can only squeeze so much lead into a similar-size case. So, if you find yourself looking for a group 24 battery that is typically 75 AH at a 20hr rating, and all of a sudden you find one that is a 100 AH rated, be skeptical. What that manufacturer most likely did is publish the 100 hr rating instead of the 20hr rating to inflate the AH rating.
Where can I find a battery's AH rating?
A battery's specification sheet is the best place to find a battery's AH rating. For instance, our MK Battery brand typically includes several AH ratings to give you an idea of what you can expect at different amperage loads.
