Ampere Hour

The Ampere Hour is the normal rating of Lead Acid Batteries. It is commonly misunderstood – for good reason – that this is the amount of Amps that the battery can produce for one hour until it becomes flat. What the rating actually means however, is the amount of current that can be drawn for 10 or 20 hours which is then scaled to 1 hour.

It sounds confusing but is actually quite simple.

Take the Power Inspired battery. From the data we see that the battery has a rating of 9Ah specified at a 20hour rate. This means we can draw 9/20ths of an Amp (0.45A) for 20 hours until it becomes discharged. However, lead acid batteries have a non-linear discharge curve. As a result, the more current we draw from the battery the less capacity it has. If the Ah conversion were linear we would expect to be able to achieve a current draw of 9Amps for an hour. However the capacity at the 1 hour rate is in fact 5.4Ah meaning we can only draw 5.4Amps for the hour.

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