Amp hour (Ah) is a unit of measurement that has nothing to do with power and it’s a totally different unit than Amp (Ampere). Amps are used to describe amp hour, but they are different units of measurement. Taking our 5Ah battery above it’s easy to say that we can draw 5 amps for 1 hour – 10 amps for 30 minutes – or 20 amps for 15 minutes, in a linear calculation, and in fact we can with a lithium ion battery. All that to say, you can pretty much make a straight comparison that any brand’s 18V (or 20V max) 5Ah battery will hold more than double the charge of any other 18V (or 20V max) 2Ah battery. What makes a 2Ah or 5Ah battery?

Usually, if a battery has at least a 4Ah capacity, it will be prominently displayed somewhere on the battery like the 5Ah in the picture. Peukert’s law, however, explains why that is not true for lead-acid batteries and the more you draw from that type of battery the less charge time is available. So, you may draw 5 amps for 1 hour, but only be able to draw 10 amps for 13 minutes, for example. You may hear something called Peukert’s law, which is applied to lead-acid batteries (the battery in your car), falsely applied to lithium-ion batteries like the ones shown in this article and video. Peukert’s law says that a battery’s amp hour rating is reduced disproportionately as more current (amps) is drawn from it. Lithium-ion batteries have a self-heating characteristic that raises voltage and counteracts the dissipation of power due to internal resistance. Amp hour describes the capable charge of a battery. Theoretically, we can draw 5 amps, continuously, for 60 minutes before our 5 amp hour battery is drained. Now, that’s under ideal conditions, perfect temperature, no vibration, and consistent power. We say a capable charge because in the real world, you never meet those ideal conditions. Just think of amp hour as a way to know that one battery holds more of a charge than another. You’ll be able to use your drills, circular saws, and jigsaws longer before the battery runs down.

DeWalt DCB184 18v 5Ah Lithium-Ion XR Slide Battery