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. 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. 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. 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.

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. 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?