Makita 18v 4.0 amp hour rechargeable lithium ion battery

What do amp hours mean if they don't tell you the run time of the battery?

With all the battery-operated tools being sold, what do battery amp hours really mean to me? Does it tell me how long the battery will run before dying? - Rich Matselboba

Chris Marshall: You're right, Rich - the amp hour (Ah) rating on tool batteries doesn't tell you in "real time" how long it will power a tool before it needs recharging, but the number does give you a quick idea of how much capacity it has. In practical terms, the more amp/hours a battery has, the more run time it offers the tool. So, a 4.0Ah battery will run at least twice as long as a 2.0Ah battery of the same voltage before it needs to be charged up again. That's pretty useful information to know if you're driving a zillion screws or running a power-hungry tool, like a cordless track saw or an angle grinder. But, the battery's age, charging efficiency and the manner in which you are using the tool all factor into how long the charge will last.

Tim Inman: An "amp/hour" is exactly what the name says. One amp(ere) of electric current – which is a measure of the amount of electricity – can be supplied for one hour. Voltage measures the pressure of the electricity, similar to water pressure in a pipe. Think about the amp as the gallons of water in the pipe. For you, it is a way to tell you how long a battery will last before it needs recharging. Other variables impact this, so it isn't always exactly the same comparing one tool to the next. It is a good gauge, though, and can help you to know whether one tool battery can last longer than another. The voltage the tool requires, the resistance or efficiency of the motor, the operating temperature and other things all impact tool load and draw. An 18-volt tool with a 1.0Ah battery would last longer between charges than an 18-volt tool with a .5 Ah battery, for instance.