It's time for a new Einhell battery when your current Einhell battery is empty. The purchase of an extra battery is also smart. So you can use 1 battery while charging the other. This way, you can switch between batteries and work without interruptions. The Einhell 18V batteries are available in different amperages. For example, an Einhell 2.0Ah, 2.5Ah, or 4.0Ah battery. You can work longer with a higher amperage, but these batteries are heavier. An Einhell battery is part of the Einhell Power-X-Change 18V battery platform. This way, you can use the batteries for different tools with the same battery platform.