Tool batteries provide power to cordless tools. They are used to equip bare tools with a battery, change to a different battery series, or replace batteries that are damaged or no longer hold a charge. Tool batteries can be used with tools of the same brand, battery series, and voltage. Voltage describes the level of power the battery provides. Tools with lower-voltage batteries are lighter and more compact but less powerful than tools with higher-voltage batteries. Batteries of the same voltage and series are usually available with different amp-hour ratings. Amp-hours (Ah) describes the amount of power the battery holds (its capacity), which determines how long a fully charged battery can perform before needing a recharge. The higher the Ah rating of the battery, the greater its storage capacity. As voltage and Ah ratings increase, so do the weight and bulk of the battery and the tool it powers.