The amount of energy that a battery can store in milliampere-hours (mAh). The higher the capacity of a battery, the more it can hold with one charge. Larger batteries also require more time to charge.

2. Capacity makes sense when comparing batteries of the same size. For example, 18650 batteries usually have 2300-3600mAh, while AAA batteries have around 1000mAh.

The two main types are Ni-MH (Nickel-Metal Hydride) and Li-ion (Lithium Ion).