This chart shows the relative energy density by weight and volume. Any battery can be measured by the amount of energy it delivers. “Amp hours” and “watt hours” are typical terms used to describe the capacity of a given battery. An ideal battery would have a very high energy density. So for real batteries, the more energy that can be packed into a small volume, the better. However, there are many other aspects of battery performance that need to be considered, such as the type of materials that are used, cost, high current capability, self-discharge, and safety. The chart shows that while Lead-Acid technology has very low energy density compared to Li-Ion, it is still used in many applications because it is low cost and durable. It is well suited for many fixed applications like UPS or energy storage, and for vehicle starter batteries. Lithium-Ion, on the other hand, is extremely light weight and has high energy density so it is ideal for portable computers, mobile phones, and other handheld devices like tablets.