When it comes to batteries, most of us do not pay much attention to them beyond size, purpose and ability to be recharged. Everyone knows about AA, AAA and D cells, watch batteries and car batteries, but when it comes to other types, few people can distinguish their purpose and performance. The key to understanding battery ratings is to learn about and decipher their specifications.
At its most basic, a battery is anything that stores energy to be used at a later time. By this definition, even a rock pushed to the edge of a cliff can be considered a battery because the energy it took to push it to the edge is stored as potential kinetic energy, which can be released by rolling the rock down the cliff. Today, however, the term battery is reserved for a device that stores electrical energy.
The first batteries were invented by the Sumerians in 250 B.C., and it is believed they were used to electroplate metals for jewelry. Since this time, batteries have changed substantially, but they still have the same general design: two electrodes composed of different metals submerged in an electrolyte solution. The first modern battery is attributed to Alessandro Volta, and it was made of a copper electrode and a zinc electrode with an electrolyte solution of sulfuric acid mixed with water or brine.
Today, most batteries are still of the lead-acid type. New batteries use electrodes made of nickel and cadmium, nickel and iron, magnesium and graphite or lithium and graphite, but they are still categorized by how they are used:
- General cells – These batteries are often used to charge small, electronic devices. AA, AAA, C and D cells are in this category.
- Rechargeable cells – Rechargeable cells have largely replaced general cells. Rather than being lead-acid batteries, these are usually lithium-ion or magnesium-ion batteries.
- Starting batteries – Starting batteries provide high power for short times and are primarily used for starting combustion engines.
- Deep cycle batteries – Deep cycle batteries are meant to be charged and discharged over and over again. These types of batteries are used for RVs, uninterruptible power supplies (UPSs) and energy collectors, such as solar panels or wind turbines.
- Marine batteries – Marine batteries combine the features of starting batteries and deep cycle batteries.
Although it is helpful to understand all of a battery’s specifications, the most important are current and capacity. Current is the strength of the electricity discharged by a battery under use, and it is measured in amperes, commonly referred to as amps. However, the battery’s rating is based on its capacity, which is measured in amp-hours (Ah).
The rated capacity of any battery expresses the average amount of current it releases over a period of time under normal use. This means that a battery with a rating of 200 Ah can deliver 20 amps of power at a constant rate for 10 hours.
Generally, batteries with highly active electrodes and a high volume of electrolytes will have higher ratings than small batteries with inactive electrode material. For instance, a standard AAA battery has a much lower rating than a lithium-ion car battery does.
An analogy that is very helpful in understanding rating is that of a moving car. In this example, the current and capacity of a battery are like a car’s speed and range. For instance, if the car moves at 20 mph for eight hours, its range is 160 miles. Likewise, a battery discharging 20 amps for eight hours has a rating of 160 Ah.
Other Battery Specifications
In addition to current and capacity, batteries have several other specifications, including the following:
- Energy capacity – The energy capacity of a battery is measured in watt-hours (Wh), and it is calculated by multiplying the watts discharged by the number of hours.
- Cycle life – The cycle life of a battery is the number of times it can be fully charged and discharged before its performance slows or fails. The actual time a battery will last depends upon the depth and rate of the cycles.
- Charge voltage – The voltage of a battery when it is charged to full capacity is its charge voltage.