Batteries die! It is a natural phenomena that occurs when the useful life of a battery reaches the point of of no longer holding a charge. There are technical reasons why batteries degrade and lose their ability to power a device that include: declining capacity, increasing internal resistance, elevated self-discharge, and premature voltage cut-off on discharge.

Today I want to write about battery capacity and its impact within the design of a battery.

What is Battery Capacity?

Battery capacity is a reference to the total amount of energy stored within a battery. Battery capacity is rated in Ampere-hours (AH), which is the product of:

AH= Current X Hours to Total Discharge

The capacity is normally tested or compared with a time of 20 hours and at a temperature of 68F (20C).

Five Factors that Govern Battery Capacity

Physical Size - the amount of capacity that can be stored in the casing of any battery depends on the volume and plate area of the actual battery. The more volume and plate area the more capacity you can actually store in a battery.

Temperature - capacity, energy store decreases as a battery gets colder. High temperatures also have an effect on all other aspects of your b

Cut-off Voltage - To prevent damage to the battery and the device batteries have an internal mechanism that stops voltage called the cut-off voltage, which is tpically limited to 1.67V or 10V for a 12 Volt battery. Letting a battery self-discharge to zero destroys the battery.

Discharge rate - The rate of discharge, the rate at which a battery goes from a full charge to the cut off voltage measured in amperes. As the rate goes up, the capacity goes down.

Battery History - Deep discharging, excessive cycling, age, over charging, under charging, all reduce capacity. Note charging your battery 1 time will reduce capacity as much as 15%-20% depending on your battery's chemistry.