Battery capacity is the amount of energy stored in a battery. It tells you how much power the battery can provide you, and for what duration of time. Sounding vague? Let me clarify further.
Battery Capacity in Watt-hour (Wh) or kiloWatts-hour (kWh)
Each battery has a maximum power limit that can be drawn from it at any given point of time. It is provided as a part of battery specification by the manufacturer. The capacity of a battery tells us for how long it can disburse power at the maximum power limit. In mathematical terms, it can be defined as follows:
Capacity = Power X Duration
This means that if we draw less power, the battery will last longer. Now, we know from a previous post that power is calculated in kilowatts (kW). So, capacity is measured in terms of kWh (kilowatt-hour).
Battery Capacity in Ampere-hour (Ah)
The very definition of a battery says that it converts chemical energy to electrical energy. Battery capacity is also measured in terms of the amount of electrical energy disbursed by it over a period of time. You know that electricity is measured in amperes. So battery capacity is also measured in terms of ampere-hour (Ah). Here,
Capacity = Electricity X Duration
ampere-hour is a more commonly used unit of calculating battery capacity.
Rated Battery Capacity
To maintain uniformity across all manufacturers, battery capacity mentioned by the manufacturers is the rated battery capacity. Essentially, the capacity tells how many amperes of electricity can be generated by the battery over a period of 20 hours. So, if you have a 100 Ah, battery, it will provide you with 5 A of electricity for 20 hours.
The capacity of the battery does not remain constant. It changes with time due to factors like self-discharge, charge/recharge cycle (in case of rechargeable batteries), temperature, corrosion, storage, etc.
Related Articles:
Peukert’s Law: How Long Will My Battery Last
Defining Amps, Volts, Watts and Ohms
