What is an Amp hour?
The unit of measurement ampere hour (Ah) is independent of power and is quite different from ampere (amps).
Battery charging capacity is measured in amp hours.
Lithium batteries rated at 80AH can supply 20 amps for 4 hours before they die. However, if the current draw increases to 40 amps, the battery will only last 2 hours.
Under ideal conditions, with perfect temperature, no vibration and stable power, you can expect a battery to charge up to its full capacity. However, there are energy losses associated with factors such as internal resistance that will prevent the perfect charge in real world conditions. If you were to put a steady 90 amp load on a battery, it would not last the 30 minutes described by its AH rating. Furthermore, if the battery is damaged it could overheat and die, meaning it will no longer hold a charge. To summarise; AH ratings are a guide that allow us to compare one battery’s charge capacity against another; enabling us to use our tools for longer before needing to recharge or replace the battery.

How to calculate the Ah of a lithium battery?

One ampere of current flowing for one hour transfers 3,600 coulombs (ampere-seconds) of charge during the hour. This is also known as an ampere hour.
Ah can be represented mathematically as follows:
Amp hour (Ah) = Current (I) x Discharge time (T)
Consider a battery that draws 60 amps (A), which is discharged in 30 minutes, to understand how the ampere hour rating is calculated:
Current = 60 A
Discharge time = 30 mins (0.5 hours)
Ampere hour = 60 x 0.5 or 30 Ah for 1 hour
Another example is as follows:
Current = 10 A
Discharge time = 5 hours
Ampere hour = 10 x 5 or 50 Ah for 5 hours
Battery ratings are displayed on the battery. If the rating is not specified, it means the battery is a starting battery that isn’t designed for continuous power.

What is Watt hours on a battery?

(Wh) is the abbreviation for Watt-hour, and it represents the amount of energy accumulated by any given battery in electrical applications.
Wh=Ah*V