An amperehour is a unit of electric charge. It is represented by Ah. It is a unit that measures electric power over time. The amperehour is not part of the International System, since the hour is not either.
The amphour indicates the amount of electric charge that flows for one hour through the terminals of a battery (or an electrical conductor) providing an electric current of 1 amp (A) for one hour (h).
The amperehour is used to measure the capacity of a battery, that is, the amount of electricity it can store during charging and return during discharge.
A battery that has, for example, a capacity of 100 Ah, theoretically can give a current of 10 amps for 10 hours, or 1 amperes for 100 hours.
At a practical level there are certain performance losses conditioned by the battery discharge rate, the faster a battery discharges, the more energy is lost due to internal resistance. Therefore, the load capacity is usually given with reference to a standard discharge time (10 or 20 hours), and for a specific final voltage.
In the case of rechargeable batteries, Ah is an indicator of the energy charge that a battery can hold. It also will power a device before needing recharging.
How many Coulombs is an amperehour equivalent?
One hour of amps equals 3,600 coulombs (ampseconds) of electrical charge transferred to a constant current of 1 amp for 1 hour, that is. In 1 second at 1 amp, the current will flow 1 load pending.
The unit of electric charge in the International System is coulomb C. Coulomb is a derived unit that is defined in terms of the ampere: 1 coulomb is the amount of total electric charge that passes through a crosssection of a conductor through which an electric current of one ampere for one second.
Charging capacity of a battery. Oh and mAh
Charging capacity is the charge that a battery can store. It is measured in amphours (Ah). In batteries with a low charge capacity, it is usually expressed in milliamperehour (mAh). For example those of a phone battery.
Between an 800 mAh battery or a 3,000 mAh battery, the second will last longer (twice) because it has more stored electrical charge.
Often, battery manufacturers indicate in their specifications only the charge stored in milliamphour (mAh). Others use only the energy stored in Wh · h (Wh).
Lithiumion batteries are commonly used for portable electronics and electric vehicles. They have a higher mah and a better battery life.
Examples of electrical capacities

An AA size battery has a capacity of approximately 2 to 3 amphours.

The average battery of a mobile phone usually has between 2,500 and 4,000 milliampereshour of electrical capacity.

A large car has large batteries. It would have a battery capacity of about 50 amps per hour.
Table of the best phone batteries according to the OCU
Samsung M30s 
6000 mAh 
OPPO A5 2020 
5000 mAh 
Xiaomi Redmi 8 (32 GB) 
5000 mAh 
Xiaomi MI NOTE 10 
5260 mAh 
Motorola One Macro 
4000 mAh 
Samsung Galaxy A51 
4000 mAh 