AC chargers know nothing of the car’s state of charge. It’s the car’s software that can stop charging at a predefined limit.

Most EVs have this setting in the dashboard (with a few notable exceptions, such as a 30 kWh Leaf). However, as chargers get smarter, communicating with the car becomes possible.

The charging amount and time are based on the EV’s usable battery capacity.

Unfortunately, not all car manufacturers publish this information; some list total battery capacity, some don’t state whether usable or total, and some don’t list any battery capacity information. For some EVs, these specs are estimated.

Charging efficiency is the proportion of electrical energy stored in the battery during the charging process.

Charging Efficiency (%) = (Energy Stored in Battery / Energy Supplied) × 100

For example, a battery receives 15 kWh of energy during charging, but only 12 kWh is actually stored in the battery. The charging efficiency is 80%.

This means 80% of the supplied energy is effectively stored in the battery, while 20% is lost due to factors such as heat or energy conversion inefficiencies.

Charging losses may result from AC to DC energy conversion, heat loss, and the normal power loss that occurs in the transfer of electricity. In addition, some EVs will use mains power to heat or cool the battery (prolonging battery life).

Charging loss means more energy is supplied than what ends up in the battery. Typically, slower charging speeds (i.e. 10A) and colder temperatures result in more charging loss.