eli5 What is the difference between W/h and mAh

Question

If you have an electric meter, it shows energy consumed in Watts per hour. This energy has an amount, Joules that is. But in a battery there is energy stored that also has an amount. So it should be in Joules as well. But the unit is different: Ah,(or mAh).Why is that and what is the difference between these two u

In: Physics

Answer 1

Watts/hour is not joules. 1 W= 1J/s and 1h =3600s so 1 W/h = 1/3600 W/s^2. That is a measurement of the rate your power usage increases. It is watts time hours that energy in jouls 1Wh = 1J/s * 3600s = 3600J

Compare that to motion. Distance in m is like energy in joules. Speed in m/s is like power in J/s and acceleration in m/s^2 is the in J/s^2. Watt/hours is like acceleration but for energy.

In electronics, you often make a system with a specific voltage. If the voltage is the same power is proportional to the current. Wires’ current limit depends on their thickness. fuses limit current So it is often practical to look at energy consumption in current * time in Ah or mAh. Technically it is not a unit of energy but one of electrical charge.

Some batteries do use Wh as the energy content. Power = voltage* current so you need to multiply the electrical change in Ah with the voltage. Batteries do not have a constant voltage, it change depending on the change level. What is written on them is the nominal voltage and you get a result that colors to correct if you just use it.

So Ah and Wh it functionally the same if the voltage is the same. What is used depends on the situation. when voltages is the same like phones that all today use li-ion batteries at 3.7V, car starter batteures at 12V, and power tools where that system has a constant voltage Ah or mAh is common.

If you look at batteries where voltage is not constant. Laptops are an example where it is often are multiple Li-Ion in series so the power supply can work at a higher voltage and have thinner cables. the voltage depends on the laptop so Wh is used. The same for electrical cars, the voltages varies between manufacturers and models so kWh are used