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
The amount of energy a battery stores is measured in watt hours or joules, It’s what you get when you integrate current(amps) with voltage.
The reason Ah or mAh is often used is because it’s a more useful measurement for devices that don’t include a boost converter. If you have a linear regulator, the current drawn doesn’t change with battery voltage, the extra power from having a higher voltage is wasted as heat.
With some loads, it’s even worse than that, as the current drawn can be higher at higher voltages. A dumb flashlight, for example.
Basically, it’s a measure of charge(like coulombs), rather than energy(like joules)
An Ampere hour is a unit of charge, the ability of a material to interact with electromagnetic fields. A Watt hour is a unit of energy, how much work the system can do. Depending on the system and the conditions it’s in, you can do a lot of work with a smaller charge, or not a little bit of work with a big charge.
A battery can tell you how much charge it’s bringing to the system, but how much work the system can do with it, the battery can’t guess. Meanwhile the electric companies get their electricity by work, so they product is energy, so they charge you for the energy you use
A Watthour is not a Watt divided by an hour but a Watt times an hour. Wh = W x h not W / h
Watts are already joules divided by seconds. So by multiplying the watts by a unit of time again you end up with measure of the same thing as Joule was originally. (You can get back to joules by dividing by 3600 the number of seconds in an hour)
Watts in addition to be being joules per second are also volts times ampere. W= J/s = VA
So a what hour is a Volt times Ampere times hour. Wh = VAh
so to get from Ampere hours to watt hours you just need to multiply them by the voltage.
Two batteries with the same mAh but different voltage will have different Watthours.
If you know the Watthours a battery claims to have and know the voltage you can just divided the Watthours by the Voltage to get the Amperhours.
Batteries are, in many ways, electricity on easy mode. They use one specific voltage. This means that a bigger battery for the same purpose will have the same voltage, but will hold more energy in the form of more amp-hours. You *could* measure this in joules, but for batteries people choose to use amp-hours instead.
In something where the voltage varies, amp hours become much less useful. You are pretty much forced to use energy (joules). Since your house is AC the voltage is constantly going up and down, not to mention it runs through transformers both inside and outside of your house that also change it.
W/h is a useless measurement. Your probably thinking of Wh or kWh
Power is energy being consumed per second
Wh is an amount of energy. If you have a 250 W device, run it for 3 hours, it consumes 750 Wh of energy. The power company charges you by the kWh, which is 1000 Wh
Ah, or mAh is a measure of charge, usually for batteries. Amps is a measure of current, which is charge/second, so a device drawing 1 mA for 3 hours consumes 3 mAh from a battery. 1 Ah = 1000 mAh
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
No. It displays energy in Wh (watt hour) because thats an energy unit. 1 W := 1 J/s is a unit of power by definition energy per unit time. So 1 Wh = 3600s × 1 W = 3600 J of energy. Thats not that much so we usually use 1 kWh = 1000 × 1 Wh.
We don’t measure battery life by how much energy it stores but by how much charge it stores. Current by definition is charge/time and 1 A (ampere or amp for short) = 1 C/s (Columb per second). So 1 Ah (ampere hour) is a unit of charge. But 1 C is a large quantity of charge so we use 1 mAh as a unit which is 1/1000 × Ah.
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