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If you want to convert between amp-hours and watt-hours or find the C-rate of a battery, give this battery capacity calculator a try. It is a handy tool that helps you understand how much energy is stored in the battery that your smartphone or a drone runs on.
Additionally, it provides you with step-by-step instructions on how to calculate amp-hours and watt-hours, so you will be able to perform all of these calculations by yourself, too!
What are battery amp hours?
The primary function of a battery is to store energy. We usually measure this energy in watt-hours, which correspond to one watt of power sustained for one hour.
If we want to calculate how much energy – in other words, how many watt-hours – is stored in a battery, we need information about the electric charge in the battery. This value is commonly expressed in amp-hours – amps (units of electric current) multiplied by hours (units of time).
Battery capacity formula
As you might remember from our article on Ohm’s law, the power
P of an electrical device is equal to voltage
V multiplied by current
P = V × I
E is power
P multiplied by time
T, all we have to do to find the energy stored in a battery is to multiply both sides of the equation by time:
E = V × I × T
Hopefully, you remember that amp hours are a measure of electric charge
Q (the battery capacity). Hence, the final version of the battery capacity formula looks like this:
E = V × Q,
E– Energy stored in a battery, expressed in watt-hours;
V– Voltage of the battery; and
Q– Battery capacity, measured in amp-hours.
How to calculate amp hours?
Let’s assume you want to find out the capacity of your battery, knowing its voltage and the energy stored in it.
- Note down the voltage. In this example, we will take a standard 12 V battery.
- Choose the amount of energy stored in the battery. Let’s say it’s 26.4 Wh.
- Input these numbers into their respective fields of the battery amp hour calculator. It uses the formula mentioned above:
E = V × Q
Q = E / V = 26.4 / 12 = 2.2 Ah
- The battery capacity is equal to 2.2 Ah.
Battery capacity calculator: advanced mode
If you open the
advanced mode of this battery capacity calculator, you can compute three other parameters of a battery.
- C-rate of the battery. C-rate is used to describe how fast a battery charges and discharges. For example, a 1C battery needs one hour at 100 A to load 100 Ah. A 2C battery would need just half an hour to load 100 Ah, while a 0.5C battery requires two hours.
- Discharge current. This is the current
Iused for either charging or discharging your battery. It is linked to the C-rate with the following equation:
I = C-rate × Q
- Runtime to full capacity. It is simply the time
tneeded to fully charge or discharge the battery when using the discharge current, measured in minutes. You can calculate it as
t = 1/C.