How To Calculate Battery Run Time When calculating the runtime of a lithium-ion battery, several factors need to be considered:

Battery capacity (Ah): The battery capacity indicates the amount of charge the battery can store and is generally measured in ampere-hours (Ah). The larger the capacity, the more charge the battery stores and can support longer runtimes.

Current consumption (A): Current consumption indicates the amount of electricity consumed by the battery per hour during use. The exact current consumption can be estimated based on the power of the device.

Depth of Discharge (DOD): Depth of Discharge indicates the ratio of the amount of charge extracted from the battery to the total capacity. The greater the DOD, i.e., the more charge is extracted from the battery, the shorter the life of the battery will be. So in order to maximize the life of the battery, the DOD of lithium-ion batteries will generally be set at about 80%.

The following is a simple example of calculating the runtime of a lithium-ion battery:

Suppose there is a Li-ion battery with a capacity of 3000mAh (3Ah) and the device draws 500mA (0.5A) during use. assuming a DOD limit of 80%, let's now calculate how long the battery can run.

First, we need to calculate the usage time of the battery (in hours). The usage time can be calculated by the following formula:

Usable Capacity = Battery Capacity (Ah) × DOD

Run time = Usable capacity (Ah) * DOD / Current consumption (A)

Run time = 3Ah * 80% / 0.5A = 4.8 hours

This means that a Li-ion battery with a capacity of 3Ah can support 4.8 hours of runtime under ideal conditions using a device with a current of 0.5A.

If you only know the wattage, you will notice that many devices use wattage to determine their primary specification. The wattage can be found on the device's specifications or label.

5W light bulb

20W for a laptop

100W motor

200W solar street light

Convert to current: Convert the power of the device to current using the following formula:
Current (A) = Power (W) / Battery Voltage (V)

The standard voltage of a common Li-ion battery is usually 12V, 24V, 36V, 48V, etc. Here we assume that we use 36V-20Ah for a 1000W device, then the runtime is calculated as follows:

Run time = 20Ah*80% / (1000W/36V) = 0.576 hours

Still confused? Then let's take another example.

Suppose there is an e-bike equipped with a 36V-10Ah Li-ion battery and the motor is rated at 250 W. We will calculate the expected run time of the e-bike under different conditions.

Calculate the current consumption:
Current (A) = Power (W) / Voltage (V)
Current = 250W / 36V ≈ 6.94A

Calculate available capacity:
Available Capacity = Battery Capacity × Deep Discharge Limit
Assuming an 80% DOD limit, available capacity = 10Ah × 0.8 = 8Ah

Calculate the Run time:
Run time (hours) = Usable capacity (Ah) / Current (A)
Run time ≈ 8Ah / 6.94A ≈ 1.15 hours ≈ 69 minutes

Therefore, according to the above estimation, a 36V-10Ah Li-ion battery can support an electric bicycle for about 69 minutes at a rated power of 250W.