How do I determine runtime if the batteries aren’t at 100% charge?

To determine the runtime of a battery system given its power consumption, voltage, and a discharge cut-off voltage of 11V, use the formula: Runtime (hours) = (Battery Capacity (Ah) * (Starting Voltage – Cut-off Voltage)) / Power Consumption (W); where “Ah” represents Ampere-hours, “V” represents Volts, and “W” represents Watts.

Side note: This is a rough approximation since there are other factors like heat, cold, and resistance.

Explanation:

Calculate usable energy:
First, find the usable energy stored in the battery by multiplying its capacity (in Ampere-hours) by the difference between the starting voltage and the cut-off voltage (11V in this case).

Divide by power consumption:
Then, divide the calculated usable energy by the power consumption of the device to get the runtime in hours.

Key points to remember:

Battery capacity: Always use the rated capacity of the battery in Ampere-hours (Ah).

Average voltage: This calculation assumes a constant power consumption, so the voltage used should be an average voltage during discharge, which might be slightly lower than the initial voltage.
Efficiency factor: For a more accurate calculation, you can include an efficiency factor to account for energy losses within the system.

Example:

Scenario:
A battery with a capacity of 100Ah, a starting voltage of 12V, and a cut-off voltage of 11V is powering a device that consumes 20W.
Calculation:
Usable energy = (100Ah) * (12V – 11V) = 100 Wh

Runtime = 100 Wh / 20W = 5 hours