Calculating and Measuring Power Consumption

The design of an autonomous system always begins with the calculation of how much power is consumed. The easiest way to measure your device is a laboratory power supply that features a voltage and ampere meter. The nominal voltage provided by a lead acid battery typically varies between 11 Volts (empty) and about 14.5 Volt (charging, voltage at charging limit). You can tune the voltage at the laboratory power supply and see how much current the device draws at different voltages. If a laboratory power supply is not available, measurement can be performed by using the supply shipped with the device. Interrupt one cable that goes to the DC input of your device and insert an ampere-meter (or ammeter). Note that a ammeter will burn itself or your power supply if applied between the positive and negative terminal because it behaves like a simple cable between the probes -thus creating a short. Many ammeters have an unfused input, so exercise caution as they can be easily damaged.

The amount of power consumed can be calculated with this formula:

P = U * I

P being Power in Watts, U being voltage in Volts, I being current in Ampere. For example:

6 Watts = 12 Volts * 0.5 Ampere

The result is the rating of the device. If the device of the example is operating for an hour it will simply consume 6 Watt-hours (Wh), respectively 0.5 Ampere-hours (Ah). Thus the device will draw 144 Wh or 12 Ah a day.

To simplify things, I will use the nominal voltage rating of batteries for calculations and not take into account that the voltage provided by the battery varies depending on its state of charge. Batteries are rated at their capacity in Ah -so it is easier to calculate using Ah instead of Wh. A battery from a big truck has typically 170 Ah -thus a 100% charged truck battery would power the device for about 340 hours during a 100% discharging cycle.