I am working on a project right now where two different DC water pumps are being powered by a single 12V DC battery. I am trying to do some performance testing to find how much energy from the battery is used over every cycle (about 4 hours) for a relatively low cost for testing. The initial thought was to attach a Volt meter but that seems too easy and I don't think that would work. Any ideas on what to use to measure the energy usage over that period or a way to measure the battery charge over the 4 hour period that would allow me to make a graph of the energy used? Preferably the data will be in Watt*hours. Any help would be greatly appreciated!
Here's 2 ways you could do it:
Simpler: Measure the battery voltage while each pump is running (it may droop somewhat), and then measure the current with a multimeter. Use these to calculate the power, and then multiply the power by the run time of each pump in a 4 hour period.
Harder/more accurate: Install a current shunt or clamp in the pump circuit, as well as a sensor for the battery voltage, log the output of these using an arduino or similar and let it run for 4 hours. You can calculate the wattage at every time point, multiply that by the sample interval (say 1 second), and then total those up to get the power usage.