# Power analysis of a simple circuit and battery lifetimes

I only do electronics stuff as a hobby so forgive my simple question.

I have a simple IR LED/photodiode detector circuit hooked up to an arduino microcontroller. It's not important but the purpose is a drip monitor. I want to put this circuit in a box and hook up a battery supply rather than my computer. I was wondering how to go about estimating the lifetime of this circuit and how long I should expect to wait before thinking about replacing the battery. Is power consumption the only factor to worrry about here.

How do I calculate the power consumption for the entire circuit? I know P = I*V = I^2*R but that's only for a single element. Is there an easy way to measure the power for the whole circuit? Also, how do I measure the power for the Arduino? It has many pins so I'm not sure how to measure the voltage and current "across" it.

• P does equal IV but it doesn't equal IR - I think you mean IIR or (I^2)*R. Anyway, have you got a multimeter or any means of measuring current? – Andy aka Mar 24 '13 at 21:57
• Oops. Yes, I have a multimeter. – ballaw Mar 24 '13 at 22:12

If you have a multimeter you can calculate dc power based on voltage measurment and current measurement but i think you are more interested in battery life? If so, connect the battery and measure the current taken. Then compare this to the ampere-hour figure of your battery - if you have a 1Ah battery this can provide 1A for 1 hour or 100mA for ten hours or 10mA for 100hours. But battery suppliers usually assume the Ah rating is for all the usable energy in the battery. It's probably more likely you'll be able to use about half of this but... and here's the small print....

How close is the battery terminal voltage to what your arduino needs? If you are directly powering it, the output voltage may not have to droop much before the arduino gives up. You maybe be feeding it via a linear voltage regulator and this will give you a bit more headroom. If you are using a switching PSU this will give you more. I can't say - only you can.

Since you are designing a battery powered application, the key is to save power by making the Arduino and the LEDs 'sleep' most of the time. In that case, calculating power usage is not as simple as taking a static current measurement using a multimeter. You have to find the average power used taking into account the ratio of time spent sleeping and active.

The Arduino only needs to wake up a few milliseconds at a time to monitor the drip or wait for an interrupt to wake up when a drip is sensed. Also make sure the LEDs are pulsed an not constantly on. With some of these optimizations, you can expect Arduinos to stay powered for over a year with AA batteries. Please try to read the links below for more information.

https://hwstartup.wordpress.com/2013/04/15/how-to-run-an-arduino-clone-on-aa-batteries-for-over-a-year-part-2/

https://www.openhomeautomation.net/arduino-battery/