Battery power calculation for my project


I've created my project which consists of an Arduino Mega and a number of components.

Thus far I have powered it over USB at 5v and measure the power consumption to be a peak of 0.2A (200 mA).

Now, there is an additional component that I have to add to the solution which uses 12v DC and consumes 40 mA. My plan is to buy a lithium 12v pack and provide power through the power jack, to calculate the peak power consumption, do I just add the 40 mA to the 200 mA giving a total of 240 mA?

If that was indeed the case and I wanted to run it for 10 hours, I would need a battery that is at least 10 * 240 mAh = 2,400 mAh.



The onboard 5volt regulator of the Mega can’t (shouldn’t) supply that ~200mA when powered from 12volt.
The regulator could overheat (depending on ambient temp) and shut down after a while (if you’re lucky).

You could use a boost converter to make that 12volt from 5volt USB, or from a lower battery voltage.

Or use a 12volt battery, and power the Mega with a 5volt buck converter (on the Mega’s 5volt pin).
In this case, the ~200mA will be reduced to a <=100mA draw.

Ok, I'm little confused. At present I am powering it over USB, the draw is 200 mA @ 5v. If I simply wanted to convert this to battery power and run for 10 hours, I could get a 5v battery with 2,000 mAh and power it through. However, from what you are saying is that I should not power it through the jack though.

You didn't tell us what 'other components' are connected to the Mega.
I assume a constant current draw.

When you power the Mega from USB, then the onboard 5volt regulator is not used.
Because USB power is already 5volt.

When you power the Mega with a higher voltage, either on the DC socket or on the V-in pin, then the onboard 5volt regulator is used.

The regulator has to drop 12volt to 5volt, so 7volt*0.2A = 1.4watt is dissipated as heat in the regulator.
That's borderline for an onboard regulator with only a piece of circuitboard as heatsink.

Several solutions for your problem.

  1. Use a (bigger) 12volt battery that can deliver 240mA for 10 hours.
    Connect it to the 12volt device and the DC socket of the Mega (and watch the temp of the regulator).

  2. Use a (smaller) 12volt battery that can deliver 140mA for 10hours.
    Power the 12volt device from the battery, and power the Mega through a 12volt to 5volt DC/DC converter, connected to the 5volt pin.

  3. Use a battery with a lower voltage (e.g. a 7.2volt 2-cell LiPo), and connect to the V-in pin.
    Power the 12volt device with a 12volt boost converter from the battery.
    You will need ~300mA for 10 hours.

Great, thanks so much for the replies. I think I will go for the 12v with DC DC step down such as this one:

SMAKN Dc/dc Converter 12v Step Down to 5v/3a Power Supply Module

As for what is being powered:

1 x HC-12 RF module
1 x PN532 NFC
1 x Adafruit Wave shield

And theb12v item is this:

Seco-Larm E-931-S35RRQ Enforcer Indoor/Outdoor Wall Mounted Photoelectric Beam Sensor with 35 Foot Range


Can you please post a copy of your idea of your circuit, in CAD or a picture of a hand drawn circuit in jpg, png?

Thanks.. Tom.. :slight_smile:

If this is intended to be a permanent project you can likely shave some 150 mA or more of that power consumption off of your project.

  1. drop the Mega. Go for an Uno, make your project work on it. It doesn’t look like you need the Mega’s many pins.
  2. drop the Uno, keeping only the processor (many Unos have this in a socket on the board).
  3. add the required external parts (filter caps mostly), use the internal 8 MHz oscillator, and see your power use drop to a few mA for the processor, plus whatever the peripherals take.

And now your battery can last 4x as long, or you can get a battery 1/4 the size.

So at present this is a prototype, if it proves to be successful then clearly there is some refinement that can be done. Never the less, to get the prototype proven it has to operate remotely on batteries.

I'll put together a diagram that shows what I have.

That showcase one I'd make into stage 2 of the project.

Stage 1: development boards on USB power, to get the schematics proven and for easier debugging of the software.

Stage 2: strip down to the basics, solder everything as compact as practical on a piece of perfboard, and battery power that contraption and use it for your showcase. Looks a lot better than a rats nest of wires that are awkwardly held together.