# How to measure or calculate power consumption

HI, I recently finished a beta version of an airsoft/paintball bomb & Capture-The-Flag gadget. It seems to require more power than I thought, eating through a 1100 mAh 11.4V LiPo in less than 2 hours...

I have one 3'2 TFT touchscreen hooked up to one Mega through a TFT shield. The Mega is also running an DF Robot Mini Mp3 player connected to a 2W/3Ohm speaker. The device is periodically flashing 3 five mm LEDs.

This is the schematic:

![](http://<a target=)">

How do I go about measuring or calculating the power consumption? I need this gadget to run constantly at least 12 hours.

You're just burning power in the on-board linear regulator.
If you have to use that battery, get a switch mode regulator.
You can measure current with a decent multimeter

Put your digital multimeter on "current" setting, make sure the probes are plugged in in the right place (most DVMs require you to put the red probe in a different place for current measurements), connect the red wire to the one going to the battery pack, and the black one to the rest of the circuit. Read the number off the screen. That is the current it's drawing.

Schematic is missing and posted image is too small to see.

When you say 1100 mAh 3S - is that 1100 mAh total capacity (ie, 3 x360mAh cells), or three 1100 mAh cells in series?

You're pulling a fair amount of power there - the mega is 50 (all Arduino boards are power hogs - they are not designed for battery powered applications, so they have things like power lights and on-board serial adapters that sit there wasting power), nother 100-200 for the screen, easy. And a 2W speaker, say we're seeing a half watt out of that on average, that's nother 100 mA...

So I think you're looking at 400-500mA right now, so it sounds right to me....

As noted above, using a DC-DC buck converter to get 5v (or 7v to use on-board regulator) from the 12v will improve battery life significantly - but your project is still a power hog, and your battery is still wimpy.

Yes, I forgot to mention the buck converter I have connected between the battery (it's 1100 mAh in total) and the Arduino. The rest of the components are all connected to the arduino pins and sourced through them, so they can't really draw that much power, can they?

Attaching the schematic since the picture hosting doesn't seem to work.

schematicREAL.pdf (281 KB)

And regarding the battery: would this be a good alternative?

Why don't you use a 5V buck converter, instead of a 7V buck converter and a 5V linear regulator, that's really inefficient.

Mixe:
And regarding the battery: would this be a good alternative?

No, there's absolutely no reason to have a 12V output. Going from around 3-4V to 12V to step it down again to 5V for the Arduino is just a waste of power (and money).

Ah. I see. So this would be more logical?

It's a 12V output one again.
And I'm sorry to pop your bubble, but 70Ah for €45 is just too good to be true ... These cheap Chinese batteries use fake or rejected cells, that are only 100-300mAh in capacity, and they advertise them as having 3000mAh or what not. It's just a scam.
Maybe that's the reason your project doesn't last, a fake battery?

PieterP:
It's a 12V output one again.
And I'm sorry to pop your bubble, but 70Ah for €45 is just too good to be true ... These cheap Chinese batteries use fake or rejected cells, that are only 100-300mAh in capacity, and they advertise them as having 3000mAh or what not. It's just a scam.
Maybe that's the reason your project doesn't last, a fake battery?

No, it's also 5V, look again. It has different outputs. But you're in no way popping any bubble. I ask to get informed, and I can totally believe the scam part. The battery I did use is an RC battery that I normally use to power my airsoft guns, and I can run them for 20+ hours on that single battery. Not continously though.

Well, I guess I'll end up using a motorcycle battery incorporated in the design. 7 Ah should do the trick.

Mixe:
No, it's also 5V, look again.

Nope, 12V too

Capacity 68800mAh
Autual Capacity 12000mAh
Output 5V/2A,12V/2A,16V/2A,19V/3.5A

Also, that 'Actual Capacity' line seems a bit (read: extremely) dodgy ...

Mixe:
Well, I guess I'll end up using a motorcycle battery incorporated in the design. 7 Ah should do the trick.

Should work indeed, and it's easier to charge as well (than a Li-Ion).
On the other hand, it's quite heavy, compared to a Li-Ion of the same capacity.

A power bank might indeed be a good solution, as you suggested, it's even easier to charge than the motorcycle's SLA, a lot lighter, and 5V already, so no need for a step-down converter.
If you buy one from a respected brand, you should be fine.

But the different voltages are listed in the part you quoted: 5V/2A,12V/2A ... And I sort of believe the "actual capacity" line. 12000 mAh would also do the trick. Worth trying.

Mixe:
Worth trying.

I wouldn't agree with that, for €45, you can get a decent 5,000 - 10,000 mAh power bank from a known brand, so you know that you get what you pay for.

I once bought one of these fleabay paks rated at 20,000mAh for not too much money. Unit delivered was certainly heavy enough but on load was lucky to deliver 2000mAh. Since it wasn't worth the hassle of trying to claim a refund I stripped it down. Inside was one small battery and 2 large steel plates !! You pay your money and takes your chance.

Hi,
OPs circuit posted earlier.

Tom...

Does the screen have a backlight? I expect that is where most of the power is going.

Many displays have a PWM input to control the light. If you give it 50% PWM, you will probably find that power is reduced by almost half and the readability is barely affected.