rpiswag
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Joined: Mon May 19, 2014 10:04 pm

Powering the Pi 2B from an AA Battery

Sun Mar 15, 2015 5:51 am

With a .5 to 2 volt Battery to USB psu that can output up to 4 amps can I power a Pi 2B with a wifi adapter Ethernet cable attached with a keyboard and mouse and a USB flash drive? I know the psu can handle it. The psu can charge 2 IPads. Is this load to much for one AA battery. I have various reasons that I can only Use one AA battery but I won't bore you.
A computer's power can't be just measured Gigahertz. It is the same thing with us humans.

jweob
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Joined: Sun Jul 15, 2012 1:56 pm

Re: Powering the Pi 2B from an AA Battery

Sun Mar 15, 2015 7:26 am

Can you provide a link showing the kind of PSU you are using?

I think you would struggle to power the pi off an AA battery. The pi 2 requires about 400mA without any accessories (http://raspi.tv/2015/raspberry-pi2-powe ... easurement). 400mA at 5V is 2 watts. For your 1.5V battery to provide 2 watts it would need to give 1.3 amps. The capacity of a good AA battery is 1.6 amp-hours (http://www.candlepowerforums.com/vb/sho ... -Shoot-Out), so the AA would only last an hour or so. If you add in accessories, losses in the PSU etc. it wouldn't even last this long.

rpiswag
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Re: Powering the Pi 2B from an AA Battery

Sun Mar 15, 2015 12:33 pm

All I need it for the pi to run for 1 minute intervals and for the pi to mostly run in stand by. I got that working I just want to know how long it will run.
A computer's power can't be just measured Gigahertz. It is the same thing with us humans.

jweob
Posts: 31
Joined: Sun Jul 15, 2012 1:56 pm

Re: Powering the Pi 2B from an AA Battery

Sun Mar 15, 2015 2:27 pm

You could use an ammeter between the battery and the PSU to measure the current drawn while it is active and the current drawn while it's on standby, then calculate how many cycles it can do before the battery capacity would be exhausted.

E.g. if the Pi wakes up five times an hour and uses 500mA for 1 minute, then 100mA while it's on standby the energy consumed in one hour would be 0.5A x 5/60 h + 0.1A x 55/60 h = 0.13 Ah. If you use a duracell battery which has a capacity of 1.6 Ah then you would be able to run the system for 1.6 / 1.3 = 12 hrs.

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pluggy
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Re: Powering the Pi 2B from an AA Battery

Sun Mar 15, 2015 4:58 pm

Except the Pi wants 5 volts and the AA is only 1.5....

Assuming a 100% efficient buck convertor, the 500mA the Pi wants would be 1.67 Amps coming from the battery. Less than an hour on all the time (if it would provide that current for that long which is dubious). Maybe 3 and half hours intermittent.

Sounds like cruelty to AA cells to me.....
Don't judge Linux by the Pi.......
I must not tread on too many sacred cows......

ame
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Re: Powering the Pi 2B from an AA Battery

Sun Mar 15, 2015 11:50 pm

rpiswag wrote:With a .5 to 2 volt Battery to USB psu that can output up to 4 amps can I power a Pi 2B with a wifi adapter Ethernet cable attached with a keyboard and mouse and a USB flash drive? I know the psu can handle it. The psu can charge 2 IPads. Is this load to much for one AA battery. I have various reasons that I can only Use one AA battery but I won't bore you.
Isn't this the same question as your other thread? I can't find it now, has it been deleted?
(Edit: No, I misremembered it).

One AA battery has a typical capacity of 2000mAh. At 1.5V this is about 3Wh. The Pi and peripherals will consume about 750mA at 5V, so that's about 3.75Wh. If it works at all then you'd expect to get about 45 minutes of operation. Frankly I'd expect 20 minutes would be extremely generous.

These calculations are trivial and simple. If you cannot work out the answer on paper then you need to study a little more.

To answer your question- why don't you try it? It's a simple experiment. Write up the results and it might be useful for others.

jweob
Posts: 31
Joined: Sun Jul 15, 2012 1:56 pm

Re: Powering the Pi 2B from an AA Battery

Mon Mar 16, 2015 1:50 am

pluggy wrote:Except the Pi wants 5 volts and the AA is only 1.5....
I am embarrassed. The current draw figures in the example calculation are too low. However, if you put the ammeter between the battery and the PSU as I suggested then you will be able to use the figure you get to do the calculation correctly. I agree with ame though - simplest way would just be to try it out.

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