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Mechduino
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Power Usage Question

Sun Jun 17, 2012 5:04 pm

This may be a very stupid question, but I am going to ask it anyway.
Roughly how long will a RasPi run while powered by a 2150mah 7.4v battery(And here is where it gets hard, any rough guesses would be fine) running through http://www.hobbyking.com/hobbyking/stor ... apter.html

Here's a link to the battery if you want to see it, it is pretty cheap but I think it would work well depending on how long it will last. http://www.hobbyking.com/hobbyking/stor ... oduct=7362

Thanks,
-Mechduino
Bluebian, the better Raspbian, okay maybe worse, well...definitely worse.

zardoz99
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Re: Power Usage Question

Sun Jun 17, 2012 5:19 pm

About 3 to 3.5 hours I would guess.

Z.

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alexeames
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Re: Power Usage Question

Sun Jun 17, 2012 5:32 pm

2150 mAh 2s lipo 7.4v

OK - we'll assume you get 85% of the capacity out of it because you don't want to kill the cells.
That leaves use with 1827 mAh.

Assume our regulator gives 85% efficiency. That leaves 1553 mAh.

7.4 is the "empty" voltage, the real voltage under light load will be higher (4.2v/cell when fully charged, decreasing during discharge.) Let's use an average voltage for easy sums of 7.9v

Multiply this 7.9V by 1.827 Ah and you get 14.4 Watt hours. But it does depend what things you have plugged into it.

as you can see in my signature, my Pi uses about 2 Watts. That's when it's idle. It goes up to about 2.5 when being "caned". So assuming 50% idle and 50% "caned", we'll say average use 2.25 Watts.

14.4 Watt hours / 2.25 Watts = 6.4 hours
Alex Eames RasPi.TV, RasP.iO

sharpapotheosis
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Re: Power Usage Question

Sun Jun 17, 2012 5:44 pm

You'll need to use some circuitry to take the voltage down to 5v though.

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Mechduino
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Re: Power Usage Question

Sun Jun 17, 2012 5:49 pm

Firstly, Thank everyone you have all been very helpful, and sharpapotheosis, the point of the converter is not only to make a convenient plug but it also brings it down to 5v, it does this since the standard size closest to 5v is 4.8v and it is better to bring it down rather than up, but thanks for trying to help, I cant wait to try this when my Pi comes.
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alexeames
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Re: Power Usage Question

Sun Jun 17, 2012 6:03 pm

The other thing to consider, if weight is an issue, is whether you can get a lipo with lower C value. 20C is pretty common now - I can remember when they were fairly new, but you won't be flying current hungry RC planes for 6 minutes, you'll be using a much lower discharge rate. The higher the C value, the heavier the cells. If you can still even get 10C, they will be a bit lighter. I doubt HobbyKing sells them though - and it may not even be an issue.

I assume you already have a proper lipo charger?

p.s. I don't know what went wrong with my edit in my previous post. The sentence "But it does depend what things you have plugged into it." should have been right at the end.
Alex Eames RasPi.TV, RasP.iO

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Mechduino
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Re: Power Usage Question

Sun Jun 17, 2012 7:30 pm

Weight is not an issue for me luckily, I just want a cheap effective battery that will run my Pi for a decent amount of time, So that I can make a tablet, though it will not be incredibly compact as it will be mounted in a book, and yes I do have a charger that will work for this, should give a full charge in about an hour and fifty minutes, which isn't bad for these larger batteries.
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alexeames
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Re: Power Usage Question

Sun Jun 17, 2012 8:04 pm

If you wanted a longer duration, you could make up a power supply yourself using one of these...

Image
http://www.ebay.co.uk/itm/390410359686? ... 1439.l2649


And put two sets of input connectors on, so you can connect two batteries in parallel - then you can "hot-swap" batteries without needing to power down. Just a thought, if you feel more adventurous. I've made two 5V USB power supplies out of these recently and they're good so far. :D
Alex Eames RasPi.TV, RasP.iO

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Mechduino
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Re: Power Usage Question

Sun Jun 17, 2012 9:18 pm

That right there is certainly something worth looking into! I'll check it out and see if it is a good alternative. Thanks for the tip!
Bluebian, the better Raspbian, okay maybe worse, well...definitely worse.

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