lwalthour
Posts: 10
Joined: Mon Oct 03, 2011 3:29 pm

Re: 9v Battery Power

Sat Oct 22, 2011 3:15 pm

I'm still in the newbie stage of hardhacks, but in theory wouldn't an LM7805 voltage regulator and 9v battery be enough to power a RPi?

LM7805:
Vout: +5 V
Vin: +7 V min, +35 V max
Iout: 1 A

Also, about how long would a 500mAh 9v battery power it for assuming a 500mA 5v load? Looks like an hour to me but I'm not sure if changing the voltage would make a difference.

[EDIT]
I have a couple of 7805's laying around, so I'm kinda tempted to see if it charges my cell phone... on the other hand, I don't want to blow anything up.

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emercer
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Re: 9v Battery Power

Sat Oct 22, 2011 11:20 pm

In a word, it is.

Why is it not ideal? Because linear regulators like the LM78XX simply "burn" the excess voltage as heat, so you're outputting 5V to the RasPi, but eating it from the battery like a 9V appliance. The ideal solution would be to replace the LM7805 with some 5V step-down switching regulator like the DE-SW050 or the ADP2105

NoSuchNick
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Re: 9v Battery Power

Sat Oct 22, 2011 11:41 pm

Since it sounds like you want to use something like this:
http://en.wikipedia.org/wiki/N.....lt_battery
There are a couple of other things to consider.
First of all, like emercer says, the 7805 will "waste" almost half of the energy, when the battery is full. However that won't matter much, since after just a couple of minutes (10-20) the voltage will have dropped considerably.
I haven't found any reliable source on how long a typical 9V battery will take to reach ~6.5V (rough minimum where a L7805 would still work) at a drain of 500mA, but that is because non of the normal 9V batteries are rated for such a strong current.
I've found facts from two different batteries. One says the total capacity of the battery is only ~350mA at a drain of 500mA, the other lists only 50mA as maximum drain. It all depends a lot on the specific battery type, but almost anything that has 500mAh capacity at 9V is not rated for a continous current of 500mA.

In my opinion the simplest way to power the pi from batteries would be 6 AA NiMH rechargable batteries and a switching converter. These should power the pi for >4 hours at a reasonable size. If you need a smaller size, you could simply go to AAA or use Lithium Ion batteries.

lwalthour
Posts: 10
Joined: Mon Oct 03, 2011 3:29 pm

Re: 9v Battery Power

Sat Oct 22, 2011 11:42 pm

Hmm, so definitely not ideal, unless I just need to macguyver something together with what I have.

So, given:
RPiB drawing 500mA
DE-SW050
9v with a capacity of 500mAh
...the lifetime would be significantly longer than with an LM7805?

Thanks for the help!

NoSuchNick
Posts: 42
Joined: Tue Sep 20, 2011 1:38 pm

Re: 9v Battery Power

Sat Oct 22, 2011 11:50 pm

Quote from lwalthour on October 23, 2011, 00:42
Hmm, so definitely not ideal, unless I just need to macguyver something together with what I have.

So, given:
RPiB drawing 500mA
DE-SW050
9v with a capacity of 500mAh
...the lifetime would be significantly longer than with an LM7805?

Thanks for the help!

Two things to note:
- The r-pi might not always draw 500mA
- if have the wrong type of 9V source you might have trouble getting 500mA out of it regardless of the duration

Other than that, the answer is yes, but it will still be a lot shorter than any 6 AA type batteries between 1.2 and 1.5 Volt (these typically have between 700 and 3500mAh)

WizardOfOZ
Posts: 76
Joined: Thu Oct 13, 2011 8:08 am

Re: 9v Battery Power

Sun Oct 23, 2011 12:08 am

A quick calculation gives 25 minutes of running time to 6.5V @ 500mA on an Energizer 522, assuming zero internal resistance. Which won't ever happen, so in reality you will get much less. With factory fresh brand name batteries.

lwalthour
Posts: 10
Joined: Mon Oct 03, 2011 3:29 pm

Re: 9v Battery Power

Sun Oct 23, 2011 1:14 am

Quote from NoSuchNick on October 23, 2011, 00:50
Quote from lwalthour on October 23, 2011, 00:42
Hmm, so definitely not ideal, unless I just need to macguyver something together with what I have.

So, given:
RPiB drawing 500mA
DE-SW050
9v with a capacity of 500mAh
...the lifetime would be significantly longer than with an LM7805?

Thanks for the help!

Two things to note:
- The r-pi might not always draw 500mA
- if have the wrong type of 9V source you might have trouble getting 500mA out of it regardless of the duration

Other than that, the answer is yes, but it will still be a lot shorter than any 6 AA type batteries between 1.2 and 1.5 Volt (these typically have between 700 and 3500mAh)

Looks like you posted while I was composing my post... sorry if it seems like I ignored you!

Bacan
Posts: 347
Joined: Sun Sep 25, 2011 10:03 pm

Re: 9v Battery Power

Sun Oct 23, 2011 5:38 am

The real power consumer will be the portable display system. Unless you just want to do a mobile data capture system, like a medical monitor for heart & temp rate.

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Lob0426
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Re: 9v Battery Power

Sun Oct 23, 2011 4:11 pm

The only reason I am getting away with a 7805 is that I am supplying it with a 12v wall wart. I will look into those switching regulators as the drain will be a lot less than the 7805 which is a linear regulator. Anyone have a URL to a circuit for 12v to 5v for either of those? I have one for the 7805 and already tested it (breadboard) using a 9v battery. It ate that battery alive real fast and there was no drain at all, just voltage tests!
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reiuyi
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Re: 9v Battery Power

Mon Oct 24, 2011 11:34 am

Look into this http://www.dealextreme.com/p/8.....ubec-45214

It's based on de LM2596 and has about 80% efficiency converting 12v to 5v. It's 3A max. For that price, you can't really go wrong. The LM7805 is trash.

The LM2596 is like 7-35volt in, 5 volt out, though I have not yet tested what the 5v drop would be like if you supplied less than 7 volts to Vin. Then again, I'm not even sure what Raspi's exact input voltage is with respect to margins. Anyway, the LM2596 is perfect for 12v to 5v conversions, and it's also useful for any battery-powered projects

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