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I want to run Raspberry Pi using a general non-rechargeable 9v battery. Raspberry Pi needs 3.5W and 5V input. Questions:

  1. Which LDO should i use ?

  2. For what amount of time general 9V battery is able to supply 3.5watt power ?

    EDIT1:Battery is Duracell 9V rectangular battery.

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  • \$\begingroup\$ What 9 V battery is it? \$\endgroup\$ – Leon Heller Apr 14 '13 at 21:02
  • \$\begingroup\$ Duracell 9V rectangular battery. \$\endgroup\$ – Parth Parikh Apr 14 '13 at 21:09
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    \$\begingroup\$ Oh the rectangular one!!! \$\endgroup\$ – Andy aka Apr 14 '13 at 22:09
  • \$\begingroup\$ I'd suggest (a) go for 2 lipo cells in series and (b) replace the 3.3V regulator on the board: raspberrypi.org/phpBB3/viewtopic.php?t=12387 \$\endgroup\$ – pjc50 Apr 15 '13 at 10:41
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The data sheet available on this page won't tell you outright but it'll hint at very bad news.

It shows that at a power draw of 0.25W the service life would be 14 hours. However at 0.5W the life falls to - not 7 hours, but 3.5 hours. This shows that pushing the battery beyond its limits results in useful life much shorter than a simple amp-hour calculation would suggest.

Double the power 3 more times and if the same pattern applies, the projected life would be more like 4 minutes than the 36 minutes that Samuel suggested.

Find a battery with your current requirements at least on its design spec. Even four AA cells would a closer match for your needs.

Alternative approach : ditch the R-Pi and use a lower power target processor. This depends on your computational needs, but there are low cost ARM processors like the TI Stellaris and others, with maybe 10% of the processor speed but capable of running at a few ma and sleeping at a few microamps for maybe a year on that battery.

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  • \$\begingroup\$ +1. In addition there is the effect of heat on the battery performance, and output voltage dropping below regulation headroom due to power consumed and batter internal impedance will prevent the RPi from getting sufficient operating voltage anyway. A few minutes is a reasonable hypothesis. \$\endgroup\$ – Anindo Ghosh Apr 15 '13 at 8:02
  • \$\begingroup\$ Yeah, heat is one of the mechanisms (in NiCd some heat actually decreases the internal resistance providing MORE power for a while!) but too much will destroy the battery - another is that the internal resistance starting at 1.8 ohm makes the terminal voltage about 7.5V to start with. \$\endgroup\$ – Brian Drummond Apr 15 '13 at 8:36
  • \$\begingroup\$ So 4 AA batteries can run the Pi for an hour ? or would it die in few minutes ? \$\endgroup\$ – Parth Parikh Apr 15 '13 at 11:56
  • \$\begingroup\$ I said "closer" to your requirement. Check their datasheets. A plain Duracell here uk.farnell.com/duracell/15035013/battery-duracell-plus-aa-pk8/… - datasheet shows voltage vs. time at 0.5A and 1A. 4 cells probably won't do it, 6 cells might give you an hour or two. But rechargeable cells would be a better bet, or LiPo as a commenter suggested. \$\endgroup\$ – Brian Drummond Apr 15 '13 at 16:25
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This isn't a good idea. Do realize you're trying to run a computer off of a standard battery. An LDO will just dissipate any excess power needed to supply the 700mA to the system. Effectively you can just assume your 9V battery will be supplying 700mA and you'll be splitting that power between the LDO and the Raspberry Pi. You'll need to select a 5V LDO that can dissipate at least (9V-5V)*(700mA) = 2.8W.

It's difficult to say how long you're expecting this setup to last, but it will be on the order of half an hour. A typical 9V battery is rated at around 600mAh, to calculate duration you just divide by the current, (600mAh/700mA) = 0.6h = 36 minutes. It could be more, it could be less, the Raspberry Pi will not likely be consuming 700mA at all times and it may even peak beyond that. You need to determine a power profile for your use case.

To get a longer life time you should use a switching regulator, use more batteries in parallel, use high capacity batteries, or work out a way to plug the computer into a wall outlet.

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Here is the datasheet for the Duracell 9V battery.

What you will find is that the battery just isn't up to the task. None of the charts and graphs show operation at the power levels that you require for the Ras-PI.

If I were to extrapolate from the graphs that are there... In the most optimistic case you could power the board for about 3 hours. In the most pessimistic case, you might get about an hour. All of this assumes a switching DC/DC converter with 100% efficiency (which doesn't exist). A practical switching converter is going to give you an average of about 75% efficiency for this project due to the large input voltage range, so reduce those estimates by about 25%.

In short, unless you have a specific need for a battery that will only last about an hour I wouldn't do this.

You certainly do NOT want to use an LDO regulator. That type of regulator is inefficient in this task and might reduce your battery life to well under 30 minutes.

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