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What would be the estimated run time off of parallel 9V batteries fed through a LM7805 5V regulator (no peripherals)?

Fearless
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2 Answers2

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9V battery has about 500mAh, since your Pi consumes about 700mA, you're looking at run times about 40 minutes, more or less depending on the type of the battery (a good quality panasonic industrial type has 570 mAh.. low end type probably more like 400).

running Pi with 9V battery and a linear voltage regulator is very inefficient, you basically turn almost half of the energy the battery has into the heat. you should have considered running off 6V battery pack or with DC-DC converter.

lenik
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  • Thanks. Is there any way to make a power source that'll last about 8 hours straight? – Fearless Jan 23 '14 at 23:14
  • something like lead-acid motorbike battery, 6V 6000mAh might work. however batteries lose the ability to recharge pretty fast, and 6V 12000mAh seems a much better solution if you intend to run it more than a few times. – lenik Jan 23 '14 at 23:42
  • how about LiPo or lithium ion? – Fearless Jan 24 '14 at 00:14
  • lithium batteries require a complicated charger, prone to failure, may catch a fire if handled roughly or overcharged. i would advise against putting lithium batteries into design that is not under your constant supervision. – lenik Jan 24 '14 at 00:19
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    Why not just use one of those USB-battery-packs? No fiddling required and you can get a 13.000mAh device for about 30€. – Plankalkül Jan 24 '14 at 17:05
  • @Plankalkül, I like the usb packs, but the Pi is over the USB mA limits, so quality(!) battery packs could have issues without using multiple USB outs and a combiner cable. For example, my monitor with usb hub requires both an output port and the input port (which it back-feeds) to power my Pi B. I've been debating switching to a Beaglebone Black primarily because it is under the USB power threshold. – lossleader Jan 26 '14 at 18:31
  • Hmm. Most battery packs I have seen generally have an output of 2000mA. – Plankalkül Jan 27 '14 at 21:08
  • Lithium ion and LiPo word okay in cellphones and laptops, why not with an RPi? I'm just trying to find an easily rechargeable, lightweight, power source. Using 8AAs or a couple 9v each time is costly >.< – Fearless May 14 '14 at 13:51
  • @Fearless Li/LiPo batteries were banned from aircargo a while ago. I'm not sure if there's anything changed since then, but I'd refrain from using them in a design that I would not like to go up in flames (I have a few Li/LiPo burned away for no apparent reason). – lenik May 14 '14 at 14:36
  • @Lenik How come cellphones and laptops can use them with no problems? – Fearless May 14 '14 at 15:05
  • not sure about laptops, since there are plenty of pictures of burning laptops on the web, but cellphones could get away with a dedicated charging circuitry, that allows them to cater to the particular brand of battery they use. if you're sure your charger will be up to the job, you may use LiPo without any worries. – lenik May 14 '14 at 16:07
  • @lenik So I really need to make sure that I have "smart" charger that can carefully monitor the battery. I've seen ones with per-cell monitoring and temp regulation. – Fearless May 15 '14 at 02:13
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Rather than a linear regulator, a switched mode regulator is much more efficient. Conveniently the MoPi board provides it and would accept 9V input with no additionally circuitry. A Model B with no peripherals and no load actually draws only 400--500 mA. A single alkaline 9V battery should provide enough charge for about an hour. A 9V lithium a bit more. Unfortunately, a rechargeable 9V battery is typically only around 300 mAh, that means less than an hour. Maybe just with a Model A. In comparison, the MoPi people show 8 AA Ni-Mh batteries (2600 mAh) to run a Pi for around 8-10 hours.

Fred
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