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


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.

  • 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 0:14
  • 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 0:19
  • 3
    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

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.

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