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How many AA batteries would it take to run a 3rd-gen iPad for one hour?
September 5, 2012 6:53 AM   Subscribe

How many AA batteries would it take to run a 3rd-gen iPad for one hour?
posted by Egg Shen to Technology (7 answers total) 2 users marked this as a favorite
 
well the ipad 3 has a 11,666 mah battery
posted by majortom1981 at 6:57 AM on September 5, 2012


It looks like the stock battery is 42 watt hours and runs for about 10 hours. That means it's using 4.2 watts, so we need 4.2 watt-hours for your one hour of life. Google says a AA battery can provide around 2 watt-hours so that's 2 AA batteries to run it for an hour, 20 to equal the stock battery.
posted by ftm at 7:01 AM on September 5, 2012 [3 favorites]


This product claims 2-4 hours of life from 4 AA batteries.
posted by Rock Steady at 7:05 AM on September 5, 2012 [1 favorite]


From what I read, the iPad can draw up to 2 A at 5 V. That's 10 W.

Good NiMH cells can provide 2.5 Wh, so you'd need four.
posted by scruss at 7:05 AM on September 5, 2012


Different AA battery chemistries have different power capacities. A disposable lithium AA battery can have something like 4.5 Wh of power. So, you could get by with one of those.
posted by Good Brain at 10:05 AM on September 5, 2012


The amount of current the iPad draws depends on how hard it's working. If you used only 2 AA batteries as ftm suggests, that would cover "average usage" but not the hard-working periods (as scruss points out, "up to" 10A (and some sources say maybe more).

The other thing is the voltage: the charger for the battery runs at 5V, not sure about the battery itself. Each AA battery gives 1.5V, so you can't get 5V from two of them.

You'd actually want 4 batteries, which will run it for 2 (or more) hours, if you're really asking. But if it's a comparison-type question that does not involve putting actual AAs on an actual iPad, it may be okay to put in your PowerPoint that 2 batteries run it for 1 hour.
posted by aimedwander at 10:06 AM on September 5, 2012 [1 favorite]


Battery chemistry is really important to the question, as well. IIRC, alkalines are strongly oriented toward trickle usage. NiCD can provide a very high current in a quick burst, but can't hold very much total power. NiMH holds more than alkalines, will deliver the power faster, and is rechargeable, but doesn't have the current-draw capability of NiCD, and is far more expensive. Lithium ion batteries (what the iPad uses) can provide big power draws and high energy density, but are VERY expensive, and the underlying chemistry is actively dangerous; lithium is really, really nasty stuff. And lithium batteries tend to degrade over time, whether you're using them or not.

So, you need to choose a battery technology, and then come up with an arrangement of cells that provides the current needed by the iPad, at the voltage it expects. Once you've got that, the iPad will run, and then you figure out how much runtime you've got. It'll probably be much longer than an hour if you're using alkalines, because you will need a ton of batteries to provide enough current. If you use NiCD, on the other hand, even three cells could probably produce enough current and voltage to run an iPad, but for only a few minutes.

Dan, over at www.dansdata.com, really knows his stuff on battery chemistry, and he could probably answer your question much more accurately. This answer is very handwavy; he can probably be fairly specific.
posted by Malor at 10:43 AM on September 5, 2012 [1 favorite]


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