Tuesday, February 14, 2012

It is a custom computer roughly the size of a TI calculator. It has 2 ddr2 ram, a ssd that hooks to the motherboard via BGA, a small screen, a mini keyboard like those on blackberry phones, an i7 3.2 GHz cpu, 4 usb, ethernetport, mic/headphone ports, an internal wireless card, and an onboard video chip. I am going to try running it with four AA batteries.Is it possible to run a mini computer on AA batteries?In short, not a chance.

An i7 3.2GHz CPU alone draws about 130 Watts. An individual AA cell provides between 2500 and 3000 mAhr, or say, a max of 4.5 Watt-hr. So add up your four AA cells, and you get 18 W-hr of energy.

Now, of course, that's not going to deliver the various different voltages your system needs (+12Vdc, +5Vdc, +3.3Vdc, -12Vdc).. is there a power supply for this unit? Your four AA cells can provide at most 6V, which itself does you go good at all... you need a power supply of some sort to allow one input voltage (a battery) to provide every voltage the PC motherboard needs. Most laptops take in 14V or so, not 6V. And even 9 or 10 of these cells won't do the job... read on.

So et's forget about what power supply you're going to use for now (the regular one for a desktop PC runs off 110V/220V AC, you can't use that one), and assume the power supply does a perfect conversion (it doesn't, most are 80-90% efficient).

So that CPU... 130W on a 18W-hr battery. Converting, that would be a 64,800 W-sec battery. So you could run that CPU (assuming that perfect power supply) for 498 second, or 8 minutes. Of course, if you factor in any real power supply, you're more like 7.5 minutes. The add memory, screen, ethernet, etc. and you're going to run for maybe a minute or two.. not long enough to actually boot any modern OS. If you used a lower power laptop CPU, you would get much farther, but it's still not a practical system. Even cell phones would not work well from AA cells.

The other problem is this: you're assuming four AA cells could actually provide that much power that fast... a 130W draw would demand 43A of current from each 1.5V cell. A typical AA cell might actually be able to provide an amp or so.. drawing 45A would be no different than a short circuit, the battery would burn up or even possibly explode.

Alkalines have a relatively high internal resistance.. they drop significant voltage as heat when subject to high current draws... that's why NiMh cells, with lower A-hr ratings, actually last longer in high current apps (digital cameras, RC cars, photoflash, DVD players, etc). But even plain old everyday NiMh cells don't usually supply this much current, you need the specialized sort they use in R/C car batteries, hybrid cars, and, well, laptop cells.

To go with the laptop cell, you need a laptop-style power supply and power management. The battery in my laptop, which has an Core 2 processor specialized for low power consumption (as do most laptops) has over 100W-hr capacity, and the ability to deliver serious surge currents as need be. However, if the CPU ran full out, that laptop wouldn't stay on for 1/2 hour. With power management in the OS, it runs 2.5 hours or so on that battery.Is it possible to run a mini computer on AA batteries?Usually devices like that require regulated voltage input, which means it expects a constant value. AA batteries slowly reduce their output as they give up energy, so it's very likely that it will stop running in short order.

As far as actual feasibility, if you get the voltage and current right, it will run, however, with that 3.2GHz i7 in there, I have a feeling it's going to draw a lot of current and your batteries will get dangerously hot and possibly leak/explode.

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