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I 'm looking for a battery solution to power up BeagleBoard-xM. Until now, I used a 5 V, 3.2 A AC/DC power supply adapter (2.1 mm barrel plug).

I 'm thinking of using a 4 x 1.2 NiMH AA battery pack (or NiCd). This way voltage will be 4.8 V, that is the minimum -xM's electrical specification for input voltage DC. If this is not a proper solution, please let me know.

My question is:

How can I determine amperage to be 2 A or greater? As far as I know, the board will only take the current it needs when attached to the battery, but low amperage may be the reason of features misfunction.

Any help, or even different solution, would be appreciated.

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    \$\begingroup\$ 750A???? I'd be a bit wary of that expansion bus... \$\endgroup\$ – Brian Drummond Mar 14 '14 at 12:05
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    \$\begingroup\$ Why not Li-ion batteries? They're easily available as packs in multiples of 3.7 V and have pretty good amperage. You could get a 7.4 V pack and add a regulator circuit for getting the rated voltage. \$\endgroup\$ – Shrikant Giridhar Mar 14 '14 at 17:19
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I am fairly new to this stuff as well but I will do my best to answer your question.

I do not believe that four 1.2v batteries will be sufficient on their own (at least not for too long) because of the inevitable voltage drop that will occur over time.

Most NiMH batteries I have come across have between 1800 and 2500mAh and four of these will be able to run your board for a while before they lose the ability to source 750+mA, but when they do it will cause the supply voltage to drop below 4.8v.

I see that the typical max current supplied by the USB ports on the board is 1500mA, and assuming outbound USB current was not factored into the typical average current draw of the board, it could mean that as much 2250mA could be needed at any given time. (maybe even a bit more, I don't know enough about the beagleboards to say). At that level of current draw it wouldn't take very much time for a voltage drop to occur.

I would instead use a switching regulator to power your board. I looked briefly but didn't find many prefabricated "boost" converters that would supply sufficient amperage, but I did find a lot of "buck" type converters that would take a higher input voltage (say 7.2v, from six batteries) and convert it to 5v 2A+ at the output.

You can also do a little research and build your own buck or boost converter instead of buying one, or take a simpler approach and build a linear regulator, but these are not as efficient and you wont get as much run time out of your batteries.

The max current draw may(MAYBE, not sure) also be something that is only approached infrequently in quick spikes. If this is the case then it may not be necessary for your regulator to source the max current, but it would mean that you should put some extra capacitors in your regulator's output stage to handle instances of increased current draw.

I hope this helps. :)

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  • \$\begingroup\$ I personally drive my BeagleBoard (classic) from a 7.4V (2-cell( LiPo pack with 5V buck converter rated 5A - it never needs anywhere that much current but it has worked fantastic for me. With a USB WiFi and USB Camera plugged in it drew about 1.5A. Standalone it was about 300-500mA \$\endgroup\$ – EkriirkE May 20 '14 at 19:16
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It would be great if you had a KillaWatt device to measure the actual current or watts (and thus calculate current) that your device is using. Then you don't have to guess. There are just so many variables in that datasheet that it is too hard to estimate what you will need.

From my own experience with NiMh AA batteries, if you use the Eneloop or the Eneloop XX you will get 2000 mAh or 2500 mAh respectively and they both can provide a ridiculous current for a small little battery. I have seen them put out 5+ amps. I don't know how many charge/recharge cycles you would get pulling that kind of current, but they can do it!

Also, they start out at a much higher voltage, closer to 1.44+ volts fully charged and stay above or around 1.2 volts for a good part of their discharge curve so you will get your 4.8 volts (and then some), but as they get near the end of their capacity, the voltage will drop.

In summary, if you could actually measure how much energy you need, then it would be easy to say whether 4 rechargeable AA batteries can do the job or whether you will need something much more complicated like the buck converter suggested in the other answer. If you can't get access to a KillaWatt to measure your power requirements, 4 Eneloop rechargeable NiMh batteries are relatively inexpensive and quite safe to use at high currents compared to Li-ion technology so it may be worth it to just give it a try!

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