Taking Scotty's assessment a little further(ok a
lot further).
You would be using a 12V lead acid deep cycle battery type.
Because you need 5V you would use a switching power converter called a DC to DC. It would consume 11-15VDC from the battery and convert it to 5VDC.
The DC to DC will be at least 80% efficient.
This means to provide your 14W it would consume 14W/0.80 = 17.5W
With a nominal 12V system the current you'd be pulling from the battery would be 17.5W/12V = 1.5A
Running this for 300 hours represents. 1.5A x 300hrs = 450Ahrs of battery capacity.
If you want to do this regularly then you must not allow the battery to drop below 60% of its capacity ESPECIALLY when strung over that many hours. This means you need more battery capacity than the raw consumption.
Actual battery capacity = 450Ahrs/0.60 = 750Ahrs
750Ahrs is what you need if you want to run more than a handful of these discharge/charge cycles before the batteries start failing.
Here is an example of the type batteries I would use.
You would need three pairs in series - hooked in parallel to feed your DC to DC.
Again two of these hooked in parallel to give 12V would give 12V 225Ahrs. Then since you need 750Ahrs you would need this three times with each series pair hooked in parallel to the other two (pairs) for a total of 6 batteries putting out 12V with a capacity of 675Ahrs.
Note this only adds up to 675 not 750. This means you will be dragging the "bank" down far lower than the previously recommended 60% limit. This will directly reduce the "banks" service life.
If I were faced with the problem I would add another set of 2 batteries in parallel to bring the bank to 900Ahrs and then I'd be getting the maximum life of the batteries and they would also be able to run more than 300hrs before hitting the 60% point.
However note the weight 62 x 6 = 372 or better(or worse) 496lbs.
You could also look at different model of battery to see if you could find a size that would land closer to 750AHrs. I have 4 of the above batteries and can say that they are about as large as one person can reasonably move about or carry short distances, so that needs to be considered too.
Last comment. If you can hook up some sort of wind or solar charger then the size of the bank could be reduced substantially. On the other hand solar costs a bundle compared to batteries. And wind can often be flaky.
You will need to consider how to charge this bank because that isn't trivial either.
What's the application?
Keith Cress
Flamin Systems, Inc.-
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