Friday, January 21, 2011

Power Strugglez

Continuing on the "ends in Z" theme...

One of the big questions for this trip has been how to provide backup power for the nodes. It's been in the back of my mind to DIY a power solution in the lab, and with the help of the interwebs (Google: LVD circuit) I don't think it would be that big a deal, however there's always too much to do and too little time. As a result, we did some shopping around, and here's what we found / tested.

Sparing you the long iterative process, we came upon two solutions.  The first, coming in at about $55USD was a computer power supply ($9), wired with +12,-3.3 to an Apple 5 charge controller ($22 in USA, $36 here!) and a 7Ah battery ($10).   We originally tried to save on the power supply by using the Ubiquiti PoE bricks to drive the charge controller, but they were nowhere near up to the task. Here's Nick hacking the power supply together (or apart, as the case may be):



The Second, coming in at $38 was a good ol' Chinese-brand UPS with the same battery capacity.  

I figured the OTS UPS would perform markedly less well given the extra DC-AC-DC conversion, but the results were more striking than I imagined:



With the same load (roughly 6W), the OTS UPS lasted less than three hours (partially because the UPS doesn't overcharge the battery quite as much) while the Apple+some crap lasted longer into the night than we cared to stay.   (X axis on graph is fraction of a 24-hour day) Needless to say we're going with the former solution.

A side benefit to the above experiments is some hands-on education with LVD circuits (because I'm certainly no electrical engineer...). A previous worry that had kept us from building the LVD circuit is that if the battery drained too far and became too strong of a load when the power returned, it would hang the routers.  It turns out, that with LVD and a reasonably strong power supply, the  internal resistance of the charging battery is sufficient to bring the line voltage right back up to 14ish volts and turn the electronics right  back on when the mains kick in.

With a little fabbing, we could probably bring the cost  of the UPS down to $25. 

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