Laptop external power from batteries (DC/DC)

[Bill McGonigle]

Trying to be budget conscious I recalled that my office UPS (APC) is  
1500VA and 865W, and has lead acid batteries inside.  It claims 2  
hours of runtime at 50W.  I got it on sale for $120 or so.

So, I pulled out the Kill-A-Watt and plugged it inline with my  
laptop, and got a 42W load mostly idle (10% CPU).  So, I pegged the  
CPU's with a tight loop and ran some disk activity and it jumped to  
65W.  Now, and this was surprising to me, I pulled out the battery  
while the CPU was at 100%, and it fell to 40W.  This battery is  
supposedly fully-charged.  I put it back in and it went back up to  
65W.   I'm very surprised it's constantly using 25W of power to keep  
the battery topped off.  That's not very green.

So, to the original point of demo'ing linux in the field without AC,  
this is likely to be a cost-effective option (must cut speaker  
wire! ... use NUT).  Among the membership it ought to be possible to  
borrow a few UPS's for a demo day, should that be necessary.

I'm curious how noisy of an inverter APC puts in those things.  I'd  
hope it's better than the one that plugs into my cigarette lighter  
socket in the car, but I don't have a scope to measure it  (I know  
it's not pure-sine).  I'll plug my microphone into it and the sound  
should give me a pretty good idea.

I'm also now curious about using a bigger battery with the COTS UPS  
since they've already done most of the work for me.  I realize the  
inverter is electrically inefficient, however assuming hot-rodding a  
UPS can work, bigger batteries are more economically efficient than  
any of the other solutions I've found, in a $/VA calculation, and  
they get pretty good runtimes out of relatively small batteries, so  
they've done most of the hard engineering work I'd be unlikely to  
replicate.

-Bill

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Bill McGonigle, Owner           Work: 603.448.4440
BFC Computing, LLC              Home: 603.448.1668
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