help dog get out - newbie project review

I'm ready to hook up some power to the Mercedes window lift motor I'm using to open my monsterized sliding glass door, and now I need an affordable
power supply.
I attempted to determine the power requirements with my own car ...
I removed the 40A fuse and put my ammeter in line, then turned on the ignition switch. I got a drain of about 1.5 to 2A. Then I ran the window lift and it ran up to 7 to 9A, so I guess it's drawing about 7A without major load. Then I kept the window switch activated after the window was closed, and the meter over-ranged at its 10A max. So, I'm thinking 20A should be enough.
I can buy a 12V/25A power supply from Radioshack for $100, but I'm wondering if there's a more cost effective solution.
For example, I can get auto jumpstarters for $30 to $60 that are rated at 300A/7AHr to 600A/22AHr for starting, but have a 12V jack for powering accessories and a 110V charger to charge them. Would these be a reasonable alternative? Could I leave one of these plugged into 110V all the time?
How about an auto battery speed charger rated at 25A for $60?
I've seen a 6A 110VAC/12VDC converter for $25. It's been too long since I did these kinds of calcs ... would that be ...
6A x 110V = 660W = 12V x 55A?
Or would it be 6A at any voltage with the wattage varying?
Any advice or alternatives would be appreciated.
Also, how do I select a relay to drive the motor from the Stamp?
Thanks, Bruce
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rat shack, ick, just use a pc power supply, you need a load on the 5v to keep it from dong strange things, but other than that it'll work. for example http://www.startech.com/ststore/ItemDetail.cfm?ProductID=ATXPOWER250&tab=b&mt=a i would say you can get much cheaper supplies, but that one puts out 13 amps on the 12v. some places you can get decent supplies that'll fit your power requirements lots better, for maybe 20 bucks, just not at rat shack.
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maybe use a fet, they don't wear out nearly as fast as a relay, if you heat sink them, and they're quite easy to connect. so, the motors arm, has it got limit switches still in place? or you thinking of adding them? maybe hall effect devices? hall effect devices wouldn't wear out as quick as a mechanical switch.
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