My laptop which is a Lenovo (IBM) T41 needs a 12V auto power adaptor. I've been using a 200W inverter, but that thing likes to run my car battery dead, not to mention that it is constantly screetching an alarm as soon as the car battery is not fully charged. I have never understood why it drains my battery when it's just charging the computer battery, (with computer shut off), but it does.
Anyhow, I was told that I need to get a direct 12V charger for the computer and eliminate that annoying inverter. I was shocked when I saw the price for this charger at $140. (Almost as much as I paid for the computer). Screw that, I already have a cig lighter socket and the plug for the computer on a wire. All I need is the schematic to step the voltage to whatever it needs to be. (which I still have to find out). Anyone know where I might locate such a schematic online? I'm sure it's a very simple circuit.
If you have the skills to (successfully) implement a voltage converter, you have the skills and equipment to diagnose the problem. Do the math. I'm too lazy to look up the numbers your your particular machine, but, to use round numbers, if your battery is 60WH, it might take
120WH to charge it from 12v stepped up to 120v, stepped down to 18V, stepped again by the internal charging inverter. That's 10AH, which is a significant percentage of your car battery when it was new.
200W inverters come in various efficiencies. Get out your meter and determine the efficiency of yours...as well as the standby consumption. You can't leave the/any inverter connected and running over night.
Best one I've seen draws 140mA unloaded. Most are 5x that or more. Turn it off when not in use. And the wall charger uses standby juice, as does the internal charging circuitry.
The screeching is more likely caused by using wire that's too small. Your meter will tell you how many volts at the inverter input. It should be almost the same as at the battery under load. If it's not, fix that.
Building a car inverter is a difficult problem for at least two reasons.
1) The input can go from 10V to 15V or so. That's too close to the required output voltage for a simple regulator. Designs exist, but they're not as simple.
2) The auto electrical system is an EXTREMELY hostile environment. It takes a lot of design effort to keep it from blowing up. Read this:
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If you're using any of the hardware ports on your computer, like audio out, ground loops exist that can fry your laptop with voltage spikes.
So, if you take a random design off the web, build it without paying attention to all the subtle effects of the physical wiring, ignore the hostile environment and don't thoroughly test it under conditions likely to exist in the electrical system, you're asking for a fried laptop. YMMV
For most of us, the 120VAC inverter is a good solution that will work on your next computer and the one after that. Just fix the voltage drop problem and turn it off when not in use.
And low cost models are available from eBay as pointed out by another poster. Some models are universal in that they come with a set of eight or nine adapter plugs that permit use with virtually all main stream laptops. The universal ones also have a slide switch available that allows setting the output voltage to be compatible with the particular laptop. All that for just a few $$$ US.
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Michael Karas
Carousel Design Solutions
http://www.carousel-design.com
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