What Felder failed to mention is that this is no ordinary battery charger.
This is a "Switched Mode" Power Supply. It doesn't have the huge heavy
transformer but uses a small light transformer running at about 40kHz to
transform the current.
The frequency is controlled by a PWM chip that regulated the input and
output by using Pulse Width Modulation. This means input voltage can be
anywhere between 80 volts and 240 volts. No transformer overheating buzzing.

A switching charger normally costs about $270, Felder is saying you can do
it for about $25 if you're willing to play.

-- Gnarlie
http://www.Gnarlodious.com/Vanagon/Electrical/Pages/Bay.html

Entity EMZ spoke thus:

> Man do you guy love to over complaicate things.  Power supplies for
> batteries are very easy to build.  All you need is a transformer
> bridge rectifier and a capacitor.  Yuo don't even need to regulate it.
>
>           Eric                   86-VW4x4
>           vw4x4@fyi.net          86-SS Syncro
>           Pittsburgh, PA USA     1936-Chrysler
>                                  92-Jetta GWC
>           www.fyi.net/~vw4x4/vw4x4.htm
>
> On Fri, 5 Dec 2003, Felder wrote:
>
>> I've been talking with another listmemember about making a 12v batter
>> charger from a pc supply according to some directions my dad found.
>> Thought I'd go ahead and distribute to the group. The idea is to
>> install this in my camper so when I'm plugged into 120v at a campsite,
>> I'm charging my batteries automatically. My dad pointed me to a
>> magazine article that detailed the process. Look for a 10 or 15 amp
>> power supply to begin with. I know virtually nothing about electricity,
>> but it worked till I fried the board with a microtorch. Will be more
>> patient next time.
>>
>> Here's how they did it. It's in September 2003 Model Aviation, and if
>> my description doesn't work, email me and I'll send you a copy of the
>> article.
>>
>> 1. Make sure it works, if you can. My first one did not and I didn't
>> know it until I had done all this (only takes 15 minutes or so, not
>> counting the trip to radio shack if necessary).
>>
>> 2. Cut ALL the wire off at 6 inches.
>>
>> 3. Remove the "motherboard"
>>
>> 4. Locate the black ground wires and clip all but two close to the
>> board.
>>
>> 5. Locate the +12 wires, and clip all but two close to the board.
>>
>> 6. Locate the -5 stuff, +5 stuff, -12 stuff and anything else and cut
>> it off as close to the board as you can.
>>
>> 7. Take a soldering iron and open a hole in one of the cropped +5 wires
>> and another at a ground hole.
>>
>> 8. Now, you should be ready with two black grounds, two reds (or
>> orange) +12v, a hole at one ground and a hole at one +5v.
>>
>> Everything else should be gone.
>>
>> 9. Now you are going to run a load between +5v and ground using the two
>> open holes. The load is what the PS expects to see in a device such as
>> a hard drive.
>>
>> 10. The article suggests that you use either a 1 ohm, 25w resistor or
>> an automotive light bulb type 1156 or similar to provide this load. It
>> says that the light bulb is nice because you can read by it and it
>> serves as a pilot light, but he actually ended up using a Radio shack
>> 50-ohm, 10W resistor (RS item # 271-133). That didn' t work for me, I
>> went with a resistor somewhere close to the 1 ohm 25w which worked
>> fine. He was worried about the inefficiency of the latter, I was not.
>> The author warns that you may need to experiement as all the PSs are
>> identical in many ways but subtly different in others.
>>
>> 11. Take the two reds and the two blacks and double them for the
>> increased current to the battery.
>>
>> 12. Cut a strip of plastic that will span the two holes in the metal
>> case of the PS (assuming you removed the plastic grommets that secure
>> all the wires through these holes, and you bridged the wires properly
>> to obviate the switch). Drill the  plastic in the two holes for Radio
>> Shack Multipurpose posts 274-661. Mount the posts in the hole in the
>> plastic you mounted behind the metal holes in the case, and run the
>> respective wires to each. Now you can have screw on and banana-jack
>> connections for 12v at 10 a or greater, depending on the power supply
>> you acquire.
>>
>> Everything worked great when I had it clipped together, the resistor I
>> chose was showing 13.4 v on my DMM. Then I went to solder the
>> connections and melted something down between the board layers which
>> hosed the whole deal so it would barely turn the fan (used a small
>> torch, should have used an iron). Now I'm out of peecee power supplies,
>> but I know what to do and I saw it work. I will get a few more when
>> convenient.
>>
>> If this isn't clear, I will either scan and email or mail you a copy of
>> the article.
>>
>> Nice fan, too.
>>
>> BTW I took the 4" fan out of the bad unit and epoxied a radio shack
>> slide switch to it to make an exhaust fan for the kitchen. I will mount
>> it in the upper left corner of the driver's side window screen frame,
>> marking and drilling the frame with three small holes at the mounting
>> holes of the fan, and leaving the lower right hole cantilevered over
>> the screen. I'll wire it into the light directly above it, behind the
>> curtain rod so the curtain can slide. Just right for a sightly warm
>> night, a day away from the car in the sun, or to exhaust the kitchen
>> steam etc.
>>
>>   I don't have one yet, but for a cool light see Harbor Freight item
>> 4274-0VGA.
>>
>> Jim
>>
>> On Thursday, December 4, 2003, at 08:09 AM, Gnarlodious wrote:
>>
>>> Entity Felder spoke thus:
>>>
>>>> My dad found a copy of a recent model airplane magazine that had
>>>> explicit instructions for turning a peecee power supply into a 12V
>>>> charger.
>>> Can you send me a copy of that article, or turn me on to a webpage
>>> describing the process?
>>>
>>> --Gnarlie
>>>
>>>
>>