# DC/DC converter common ground



## vpoppv (Jul 27, 2009)

I wanted to use a smaller batter as my 12v accessory battery, so I bought a cheapo DC/DC converter, 10 amp, 72v-12v. It only has three wires! One for 12v, One for 72v, and one for common ground. Since I know you are never, ever under any circumstances supposed to connect your pack to the car chassis, so I bought junk! So my question is, in what possible application could someone use this??


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## Guest (Sep 4, 2011)

What did you buy? Link to the product would help.


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## vpoppv (Jul 27, 2009)

http://www.ebay.com/itm/REALLY-12V-...360?pt=LH_DefaultDomain_0&hash=item20bb9a6178

It says right in the listing "common ground" and it is obvious from the picture that there are only 3 wires, but I never quite realized what I was getting anyway....


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## Jan (Oct 5, 2009)

vpoppv said:


> ...but I never quite realized what I was getting anyway....


There where more signs...


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## Guest (Sep 5, 2011)

It is an E-Bike converter. Low voltage setups. You got what you paid for on this one. Get a real DC DC converter for your car. If you don't need much in the way of output amps then a good Kelly DC DC will do you good. There are others. But get a good one. Not some wimpy bicycle one. Unless of course your putting it on your bicycle conversion.


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## Guest (Sep 5, 2011)

> but I never quite realized what I was getting anyway....


This type of thinking will bite you in the behind every time. Know what your doing before spending your hard earned money. Now you must spend even more.


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## vpoppv (Jul 27, 2009)

gottdi said:


> This type of thinking will bite you in the behind every time. Know what your doing before spending your hard earned money. Now you must spend even more.


 Naw, it won't cost me more, the battery I have has worked fine for 2 years, it'll work for a few more. I just wanted to save a little weight. Thanks for the advice on the Kelly.


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## Sunking (Aug 10, 2009)

So why wouldn't it work guys?

Just because there is a wire called ground does not mean it has to be connected to the chassis. Just use it as the common return and wire it to a block for distribution.

The only problem I see with it is some of the 12 volt load devices having there chassis bonded to the negative polarity which in effect would bond the common to chassis.


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## DJBecker (Nov 3, 2010)

Sunking said:


> So why wouldn't it work guys?


Because it's not an isolating converter.

An isolating converter will have at least four wires.

This is a simpler buck converter, with an inductor rather than an isolating transformer.


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## EVfun (Mar 14, 2010)

Sunking said:


> The only problem I see with it is some of the 12 volt load devices having there chassis bonded to the negative polarity which in effect would bond the common to chassis.


Usually a LOT of things in a motor vehicle will be frame bonded. It would be hard to lift the ground off the frame. Most of the lights and accessories where designed to find ground to the body through their mounting.


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