How can I connect 2, 12v car batteries together and still get 12v?

Several years ago, I connected two "dissimmilar" 12-volt car batteries. The trick is, and I am supprised to see so many posts that do not mention it is what can happen. Everyone mentions "SIMILAR" batteries.

Yes you can do it, but with an understanding. To keep one battery from discharging into the other you need some form of an industrial grade diode(s) in series with one of the battery cables. Diodes only conduct in one direction. Otherwise you'll get a viscous cycle of one battery depleting the other.

Once you understand that then you'll get your 12-volts with the blessings of a higher current output. When using diodes, you might have to use several in parallel to handle the current. Make sure to match the rating for the application.

I did this by installing several stud mounted diodes on an aluminum plate connected to the negative side of battery one. DO NOT LET IT TOUCH VEHICLE GROUND! Then I connected the other ends with an aluminum bar about 1/2" X 1/4" in width drilled to accept a screw inserted through the tops. An example of these can be found at http://www.alibaba.com

I have not seen them lately, but I have also not been looking, there were kits on the market for the alternator that charged both batteries at once.

Connect them in parallel, negative to negative, positive to positive.

Quite simply.

They key is that you have to connect the batteries IN PARALLEL.

This means you connect the positive terminals of the batteries together, and the negative terminals together.

If you connect any number of batteries together in this way, the available voltage will always be 12V.

It's easier to see in a picture, so I hope this link will help.

http://www.sciencelobby.com/dc-theory/images/series-parallel-batteries.gif