This instructable will show you how you can still use your 6 volt lantern when you only have D cell batteries. A single D cell battery is 1.5 volts so 4 of them is 6 volts. You will need an old 6 volt lantern battery, 4 D batteries, and some materials to make a spacers. (I used a water bottle, some cardboard, and some metal clothing snaps).
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Start by gently prying open the top of the battery case, being carefull not to damage or tear the metal.
Carefully pry up the cover with the springs, and you will find 4 "oversized D Batteries" in series to add up to 6 volts. There is a piece of cardboard with metal strips at the bottom to bridge the positive of one battery to the negative of the other, and a strip on the top to connect the positive of one bridged set to the negative of the other. There is also a metal spacer that is pictured in step 3.
As you can see, a D battery is shorter (by 1 1/8 inch).
I used the bottom of a water bottle with some cardboard on top of it as a spacer. I then reinstalled the metal spacer and the cardboard with the strips of metal.
Because the negative side of the d batteries in indented, it did not make contact with the metal strips so I had to make spacers out of the metal button snaps I had laying around. I used dielectric grease to hold them in place. Other things can work if you don't have snaps. (If you got this far, I have faith in you)
Next you install the batteries alternating Pos, Neg, Pos, Neg, so that you end up with 2 sets of batteries bridged at the bottom.
Now, install the top cap. It is very important that the bridge bar on the cap is perpendicular to the strips at the bottom, so that you connect the 2 "bridged sets" together, Positive to negative. Test with a meter shows just over 6 volts.
It may take a bit of tweeking with spacers, and making sure everthing contacts, But once you get it, your good to go. You can swap them out over and over again.