Re: 6-volt Lantern battery or 4-D cells?
A 6v, 4-4.6Ah gell cell fits in place of the regular primary lantern battery. It's a hair wider and the corners are more square so its a little tighter fit in that respect. It's not as deep, and I usually put foam pads on back & front to keep it mostly centered. They body is slightly taller but the overall hieght is less at the top of the terminals. You have to solder wires to the bulb holder since the contacts don't line up. It's about $10-12
In regard to charging 6v batteries with a battery charger. Max charge rate for a 4.6Ah gell cell is about 800ma, although the manufacturer specifies max voltage AND current (usually about 1 amp, 14.5v). I use a wall wart rated 1000ma. or less and an LM317 voltage regulator set at 13.9v max. This is a safe float charge voltage so that the battery can be left on this for some time, although overnight is what I normally do.
Wall Mart often has $5 lanterns with the 4D adapters in them. I had several at one time from gell cell conversions but I have given most of them away. And I'll keep the 1 or 2 I have left. HTH, RussH