There are different versions of D-cell spacers that are made to take Eneloop AA batteries. These are the ones to get.

The spacers in this set have a twist-on end cap at the negative end. That end cap has a large, 13/16" (20 mm) flat bottom contact surface. This means that you don't have to depend on the bottom of the AA battery itself to make contact with your flashlight or other item that uses the battery. You have a nice, large contact that almost everything will be able to use.

The top, positive, end has a small rivet that contacts the battery and provides a conductor to the outside of the spacer.

The battery is totally enclosed within the spacer and will not fall out accidentally. The Eneloop batteries provided slip into the spacers with ease and they are a nice fit when the cap is on. You cannot hear the battery rattling around inside the case.

Per the instructions the batteries must be removed from the spacers to charge them. This is no trouble at all. The batteries are made in Japan and the cases are made in China. Both seem to be excellent quality.

Another issue is the idea of using an AA battery in place of a D-cell. I'm not a heavy user of my flashlights and other items that take D-cells so I find that my Titanium 12,000 mAh NiMH batteries just die in the flashlight long before I actually use the capacity. These Eneloop batteries are LSD (Low Self-Discharge) so they should retain their charge much longer while they just sit, unused, in the flashlights. Of course this means that the flashlight will work when I need it. High power batteries are good if you actually use the power, but if your batteries sit for long periods these Eneloops are probably a better choice. If you need longer life, carry spares.

Loren