How to change a camera's battery on a tripod without moving the camera?

I'm planning on shooting star trails with my Nikon D3100 and on all the websites I read they tell you to make sure you have spare batteries and a way to change them without moving the camera since exposure times can be minutes long. I can't seem to find anyone that knows how to change the battery though without moving the camera, which would ruin the shot. The camera will be on a tripod. Any help with the battery is appreciated.

If your tripod has a quick release plate you could take the camera off and return it with hardly any change of position. Although it is awkward, the battery compartment should be accessible with the camera on the tripod. However, switching batteries during an exposure will disrupt the exposure. It is important to start with fully charged batteries, and swap them during the session before the charge gets very low before taking the next long exposure. Touching the camera during the exposure should be avoided, because it is likely to contribute vibrations.

One option would be to get an AC adapter to eliminate the battery during losng exposures. Another option to one lengthy exposure would be to do a series of shorter exposures, and use a stacking program. You can find shareware stacking programs for free on line. Many people prefer this method because of the battery issue, and it cuts down on long exposure noise.