jeweler53 wrote:
I have 4 different batteries, all Nikon brand. They aren't "new", having come with the D300 and a D200 I purchased. They work just fine in the D200, and hold their charge for weeks, if not longer. I've also cleaned the batter terminals both in the camera and on the batteries.
If the batteries don't discharge in the D200, then suspicion is placed (at least initially) on the D300. These cameras are complex devices, and sometimes things go wrong, and sometimes their brains just get "scrambled." The power switch is not a hardware device that physically breaks the supply circuit, but rather triggers a software-controlled camera shutdown process.
If the two-button reset didn't work, the next step is to remove the battery entirely and let the D300 sit for an extended period...maybe as long as 24 hours. Then replace the battery, do another factory reset and see if the problem is corrected.
The second possibility is that there might be a problem with the firmware...such as the prior owner improperly attempting a firmware update. Since the camera turns on, and since it apparently functions properly while the battery is charged, my next step would probably be to download and attempt to reinstall the latest firmware. Be sure to follow all the instructions on the download page. The process is neither complicated nor difficult, but it does need to be followed precisely. Instructions are provided on the site, and prompts appear on the camera screen during and at the end of the process.
The third thing that comes to mind is that the camera may have received some physical trauma during its life. It may have been dropped or been the victim of some other improper treatment. In this case, circuit boards or internal connectors may have been damaged. Some independent camera shops may still have the ability to repair it for you. Several years ago, I traded for a D300 that worked fine except that the aperture interface ring had been damaged. I found a junker body that had a good part, and a local shop was able to install it for me. Be aware, though, that opening the camera requires replacement of essentially all of the rubber grips. They do not go back on gracefully. Nikon no longer has these available, but some 3rd party ones that work pretty well are available. So let's hope it doesn't come down to the need for repair.
Good luck as you work to a solution.
Edit***After posting my reply, I reread your original post again. This time, I am interpreting that your issue is not with the main camera battery, but with the internal time and date battery. As proud as I was of my initial reply, my answer may not help if that is the problem. In that case, I'll refer you to the statement in the manual indicating that it requires several days for the internal battery to fully charge from the camera battery. This means that you need to leave the battery in the camera for several days until the backup battery can reach a full charge. Of course, like any other battery, that internal battery will eventually wear out and fail. I am not sure where it is located inside the camera or whether it can be replaced without major disassembly. If that is the actual problem you are experiencing, try leaving the EN-EL3 battery in the camera for several days and see if the results are any different.