Articles - Battery Power: E) Turning Battery Power On or Off
Battery power should normally be turned off when you are not using your locomotive (e.g. at the end of an operating session). The battery can hold its power for many months (my personal experience).

If you have completely dead rail, you need to wave a magnet over the reed switch that is installed in the BPS circuit board to turn the power on. You usually mount that board near the top or side of the engine, so that it is within 1/2" of where the magnet can trigger it. If you have track power, the BPS in the engine will automatically turn it on, as soon as at least 6 volts of AC, DC, or DCC power is detected. Even a 9-volt battery briefly touched to the rails will turn the unit on.
External Reference:
To turn off the battery power, first remove all power from the rails that the locomotive sits on (if applicable), and then turn off the battery power via the BPS. With the Soundtraxx Tsunami decoder, pressing F5 on the throttle turns the power off. With the new NCE "Dead Rail" decoder, that same functionality is now available for non-sound decoders.

Since I have an older version of the NCE decoder, I have simply installed a second reed switch (identical to those on the BPS board; see link above, right) that I wave the magnet over to turn off the power to the battery. An example installation is shown below in my American Models FA2 locomotive, where the "off" reed switch is glued to a styrene "stand" just inside the locomotive.
E) Turning Battery Power On or Off
To remember where these are located, I made myself a cheat sheet that I taped to the back of the S-CAB throttle.
E) Turning Battery Power On or Off
Update: the latest version of the BPS board now includes both the on and off reed switches built into the board, for that very same purpose. See the photo below, courtesy of S-CAB.
E) Turning Battery Power On or Off