This article is from http://www.dccwiki.com/Energy_Storage The DCCWiki site is a fantastic resource for information. A huge thanks goes to Mitch Schwenk for maintaining the Wiki.
Some mobile decoders support an additional energy storage module to provide backup power when the electrical supply from the rails is interrupted by dirt, bad contacts, insulated frogs, etc. This can prevent a locomotive from stalling when travelling over dirty track or turnouts with large plastic frogs, especially at low speed. Short wheelbase locomotives such as 0-6-0 steam locomotives are more susceptible to this problem due to their inherently limited number of electrical pickup wheels. Locomotives without brass flywheels can also benefit from energy storage. Sound Decoders can benefit from energy storage systems because they help prevents dirt and bad rail pickup causing audible crackling or static coming from the loudspeaker. Typically this function cannot be integrated into a decoder because a useful amount of energy requires a sizeable capacitor. The capacitor (and sometimes additional components) must be installed in a separate location in the locomotive and connected to the decoder via a couple of wires. Other commonly used names are "stay alive capacitor" and "electronic flywheel".
How It Works
An energy storage module works by charging up a capacitor while the DCC signal from the rails is available. At some point later when the signal is interrupted, the capacitor takes over and supplies energy to the decoder which in turn supplies the motor. It can only do this for a short period since the capacitor will rapidly discharge. The capacitor must be charged from the rectified power supply, i.e. after the decoder has converted the digital DCC signal from the rails into Direct Current. Some additional components are required if a large capacitor is used. First, a large capacitor has a very high inrush current when it is discharged and placing a locomotive onto a layout with such a capacitor might trigger the booster's short circuit detection. To prevent this happening a current limiting resitor is needed. This however causes its own problem in that it limits how much current the locomotive can draw. A diode can be added to overcome this limitation. Secondly, a large capacitor can store energy for quite a period of time (many minutes) if there is little current draw. A locomotive that was removed from the track and then returned later can remember what it was doing before and set off on its own. While it's away from the track, the voltage of the capacitor may drop to the point where there is insufficient power to turn the motor but enough to retain the decoder's memory that it was moving. To prevent this a discharge resistor can be added across the capacitor's terminals to slowly discharge it when not in use. This effectively resets the decoder so that its speed is set to zero.
The storage component is usually an electrolytic, tantalum, or double-layer capacitor. The common type is an electrolytic because they are low cost and readily available. Double-layer capacitors (often referred to as gold capacitors) offer much denser energy storage than conventional capacitors but at a higher price and limited voltage rating. Usually at least four double-layer capacitor "coin cells" must be combined for this application. The benefit is the storage capacity can be 10 to 100 times more than with electrolytic capacitors of the same physical size. Despite the name, "gold capacitors" contain little, if any, gold.
There is no NMRA standard for wiring energy storage modules to mobile decoders and there are no known standard wiring harnesses or connectors that include the necessary connections. On many decoders the user must solder additional wires directly to circuit board, often requiring removal of some of the insulating plastic sleeve. More proficient soldering skills are needed to perform this task than are typical for installing a mobile decoder. DCCconcepts and Zimo use the convention of black wires for the energy storage ground and blue wires for the energy storage positive. TCS uses a similar convention but the ground wire is black with a white stripe. The Lenz system uses 3 wires for U+ (blue), charge (pink) and ground (brown).
Analog Conversion Mode
Energy storage is not typically supported in Analog Conversion mode because there is no way for the decoder to distinguish between a command to stop (0V sent by an analog throttle) and a dirty section of track (0V received by the decoder). A decoder fitted with energy storage and with analog conversion enabled will still work normally, it will simply stop immediately when it receives no power. Theoretically, it could be supported with the decoder assuming 0V means coast for a short period and then stop. The disadvantage being that there would be no way to cause an emergency stop.