Energy Storage

DCCWiki, a community DCC encyclopedia.

Short Definition

Decoder accessory to store backup energy

A decoder with an energy storage capacitor attached


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 help prevent a locomotive from stalling when travelling over dirty track or turnouts with large plastic frogs, especially at low speed. Short 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 louspeaker.

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 manufacturers use different names for the same idea, for example Lenz refer to is as their USP Power module. Zimo refer to it as simply "energy storage". TCS use the name Keep-AliveTM.

Other commonly used names are "stay alive capacitor" and "electronic flywheel".

Video demonstration of TCS Keep-Alive

How It Works

An energy storage module works by charging up a relatively large capacitor while the DCC supply from the rails is available. At some point later when the supply is interrupted, the capacitor takes over and supplies 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 supply, i.e. after the decoder has converted the AC supply from the rails into DC.

Some additional components are required if a large capacitor is used.

Firstly, a large capacitor has a very low resistance when it is discharged and placing a locomotive onto a layout with such a capacitor might trigger the command station or 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 hold 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 zero.

Storage Components

The storage component is usually an electrolytic capacitor, tantalum capacitor, double-layer capacitor. The most common type is 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.

Regardless of the type, all of these capacitors are polarized, meaning that they must be wired the correct way round.

For HO or OO gauge a capacitor as small as 470μF will provide some benefit but sizes of 2200μF and above are preferred. 2200μF will be able to supply power to a locomotive for a few tenths of second, enough to get it over a piece of dirty track at low speed. The endurance of larger capacitors of over 10,000μF can be measured in seconds.

Many locomotives will have insufficient or oddly-shaped space available to install a large capacitor but it is possible to use multiple smaller capacitors instead. They can be wired in parallel to increase the energy storage capacity or wired in series to increase the voltage rating. A combination of series and parallel wiring is also permitted.

The capacitor must be rated for at least 25V for the smaller gauges and 35V for the larger gauges (O and above). This rating is the maximum voltage that the capacitor can withstand without damage. If multiple smaller capacitors are used and they are wired in series then only the sum of the ratings needs to be at least 25V. For example two 16V capacitors wired in series would be suitable for the smaller gauges because combined they have a rating of 32V. If the capacitors are wired in parallel then all of the capacitors must meet the voltage rating requirement.


Schematic of a typical installation

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.


DCCconcepts, Lenz, TCS and Zimo make mobile decoders that support energy storage.

DCCconcepts refer to energy storage as "Stay Alive", they supply some decoders with the storage module included in the package or they can supply them separately. All of their current decoders support Stay Alive and all of them come pre-wired with two additonal wires to make it simple to attach their module without having to solder directly onto the decoder's circuit board.

Lenz Gold series decoders which support USP also support energy storage using their POWER1 or the larger POWER3 modules.

TCS make energy storage modules and decoders with integrated on-board storage. All of their decoders manufactured after February 2012 are Keep-AliveTM ready or have it on-board. Some of their decoders require soldering wires together to connect the storage module (-KA series) or simply plugging them together with a quick connector (-KAC series).

Most Zimo decoders support energy storage using their SPEIKOMP kit and they supply instructions for building completely DIY modules allowing greater flexibily for capacity and physical size. Some of their decoders have the built in support circuitry and only require the capacitor itself to be added whereas others require additional external components.

Comparison of Energy Storage Systems

Decoder Interface
Max Capacity
to Pads
DCCconcepts No No SA3, xxxSAP3 All Yes No
Digitrax Yes No PX108 Yes Yes
PX112 Yes Yes
ESU Yes No PowerPack-mini Yes Yes
PowerPack Maxi Yes Yes
Lenz Yes No POWER-1 GOLD, GOLD Mini Yes Yes
POWER-3 Yes Yes
NCE No No No Halt Yes No
Soundtraxx No No Current Keeper Yes No
TCS Hardwire No No KA1 All -KA series decoders Yes No
KA2 Yes No
KA3 Yes No
KA4 Yes No
2-Pin Quick Connector No No KA1-C All -KAC series decoders Yes No
KA2-C Yes No
KA3-C Yes No
KA4-C Yes No
On-board No No Integrated All KAT and KAM series decoders Yes No
Zimo No Special Port No Yes 10,000 SPEIKOMP, ELKOSODR MX618, MX620, MX621, MX622, MX63, MX630, MX64, MX646, MX647, MX648 470, 6802 No Electrolytic Yes
25V Limited Port No Yes 10,000 ELKOSOMT MX631, MX632, MX644 470, 6802 No1 Electrolytic Yes
16V Limited Port No Yes 10,000 ELKOSOPL MX633, MX643, MX645 680, 18002 No1 Electrolytic Yes
16V Unlimited Port No Yes Unlimited ELKOSOGR MX695, MX696 10,000, 20,0002 No1 Electrolytic Yes
GOLDSOR 140,000 No1 Double Layer Yes
GOLMRUND 140,000 Yes Double Layer Yes
GOLMLANG 140,000 Yes Double Layer Yes
GOLMREG MX645P22 + ADAPLU22 140,000 Yes Double Layer Unknown

1 Modules are not supplied ready-made but since only a single capacitor is requried so there is nothing to make unless multiple capacitors are to be combined when more storage capacity is desired.

2 More capacities are available by combining capacitors in parallel

3 All DCCconcepts decoders ending in SAP include the Stay Alive module in the package

4 DIY indicates that the manufacturer supplies instructions to build energy storage module from scratch

Double Layer capacitors are often referred to as "Gold Caps"

List of Supported Mobile Decoders

  • DCCconcepts: All mobile decoders (specifically those listed as xxSAx)
  • Lenz: GOLD series and GOLD mini series
  • TCS: All -KA and -KAC (all decoders manufactured after February 2012)
  • Zimo: All MX618, MX620, MX621, MX622, MX63, MX630, MX631, MX632, MX633, MX64, MX640, MX642, MX643, MX644, MX645, MX646, MX647 families