Small scale locomotives have very little physical space available for extra stuff.
What space is available is typically taken up with motors, drive shafts, decoders, lights, wiring and extra weight.
A dummy locomotive can accept a small battery and possibly the AirWire C15 CONVRTR.
But a piece of rolling stock can be used to hold the battery and the CONVRTR.
Most Popular Battery Voltage for Small Scale Locomotives
A popular battery voltage for HO locomotives is the 11.1V rechargeable Lithium-Ion battery pack.
You will probably see them advertised as LiPo battery packs.
This battery can direct drive an AirWire M15 motion decoder or the C15 CONVRTR.
The battery voltage is close to the recommended HO voltage of 12 volts.
Your locomotive's top speed might be slightly slower, but for most hobbyists, the locomotive's top speed is more than adequate for their layout.
These batteries come in many different dimensions and capacities.
A boxcar or dummy locomotive is used to hold the battery and an AirWire CONVRTR.
The LiPo batteries are relatively inexpensive due to their widespread use in cars, drones, boats and planes.
There are many online sources and many local R/C hobby shops sell LiPo batteries and chargers.
These packs have some disadvantages.
The standard LiPo battery pack does not have any built in protection.
Also, this type of battery has two different connectors - one connector is for charging and the other connector is the power output connector.
Finally, a special charger is required that has the appropriate connectors to charge the LiPo battery.
Other Suitable Batteries
Another type of battery is the single cell Lithium-Ion flat battery.
These come in many different sizes and capacities.
However, their voltage is only 3.7 volts.
This voltage is too low for locomotives.
However, 3 cells in series equals 11.1V.
And the capacity of the cell is the capacity of the pack.
Their biggest benefit is they include built-in protection.
An inexpensive Lithium-Ion charger is needed, rated at the voltage of the pack.
A JST-2 type connector is commonly used.
Click on the Boost Module link for how to use a boost module to boost a low voltage input to a higher voltage output.
How Long Does The Battery Last?
To determine how long the battery will last requires knowing the locomotive average running current and the battery capacity.
The battery capacity is shown on the side of the battery.
For small batteries it will be a numeric value followed by the initials "mAh."
The initials mean "milli-Amp-hours."
In the photo above, the Tenergy LiPo battery is rated at 1000mAh.
This means that a load of 1000mA (or 1 Amp) can be drawn from the battery for ONE hour before the battery is depleted.
Several types of battery capacities are shown on the graph below.
The graph shows the run time on the vertical axis and the locomotive current on the horizontal axis.
To use this graph, locate your average locomotive motor current on the horizontal axis.
Run your finger vertical until it intersects the appropriate battery capacity.
Move your finger to the left and read the run time in hours.
For example, if your locomotive averages about 300mA when running with the lights on and sound at full volume, an 800mAh battery will last about 3 hours.
This is strictly an average.
Heavier loads, steep grades and/or high speed running will deplete the battery quicker.
That is why any quoted battery run time must always be prefaced with "It depends on..."
The conclusion to take from the graph is that battery selection is a compromise between run time, average locomotive current and battery capacity.
And battery capacity dictates the size of the battery and the required space.
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