It sounds possible to recharge a battery with track power but the small surface area of wheel treads on rail is not the best way to recharge a high output battery. Besides, why would you want to have track power with a wireless/onboard battery operation? It's just as easy to install a charging jack in the loco and charge with a proper plug. With Lithium-Polymer rechargables, you can run for hours before needing a charge. The last time I offered advice about this, I was shot down by those who "knew more". I have yet to see their r/c locomotives so I will offer the advice again. Woodie
I often thought the same thing but I've been told that the track voltage would have to be quite high (and fast) for a charger to work successfully. Don't know that. Just was told.
Funny, I was scrolling down to add some information related to just that. Different types of batteries have different charging, retention and memory properties (as well as cost, density and safety/environmental factors). In general a higher voltage is needed to charge many types batteries, but more importantly extra power is needed if you want to have a net positive effect; You want the batteries to be charged at a faster rate then they are discharged, in order to compensate for rough areas of the track where only discharging occurs. In addition the "fast" charging used for some types of batteries uses a lot of extra power, generates a lot of waste heat, and reduces the life the battery. A possible solution is a capacitor called an "Electric double-layer capacitor". In the last few years you may have noticed very small toy RC cars and helicopters which follow the "charge for 30 seconds on the controller, run for 5 minutes" pattern. These are generally powered by one of these (They are also used in various UPSs and other electrical smoothing systems because of their nearly unlimited cycling count) These are "super capacitors". In sci-fi terms they are nano-tech sponges for soaking up and releasing electricity. Electrically they have very low resistence, which allows them to charge or discharge very fast while creating little waste heat. Their voltage is very low, but they can be connected in a serial fashion just like 1.5V cells. While they cannot be overcharged, they do need some electrics to control discharge. Energy density is some what lower then chemical cells, but not by a large factor. These cells can also be cycled (charged/recharged) millions of times - they almost never need to be replaced. However retention time is quite low - without a charge they will lose their charge many times faster then chemical batteries - days, hours or even minutes depending on the type. While they are environmentally friendly, the high discharge potential poses a potentially fatal electricution hazard. In terms of a practical system using these super capacitors instead of batteries could result in a train that runs on mostly dead track (no wiring or cleaning needed), but must periodically pass over or stop on an isolated high voltage 'charging' section - the track beside a watering tower could be electrified so that a G scale steam engine equipped with these capacitors could stop and "take on water".
