Series-connected battery packs

​These batteries are readily available, widely used in RC aircraft, cars, drones and are an option for outdoor layouts, particularly large scale (O-scale and larger). 

Since an operating voltage of 10 to 12 volts works well for most small-scale locos, why not use a 3S 11.1 volt battery?

​​​Safely charging series-connected batteries is complicated by a critical need to equalize cell voltages. Failure to do so risks a battery fire.

Commercial LiPo Chargers

These products are designed for charging battery packs with series-connected cells; 2S, 3S,  .    .  6S  .  etc. They typically support both LiPo and LiFePO4 chemistry and come with a collection of connectors. Critically, they perform cell balancing which requires monitoring each cell voltage while charging. 

One set of terminals is used for charging; 4 pins for 3 cells, 5 for 4 cells, etc. Two heavier terminals are used for battery output (i.e. the load).