Building a battery tenderThe finished product: 
IntroductionIf you run R/C without track power, you have your batteries either inside the locomotive, or in a trailing car. One annoyance of having a trailing car you cant cut is that you always have this boxcar following you around. More prototypical is having an oil tender follow your locomotives around. At least this is always supposed to be attached to the locomotive.
This project is a freelance oil tender for your locomotives. It contains a pack of 10 1.2volt R/C batteries. If youve never soldered batteries before, dont worry, the technique is straightforward.
You need:
Hartland Locomotive Works shorty tank car10 sub-c R/C car batteriesAbout 12" of red and black wire, and about 4 inches of another color.3 insulated wire ends, that will allow you to use an alligator clip, yet that wont short.Locomotive wiring plug.Couplers of choice**You may also want:**1/4" x 1/2" BasswoodPopsicle sticks02B Strap step1120 Stake pockets07 NBW07A NBWChain or strap to hold tank down12120 Brake wheel if desiredOff we go!
Solder up your battery packs. You can get battery lugs from any hobby shop that specializes in R/C cars. Alternatively, you can use pieces of silver braid, or pieces of low-resistance wire. You need a large soldering iron. I use a 140w Weller gun type iron. You want to melt the solder, and make it flow, but you dont want to heat up the battery for more than about 5 seconds.
First, solder up 2 rows of 5 battery cells a piece. Remember, solder one positive end of one battery to the negative end of the next battery. Reverse the orientation of the batteries so that the lugs line up between the top row and the bottom row. Stack and tape as shown. I use security tape (the tape with fibers in it because I dont want the pack sliding around and possibly breaking a solder point.
Test fit into tank. As built, the battery pack just fits inside the tank.
You can see the tape around the battery pack inside the top.
Drill a 1/8" hole in the bottom of the tank at this location, and in the floor of the car right below this point. This is where the wires to the engine will feed through.
Now solder up your wires. Most R/C battery chargers can charge a maximum of 6 cells, so you will need to wire up the pack so you can charge 5 cells at a time. Solder a red wire to the positive lug at one end of the pack, and a black wire to the negative wire at the other end of the pack. Solder a neutral color wire (orange in this article) to the battery lug between the cells in the middle. You can see this wire coming from behind the pack in this photograph. This wire will be the negative lead when you are charging the red wire batteries, and the postive lead when you are charging the black wire set of batteries.
Also, you need to solder a red and black wire to the lugs that will feed out the bottom of the tank to the engine plug. These are the 2 wires that trail off to the left of the photo.
Ready to go together. Slide the battery pack into the tank, and route the 3 charging charging wires out the tank top. Run the 2 engine wires through the hole in the bottom of the tank. Glue the tank together with something like Goo so you can take it apart if the battery fails.
Solder the 3 charging jacks to the charging wires. Make sure that these wires cant short out when they are pushed into the dome of the car. Test fit the dome into the hole. You may need to use some sticky tape or foam or something to keep the dome in place. We dont want to glue the dome as we charge the battery pack through this hole. Also, solder the locomotive harness plug to the 2 wires you routed through the hole in the car floor.
Detail, paint and weather to your hearts content. Just popsicle sticks over a basswood frame a bit of india ink for weathering, and some Ozark castings. Airbrush on some rust and grime, and you’re done!