Lego Trains With LiPo Batteries

After adding shelves to our loft, my kids and I recently got back into Lego Trains. We have three locomotives, which makes for 18 AAA batteries - which it's hard work to keep charged, never mind the outlay in batteries..! Lego's own rechargeable battery box (8878) is retired and £100 on Amazon without a charger (another £25), so I looked into making my own solution.

I started with the batteries, and found 4-off 800mA 3.7V LiPo batteries and a charger for £15.19 on eBay. Only two are needed per train but having two to use means one pair can charge while the other is in use.

If you're not familiar with LiPo batteries, then please read something like this introduction before going any further.

With the batteries decided upon, I had to think about how to connect this to the IR reciever. I'd seen a solution that involved cutting up a Lego cable which is great for space saving in a MOC, but that's not important to me as I'm using Lego's standard trains. I didn't want to dismantle a genuine Lego battery box (yet), and a browse around eBay found a Hong Kong company selling knock-off battery boxes for £5.89. This arrived in ten days (even over Xmas). Despite being knock-off, it was great quality and had no problems connecting to real Lego.

Finally, I needed a couple of JST sockets for the batteries to plug into. I happened to have a couple lying around, but they're easy to find on eBay for no more than 0.99p delivered as a plug/socket pair.

The general idea is to use the space inside the battery box to hold two LiPo cells, and connect them to whatever control board is between the batteries and the power connector.

The battery box comes apart easily enough: the usual two screws at the bottom to open it up, then a screw in each corner (red circle shows where) of the battery and switch assembly from the top surface (mine only had three screws, presumably to reduce cost). This revealed a circuit board, held in by two more screws. After unscrewing these, I carefully prised the circuit board from the plate. Some things to note:

• The power button is loose so do this with the circuit board facing up.
• In my box, the board was in at an angle: the end nearest the button is higher up than the connector end. Not completely sure if this was intentional or not...
• Take care not to bend the connector pins as the slide out of the plate.

I then desoldered the wires from the circuit board in case I wanted to convert it back (or use the battery holder for other things) but you could cut the wires if you wanted to use them to connect to the LiPo batteries instead.

I put all the screws back in their holes in case I need them again one day.

It was now clear how the final unit would go together: two LiPo batteries in series connected to the control board.

First, I soldered a black wire from one JST socket to the red wire of the other, then put some heat shrink over that joint to insulate it.

Next, I soldered the remaining red lead to "VCC" on the circuit board, and the black lead to GND. The leads of my sockets were too big to go into the circuit board, so I surface-mounted them. Since this means they're attached parallel to the circuit board, the need to point toward the middle of the board; if your wires fit in the holes that would be a better solution. The pictures don't show it, but I reinforced the joint with some hot glue to give some strain relief.

If you're not happy soldering, then cutting the original wires, twisting them together and insulating them with tape or hot glue should work.

I attached the circuit board back to the top plate, taking care to put the connector pins back in gently, and to make sure the mechanical buttons and slider still worked.

Then I connected the batteries and switched it on... the green LED lighting up was a good sign!

There's plently of room for the batteries in the box, though the wires were quite springy so the box won't stay shut on its own. This uncovered one issue with this solution: the screws that hold the box closed attach to the battery holder, which is no longer present. My solution was to use tape on each end of the box.

Six AAA rechargeable batteries in series give 7.2V. Despite their 3.7V rating, a single LiPo will deliver 4.2V when fully charged; though mine were at 4.1V so together gave 8.2V. I certainly noticed the difference with the extra volt. I haven't run time trials on it yet, but I'll update this when I do.

I've already ordered a second battery box. Total cost for two boxes will be £15.19 for batteries and charger, 2 x £5.89 for the boxes, and £3.96 for the JST sockets; that's £30.93, or £15.47 each. A bit of an improvement over £125!

With the design as it is, I need to remove the tape to open the box and disconnect the batteries to charge them. My next step would be to drill out two opposite corners, add something to the box below those holes and use a couple of screws to hold it closed. An even better solution would be to adding a charging cable so I can charge betteries in situ, along with a warning buzzer for when either LiPo drops below around 3.2V. I'll update this instructable whatever I do next.