Introduction: Make a Tiny Battery for LEGO Power Functions

About: I may be an electrical engineer by trade but that won't stop me from tinkering in the domain of mechanical engineers and artists:P

Hello fellow LEGO RC enthusiasts!

This instructable is going to open you the door to the world of minifig sized LEGO RC models by showing you how to replace the huge power box that LEGO has given us with a six (6) times smaller equivalent!

So where is the catch? The substitute I am going to show you in a few moments runs out much faster but don't worry. The power motors can develop remains the same!

This instructable also packs a little side quest: no LEGOs can be harmed / everything has to be 100% revertible. Since LEGOs have always been sort of sacred to me, this was a must and due to design of the original power box, a much harder task than I have imagined.

What you will need:

  • original LEGO power functions battery box
  • dead 9V battery or pre made 9V battery adapter
  • full 9V battery
  • some screw drivers (small cross and two small flat)
  • soldering gun and solder
  • 2x screw terminal
  • pliers

Step 1: Taking Apart the Original Battery Box

If no LEGOs can be harmed, cutting a power cord off one of my motors is a big NO. At least for me - if you don't care about your LEGOs just jump to step 3 to see which pole (+/-) goes to which wire. The same goes to all of you who have a broken LEGO motors / LEDs lying around somewhere.

This leaves us with taking apart the original battery box and salvaging the connector. The first thing we have to do is removing the two screws (you can't miss them but in case you do, take a look at image 1 above :) and removing both lids. Next we have to separate inner battery holder from outer hull. this is done by holding power box firmly at connectors on far sides with one hand and grabbing the ribs (battery separators) with the other. Quite some force may be required to pull the two pieces apart.

You now have access to the simple circuit board hidden inside your battery box! to make following steps easier, separating the circuit board from the plastic container is recommended. You can either use a pair of tweezers to cut the wires or solder it off. If you are planning on putting it all back together I recommend replacing the short black wire with something longer but that is entirely up to you.

You now have two options: a) solder off the strip of wires and extend them by soldering on longer wires (a legitimate way but almost impossible to revert back to original state...). b) open up the connector.

If you go for option "b", you will need two thin flat screw drivers and patience. What you have to do is gently pry the small chunk off the main part. Looking at pictures you will see signs of wear on one side - that is where you have to start opening it. Once you manage to free one side of it you will have to keep lifting it (very gently) and simultaneously pry other side inwards using other screw driver. If that doesn't help, just pull harder on first side (but not like a bull - damaging this part may make it more or less useless!).

If something bad happens to your connector despite all your efforts you will have to use hot glue to fix the new wires in place.

Once you have successfully taken apart your connector just pull off original wires and replace them with new, longer ones. I used rainbow wire strip as it fits in perfectly (note however that not all rainbow wire strips are the same so some may not fit!). If you want to use classical wires, they have to be aligned as if they were a part of wire strip and pushed inside metal "jaws" individually (you will know what "jaws" are when you see them - they are close impossible to be pictured well - see image 6 of this step). No matter what kind of wires you chose to use you have to make sure that "jaws" made contact with copper core of wires! Do this by using multimeter or battery-LED circuit. It is much harder to fix it after the connector is closed up again so just press all wires downwards again.

When contacts are tested all that is left to be done is to put the small connector cover back on and you are ready to move to the next step :)

Step 2: Taking Apart the Dead 9V Battery

If you prefer the quick and boring way (or if you don't have a soldering gun), you may buy a pre-made 9V battery adapter here (as well as by many other retailers).

The rest of you can get an used up 9V battery (search a battery disposal container at your local shop if you don't have any lying around your house). Do note that used up battery is preferred or you may be shocked (pun intended) by what you find :) Wasting a perfectly good battery really has no sense anyway if you ask me... This works because two 9V batteries fit on each other (do not ever try this - it is a bad idea that may result in fire!).

What you want to do is take pliers, find a seem on battery and strip it up (don't worry, it win't shock you even if it isn't empty). After doing so the connector part has to be ripped off (it should come off quite easily). After isolating paper and all possible waxy substances are removed, you can proceed to soldering on wires. leaving the long metal piece on for now is recommended as it allows for easier handling (it can be removed later on).

The little holes on the inner side if connector should be filled with solder and while solder is still liquid, the wire inserted in it (keep heating solder while inserting wire or insert wire first and add solder later if you have a helping hand or something to hold wire in place).

Test the contacts, fix them if necessary and you are good to move on to next step.

P: My battery doesn't fit in the adapter.

S: Use pliers to slightly widen the "cup" part of adapter. If this doesn't help do the same on battery.

P: I can't solder wires on the connector.

S: Are you sure you filled cup wit solder? You may also try cleaning waxy substance off first.

Step 3: Putting It All Together

It is now time to wire it up and test it! You may want to do a quick puzzle of which wire should go to (+) and which to (-). In case you don't- here is the solution: looking at the image above orange and yellow wires are negative (-) and green and blue wires are positive (+). If the switch on battery box was on other side, yellow and green wires would change roles but that would just complicate wiring and is also useless for RC. If you really want an option to reverse direction manually, I recommend using LEGO's switch.

You may solder wires together or use screw terminals (don't just roll wires together as this is just asking for short circuits, unwanted heating and (god forbid) fires). Optionally you may add a switch to turn power on/off (not a bad idea).

Step 4: Usage

For use with ultra small models I recommend using double sided tape to stick battery to a hidden part of your model. To get you started I attached two pictures of a tank I built (and failed miserably, especially since I was aiming for high level of detail... :|

I have also shared a LDD (lego digital designer) file of the smallest LEGO RC crawler yet! The battery you just made is stuck on the bottom and off you go. It is 5min tested, fits under most of my furniture and my dog hates it :)

If you enjoyed this instructable don't forget to like it. Share your RC constructions enabled by your new ultra small battery. If I wasn't clear at any part, please do point it out so I can fix it.

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