Lego Battery Pack Boost

Introduction: Lego Battery Pack Boost

About: I am an electronics engineer, doing a PhD at the VUB in Belgium. I have a passion for making things, both useful and cool.

Lego technics are truly amazing, especially when they are equipped with power functions. However this gives them one big weak spot: the batteries. I always use rechargeable batteries, because they are reusable, but they only have a nominal voltage of 1.2V instead of 1.5V with non-rechargeable batteries. As a result, the total voltage (with 6 batteries) is only 7.2V instead of 9V! Even with non-rechargeable batteries this voltage will drop quite quickly, while they are far from empty.

To resolve this issue, I modified the battery pack with a boost converter. This will take the voltage of the batteries and boost it up to 9V. As a result, the motors will run at full power as long as the batteries aren’t completely dead! The build consists in modifying the boost converter to fit inside the battery pack.

Let’s put the power back into powerfunctions!

Step 1: Parts & Tools

Dremel / Saw
Soldering iron
Small drill


Lego battery pack
5k & 100k resistor
Small boost converter

Step 2: Modify the Boost Converter

The output of the boost converter can be adjusted with the on-board potentiometer, but it is quite big, so we will start by replacing this. Desolder it, and replace it by the 5k and 100k resistor as shown in the picture. When measuring the output, we should get around 9V.

Next, we will need to place the diode and coil more to the inside of the board. This is easily done by heating the solder pad and shifting the components, as seen in the picture.

Finally, we need to trim the pcb to fit inside the case. This is done by cutting the top of the board off with a saw or dremel. The lead of the resistor that is also connected to the diode is now Vout+. As such, it should be soldered to the Vout+ pad (this is necessary because we removed this trace by chopping of the top of the pcb).

It should now fit inside the case, and still give us the 9V output like before.

Step 3: Modify the Battery Pack

We will fit the boost converter in a small compartment in the battery pack. This means we need to reroute the cables to this compartment.

Start by taking apart the battery pack. This is done by unscrewing the 2 small screws at the top and sliding out the part that holds the batteries. We will need to modify the part that slid out.

Use a sharp knife to cut away a small section of the barrier between the battery and the empty compartment. This should be the size of the coil.

Take a small drill and make 3 holes in the side, as shown in the picture. These will accommodate the ground wire and the 2 power wires.

The red cable is routed in a groove, we will need to add 2 more. Use a rotary tool, or a sharp knife and make some grooves to route the cables to the other side of the battery pack. Don’t count the empty grooves that are already present, this is where the enclosure slides in.

Now for the wiring. Solder an extra black wire to the first pad and route it through the grooves and hole into the compartment. Solder this to Vin- on the boost converter. (this is also connected to Vout-, so we only need 1 ground wire)

Desolder the red wire from the output terminal and solder it to Vin+. Add another red wire to the output terminal where you desoldered the first one. Connect this to Vout+

Step 4: Test & Enjoy

Put the battery pack back together and insert some batteries. Flip the switch, and measure the output: 9V! Now all that’s left is installing the battery pack in one of your awesome creations and enjoy the power.

I hope you liked this instructable, if so: take a look at my other projects :)

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    2 Discussions


    4 years ago

    Sometimes you get to wonder how the staff could have missed fantastic projects like this. Although I rarely use Legos these days I still remember the struggle with half-empty batteries and inconsistent power. Thanks for sharing this simple yet powerful fix!


    Reply 4 years ago

    Thanks! I always try to make them as clear and fun as possible :)
    My nicest lego technic models are simply on display and I got this idea I couldn't resist to make :p