Custom Battery Power for the Power Functions System

Written by TJ on April 5th, 2011

There has always been a trick to getting a custom battery box to work with PF (power functions) components. The issue is basically that there are 4 wires used in the PF cables, and depending on where you are in the circuit, the outer set or the inner set can be used (but not at the same time).

The system is mapped out in detail here: Many, many thanks to Philo for this diagram.

Implementing this information to make a custom PF battery box is what I’ve figured out recently. I wanted to make a custom (non-LEGO) battery holder that would take 8x rechargeable AA batteries. That would give me a total of 9.6 volts (rechargeable AA batteries are typically 1.2 V per cell as compared to traditional alkaline batteries that are 1.5 V each) and essentially more and longer lasting power for my models (in comparison to using a standard LEGO battery holder that only takes 6x AA batteries).

Below is a schematic showing the set-up for a custom battery box.

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You can find off the shelf non-LEGO battery holders in a variety of sizes and capacities. For AA batteries, I’ve found holders that take 4 each and also 8. In my example below, I’ve used two 4x AA holders and wired them together in series.

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This is for my UP 844 MOC. The 4xAA holders fit snugly into a space that is 8-studs wide.

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When I first started to play around with this, I somehow misinterpreted the information and ended up wiring the batteries up like this:

do NOT do this
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DO NOT DO THIS! This will make smoke come out of the PF IR receiver box.

My PF receiver is now a pristine example of what a PF receiver should look like from the outside. Inside, it’s toast.

4 Comments so far ↓

  1. Apr
    Tobbe Arnesson

    Oh noes, the magic smoke escaped! :)

  2. May

    I don’t understand why you destroyed the cable for IR reciever. You just take your battery pack (I use 4 AA) and then connect it to the old broken lego battery pack for 9V. It have railing with power output and switch on button the only think that you may solder is the AA pack to the old pack. It take slightly more space but you can always cut the useless pieces. And still have studs :P and a button.

  3. Jun
    Rich Coyle

    So I’m NOT the only one who “somehow misinterpreted the information”. Although I did it with the battery connection. Using a connector from a Lego jumper cable I connected two conductors/wires to the positive & similar for the negative. The results were the same “a toasted IR”. My problem was that I did it TWICE”. But I have an excuse for my “dumbness”. ‘Cos I’d done another “Non-Lego” wiring modification. I thought it was the source of my mistake. In order to eliminate the Lego switch (for reversing direction of the second train motor) I’d dismantlement a motor’s Lego connector and reversed the inner two wires. I’d ended up assembled the Lego Connector back-to-front and mistakenly believed that this was the source of the first burn out. It may have had a part to play but I think the IR was toasted before it could have. Thanks to your misfortune & posting I can now wire my “third” IR correctly.

  4. Dec

    So did you make it work in the end? Would this also be possible using a lego pf extension wire connecting it somehow from you custom battery box to your PF?

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