· Custom 9V Batteries

May/June 2005
 

Let your RCX/Scout run off a 9V adaptor so you never run out of power!

This tutorial shows the process of making two "fake batteries" that can be placed inside any RCX or Scout and then can be connected to any wall outlet using a 9V power converter.

Now that you are aware of the risks, let's start building this thing. You need:

Materials:
• Two wood sticks with the diameter of a standard AA battery
• Some epoxy clay to adjust the size
• A metallic (conductive) wood screw (for the positive contact)
• A metallic (conductive) tack (for the negative contact)
• Some glue
• Some cable (preferrably two different colors)
• A 9V adapter and a compatible plug
  Tools:
• A hand saw
• A hand drill
• A screwdriver
• A hammer
• Some pliers
• A multimeter

Cut the wood sticks to the correct size (or a little smaller than needed) to make the fake batteries' body.
Take one of them and make a small hole with the hand drill (very thin). Remove the isolation from one end of the first cable (I used red) and make a loop with the blank wire. Put the screw all the way through the loop and screw it into one end of the first battery (Look at the upper part of the drawing).
Make another wire loop in the same way with the other cable (I chose black) and drive the pin of the tack through it. Punch the tack into one end of the second battery (See lower part of drawing). You might want to use some glue so the tack does not slip back out.

Test the batteries' size by inserting them in some device that accepts AA batteries (like some remotes). Do not apply any voltage up to this point! If you notice they are too short for the battery holder to hold them firmly, take some epoxy clay and add it to the unused end of each battery and mold it to fit the size. Try adding the little button on the "negative battery" that you can see on the positive contact of a normal AA battery. This will help our fake batteries to fit perfectly into any standard battery holder.

Next, you need to attatch the two loose ends of the wires to the contacts of a power plug (I got mine from an old discman). Please check the polarity of the plug (which contact is connected to the central pin of the plug, which one to the outer shell) and the power adaptor. Usually, the positive contact is the one on the center pin, but this may not always be this way. Again: I assume no responsibility in the case you damage your LEGO robot and/or the power adaptor by connecting these contacts in the wrong way!

Your fake batteries are now ready. Insert them into the leftmost and rightmost battery holders in your RCX/Scout. Caution: You must place them just as you would do with normal batteries. On my RCX and Scout, The positive contact is on the battery on the lower end of the RCX (closer to the motor outputs), while the negative contact is near the IR panel and sensor inputs. This may not be the same on your LEGO brick! Measure the voltage between different contacts (with normal batteries inserted) and attach the two fake batteries' contacts to the LEGO brick's where you measured the highest voltage (about 7 to 9 volts).
Attach a power adaptor to the power plug of your fake battery pack. Make sure the power adaptor delivers between 7 and 9 V DC! Measure this using a voltmeter before connecting!

If everything went OK, you should now have a perfectly working RCX/Scout, being powered by the wall outlet! You will never run out of batteries again! Just remember: If you disconnect your power plug from the wall outlet for more than a few seconds (e.g. for exchanging your fake batteries for normal ones for use in a mobile robot or vice-versa) or the electricity goes away for some reason, your programs on the programmable LEGO brick will be lost, and in the case of the RCX, you will need to re-download the firmware.
As always, any feedback would be welcome! Click here to sign the guestbook or send me an e-mail!

UPDATE (September 17th, 2005): If you either don't have the time or the tools to make an entire fake battery, you can use the design shown here. Instead of building the batteries out of wood, you simply put two metal paperclips on the positive and negative contacts of the RCX/Scout. Make sure you don't make them either too loose (causing them to slip out, lose contact and eventually causing a short circuit) or too tight (making it impossible to remove them when necessary). Of course, the same warnings stated above apply for this design.

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