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Li-ion "AA" batteries for LEGO 4.5 V motors

I own a few lego 4.5V motors ( Motor 4.5V Type III 12 x 4 x 3 1/3 and Motor 4.5V Type 2 for 2-prong connectors ) and also some lego lights ( Light Brick 4.5V 2 x 2 with 2 Plug Holes ).

As a power source for these (long discontinued) 4.5 V motors and lights, LEGO offered two different battery boxes, each holding three 1.5 V "type C" cells. Obviously, anything that holds three "type C" cells is rather bulky. And heavy too. Also, rechargeable "type C" cells are not very common.

This is my solution:
alternative 4.5 V battery

The magical part here is a TrustFire Protected 3.7V 900mAh 14500 Lithium Battery . It's almost the same size as an "type AA" cell but it's a Li-ion cell with a nominal voltage of 3.7 V. You need a special charger to charge these batteries, however it's quite cheap.

Buy a battery holder and a 2-pole ON-OFF-ON switch (less than $2). You can get these or similar at your local electronic parts shop. Solder the battery holder to the switch using a short piece of two-lead cable and solder another two-lead cable to the switch as shown below:
Screw the original LEGO 4.5 V plug to the end of the cable. Finally, drill a hole in a LEGO plate and screw on the switch.

The 4.5 V LEGO motors run surprisingly well with the above battery and the battery holder fits into 2x8 L. This way, you can build e.g. a small engine that carries its own battery and does not need the bulky battery carriage that was originally used with 4.5 V LEGO trains:
tank engine tank engine tank engine
You can get the instructions for this little tank engine on my MOC page.