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Interesting project Lee. I believe you will find the right combination for your scratch built autos.

Bill
 

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I have all the confidence in the world that you will succeed. ;) But I think I know why the top of your head is bare! :) I know I'd be pulling my hair out.
 

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The N20 gear motors seem to have prevision metal gears. With an optimum gear ratio, it could direct drive a rear wheel with a flange. No need for a live axle. Or use two to independently direct drive both rear wheels - a natural differential. But that presents a voltage problem given their 3 V to 6 V operation range. A single LI-Ion cell (18650, RCRC123) provides a nominal 3.7 V and 4.2 V fully charged. Larger than an AA but in line with 2 AAs, size wise. Single cell Li-Ion battery management models are the size of a thumbnail made for charging via USB at 5 volts.

Flange.png
 

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Thanks. I have some N20s on order to try, and some 8-20mm motors with small gearboxes installed. There are so many things to try. The problem with small batteries is that, yes, they fit, but they store so little power.
 

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Lee, when you jump in, you use both feet! I'm really enjoying all the research you're putting into this project, and I'm eagerly awaiting the results of your truck conversions.

I wonder if you could use the differential of some of the RC cars to improve the cornering ability of some of these creations? I suspect that in a sharp corner, the wheels slip may be a significant factor in the ability to maintain speed.
 

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Thanks. I have some N20s on order to try, and some 8-20mm motors with small gearboxes installed. There are so many things to try. The problem with small batteries is that, yes, they fit, but they store so little power.
The 18650 is larger than an AA. It will provide a nominal 3.7 V at max capacities circa ~3,500 mAh (there are lower capacities in the same size cell). After the initial drop from 4.2 V, the 18650s will provide a stable voltage until it drops off quickly when exhausted - unlike alkaline batteries' continual loss of voltage. The 18650 is very common in all sorts of BT speakers and other small devices so there's lots support. The battery number is its dimensions: 18 mm diameter x 65 mm long.

BTW: there are all sorts of capacity claims for these batteries. 3,500-3,500 mAh is the max (physics). Anything above that is BS. When buying these, go to a reputable battery retailer: (e.x., batteryjunctionm, digikey, etc.). Anything listed on eBay or the like is suspect in capacity and quality (factory seconds, fake labeling). From prior research, I like the Panasonic NCR18650B @ 3,400 mAh and the Sanyo 18650GA at 3,500 mAh.

There's a myriad of BMMs for the 18650s. The BMM depicted below is 29.9 mm (1 3/32") x 17.4 mm (11/16"). Be wary of the PCB USB charge port. It may not stand up to repeated insertions. You could wire a micro DC power jack for reliability (3.5 mm) - plug and play (and remove the USB port to reduce the BMM height).

18650.png BMM.png
 
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