- Tue Mar 06, 2018 12:51 am
This depends a great deal on the conditions. On level pavement there isn't much drag from the actual tires. The majority of the power loss in the system is the motor and gearbox drag. The specification for the Johnson motor used in most of the stock Power Wheels (Fisher-Price brand) states that the no-load current is 1.2A. My measurements show that to be pretty accurate, and that inside a stock gearbox the same motor will consume 1.5-1.7A, depending on gearing (eg. 15T vs. 19T). Obviously there's additional drag pushing a vehicle. (in comparison, the 36V Mustang consumes ~6A (216 watts) with the wheels off the ground, and over 1000W up our local hill at full speed -- there is a lot more gearbox + wheel friction at 3x stock speeds, plus spinning 4 motors).
If you put on two gearboxes, you'll effectively burn twice as much power in that scenario since each motor+gearbox contributes drag that has to be overcome.
However, In conditions where you are going through tall grass or up a steep hill, you may actually be more efficient with two motors. The motor's peak efficiency is typically at about 80% speed but the peak power output is at 50%. One of those (Johnson) motors outputting 150W of useful work will run at 10,000 RPM( vs. 15.7k no-load) and will waste 120W of power (total consumption from the battery is ~270W). Two motors outputting 150W of useful work will run at 13.7k RPM and will waste 66W of power (33W per motor)-- using up a total of 216W (note that it runs faster as well).
Note also that the first scenario -- outputting 150W with one motor -- will also burn up the motor fairly quickly (likely failure in <60 seconds of load), AND the efficiency goes down ever further as it heats up. The 2-gearbox scenario one will run ~continuously with no problems.
I think it's probably fair to also point out that if you're on level pavement and running steady at ~2A (one gearbox at nearly no load), you can go for a few hours on the stock battery -- it's the acceleration, hills, etc., that end up wasting most of the runtime. Moving up to two gearboxes isn't going to be a huge impact, and buying a larger or 2nd battery is probably a good way to go to mitigate that.
F-150 - 24-36V with homebrew 100A variable-speed motor controller, 4x 775 motors (no, it's NOT all-wheel-drive--look up 'quad damage' in the gearbox section for more info!)
Pink Rocket Princess Mustang - 15mph, 36V, 4x 775 motors, homebrew PWM controller rev 2....
Hurricane - 24V w/50A step-down controller, 775 motors