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Short version:   My 18V control unit burned out within 20 minutes of my son driving his modified power wheels on the grass, even though it worked fine rolling on the pavement for 30-45 minutes earlier in the day.  This was the first day it was installed.  Bad unit or common?

Long Version: Let me begin by saying that I gradually lost my mind in this process. I recognize that now, but I’m still here so I’m figuring the best way forward.  My 3 year old son and 6 year old daughter each have a power wheels jeep.  I replaced the short life batteries in each of them with a 35ah deep cycle battery, seen here https://electricscooterparts.com/batteries2.html. They last for days and the kids lead me on 5 to 6 mile bike rides in them. 

The lack of traction on the plastic wheels combined with the rattling noise bothered me so of course I upgraded to pneumatic wheels for grip and quiet.  See attached picture.

The next noticed issue was a lack of power when climbing hills, with the 550 motors inside overheating and shutting down resulting in me dismounting the bike and pushing them up grassy hills.  A bit of an annoyance, so off we go to install new 775 motors.  I had read of success with updates to the 18v systems so I went with the Traxxas 16.8V motors, seen here https://www.amazon.com/gp/product/B001SETT0Y/ref=oh_aui_detailpage_o03_s00?ie=UTF8&psc=1 , in case I wanted to upgrade later.  Using the stock 12v wiring system they worked great, still had a top speed of 5 mph (used my GPS speedo on my phone to verify) and now climbed hills with ease. 

Fast forward to a week ago when my daughter, racing her brother, managed to run into one of only 4 trees in a 5 acre field.  The impact damaged the throttle pedal and selector switch, so I decided it was time to go ahead and do the 18V upgrade.  Rather than switching batteries, I went with a 12V to 19V step up converter, seen here https://www.amazon.com/gp/product/B01N4109JD/ref=oh_aui_detailpage_o02_s00?ie=UTF8&psc=1 . It was rated for a max current output of 15A and 285W.  I combined this with an 18V 250W Electric Scooter/Bicycle Speed Controller SPD-18250, Standard Foot Pedal Throttle THR-90, Heavy Duty Motor Reversing Switch SWT-728, and 2 Wire Key Switch SWT-222. 

Test drive went ok, it seemed sluggish going up hills compared to the 12V setup, but on pavement had increased to 8mph.  He drove it on pavement for a good 30-45 minutes, including some time with the neighbor girls as passengers (ladykiller already).  Later that day I let him drive it in my back yard, which is grass with a slight incline.  Again, it had a lot of trouble going up the hills, but did not quit running while doing so.  He backed it against the fence and then it simply stopped working.  When I checked it out, the control unit was too hot to touch and had a distinct burnt electrical smell.  I later took it apart and noticed a few hot spots that had burnt through.  See attached photos.

I assume I have one of three problems. 

1: The output wattage from the voltage converter is too high for the controller, even though the output amperage is only 15A. 

2: The controller cannot handle two 775 motors in parallel, resulting in burnout. 

3: I have a bad controller. 

As I had to make some physical changes to accommodate the parts, I don’t want to go forward with my daughters upgrade until we know the issue. Any thoughts from a tech on this?

Awesome, thanks for the quick reply.  Currently the output from the controller connects to the selector switch input, should I simply wire the controllers in parallel to the selector or do you recommend wiring this another way? Do you think its worth a shot re-soldering the PC board in an attempt to repair the controller?  

For anyone who searches for the how to modify power wheels, I thought that I should update this thread with the final version I ended up with.  The 18V controllers simply did not have enough power to run the Jeeps, even with 2 of them in parallel (which worked by the way, just not enough power even with each one powering a separate motor).  Instead I switched to a single 24V 500W controller and then used a variable resistor on the throttle to limit the output voltage from the controller to 18V.  This limited the speed and prevented the motors from burning out.  I then added a simple foot pedal for a brake, and wired it into the control unit.  I ran the brake light output through a relay before attaching two 24V LED lights to the rear of the Jeep.  The relay was used to short the motor leads, making a motor brake for the vehicles that was applied only when the brake lights were lit.  Wiring it through the control unit prevents the kids from causing damage by pressing both pedal simultaneously.