001 Switch PowerWheels to drill battery

My kid's 4-wheeler powerwheels battery has stopped holding a good charge. After charging overnight, it will just kind of crawl along and can't go uphill. Those Fisher Price batteries are expensive and "single use", so I decided to swap it out for my Milwaukee M18 drill batteries. These batteries are just as expensive, if not more, but I use these in my power tools anyways.
Original idea was from Dude Dad, check it out.
• EASY - How To Modify 1...
M18 Battery adapter: amzn.to/2YgIVVa
Motor controller: amzn.to/2H8ZNaq