We bought a Barbie Power Wheels Jeep for our oldest daughter many years ago. And since the battery died it has basically been pushed around, moved around, and left out in the weather. Fast forward a decade from when be bought it, I decided to see if the thing still functions. I hooked a battery charger up to it and found that both wheel motors still function perfectly. I was going to buy a new battery for it because my youngest wants to drive it, but found they cost $70. Instead I decided to retrofit the vehicle with an old car battery. I discovered that the battery I had fit perfectly under the seat between the two rear motors. The side-post battery worked perfect, and only some very small modifications where necessary.
To get the power to the vehicle I could not bypass the old plug in the front of the unit so I used some electrical wiring and put a wire through the underside of the vehicle and into the front.
On only a slight charge she was able to drive up and down the driveway for about 15 minutes. I figure on a full charge she might be able to drive this for about an hour, possibly longer; which is 2-3 times longer than with the regular Power Wheels battery.