There are a couple of articles about the Power Wheels batteries being nothing more than SLA cells that can be replaced.  That's a fact I also discovered several years ago.  I have also been able to overcome the two 6-volt ride-on toys in order to use a single 12volt source.  I have plans here on eBay to "upgrade" the toys to do such.

These are wonderful toys for young children to learn simple driving techniques while they are young.  The kids, however, are often disapointed when the batteries go dead after 20 minutes.  Hence the reason I wrote my upgrade eBook.  I've Jeeps with 90 minute run-times.  Putting the right batteries in the cars is the key to some brighter smiles.

Please note that any alteration to a Power Wheels or Peg Pergo vehicle will void the warrantee you have.  It is also reccomended that any change you make be thought out and tested well before any young child touches the vehicle.