Rechargeable Battery for 6-Volt Battery Operated Vehicles

I recently bought a used ride on train for my toddler, but the catch was it needed a new battery. The one thing about this brand is that the batteries are a little high maintenance. It has to charge for more than 18 hours before you can unplug it or it will run the battery down. Also, It drains very easily when not in use!! My nieces have the cheapo brand barbie jeep and their battery isn't nearly as demanding. So in about a year or two I will probably upgrade to another brand.