Batteries are available online at the Lego Education Store (http://www.legoeducation.us/store/). Battery product code is W979798. Batteries are $55 each and Lego ships the day after the order is placed via UPS or US Postal Service and takes about three days. You can also spend a fortune for one-day or two-day delivery.
Thanks, John for bringing this up. I think my team might have the exact same problem. I was going to have alkaline batteries ready for the next session to verify the theory. Then I realized that it is not easy for my team to swap out the battery. They built the chassis under the brick.
I called the Lego store and found they don't carry battery. Is there any other way to find the battery locally? I'd like to swap out the battery with a new rechargeable ASAP.
Everyone, Just a heads up here. Over the past two weeks, the pair of robots we use have started to exhibit some strange behavior. For example, turns were becoming increasingly inconsistent and the robot would sometimes move in jerky movements at slow speeds. The robots are 3 and 4 years old and initially we thought it might be a motor problem or possibly the new NXT software. Our teenage mentor suggested that the Lithium Ion batteries may be going bad. While we were initially skeptical, since both batteries showed full charges, we replaced the rechargeable batteries with alkaline batteries and the robots were back to their "normal" selves. Aha!
The lesson here is if you're using a robot with a rechargeable battery that is a few years old and you see some strange behavior, try swapping out the rechargeable with a new set of batteries. It could save you and your team from going mad.
-- John J. Barrett Industrial Medium, Inc. 1616 Anderson Road McLean, VA 22102