Wow, not how I would have designed it near a town where techie pranks are expected. You mean any decent wanna-be engineer can take a shot at depleting every transponder in town? Even if they somehow encrypt the handshaking sequence, attempting the handshaking sequence should cause the transponder to do something, thus depleting it a bit.

Really, I think I'd have included a wire to the cigarette lighter to allow for recharging, and a battery you could remove and replace yourself when it wouldn't take another recharge. Having to get another transponder when people know all about recharging tech from their cell phones seems kind of annoying.