I keep an original truck key in my gear support drawer at home and make a new one every few years anyway as the key wears down. I learned this with my wife's car, the key wore slightly and one day the security light came on as the electronics (hall effect sensor) in the ignition switch mis read the key and turned on the security light and wouldn't let it start.
I've carried coin cells before and they always come apart in my pocket. I've standardized on AA cells pretty much now so the Arc AAA and a couple other items will be rotated out soon. I'm alsmot to the point where I have no gear running on anything else but AA's. I'm getting furthur off topic but I'm standardized on Sanyo Eneloop AA's for all my gear and everything has two battery packs or sets of batteries so one can be in the gear and the other in the charger.