well, probably some kind of NFC device can be used like this... it would need get enough power to actuate a hard switch, like, think of a very small solenoid attached to an antenna that you just need to send a signal at that frequency, then the hard switch closes and connects the device to the battery and it automatically powers up

years, i suppose a brand new fully charged lithium ion battery is gonna probably have enoughu charge to be useful for at least a year, maybe two, without anything connected to it... a lead acid battery has probably got a lower leak rate but much bigger

i don't know exactly how long the batteries hold charge without anything connected, but all the devices i've bought in the last few years have come charged and i believe they charge them 100% and typically they were around 60-80% on first power up, so i'd guess that's losing about 20% per 6 months, so yeah, a couple of years is feasible, if you can make an induction trigger to switch the power on