The battery in the remote key I was using has almost run out. The unused battery in the matching remote key won't work. Does an unused battery also lose energy after 5 years?
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The shelf life of a Lithium coin cell battery is ~10 years, if stored at normal room temperatures.
All batteries lose a bit of power over time so whilst it should work perhaps it won't work for another 5 years.
If you have a DMM (digital multimeter) check the voltage of the battery. It should measure 3V DC. If it is <2.5V DC then perhaps the key won't work
Here's the 2012-2015 Honda Civic Key Battery Replacement guide that may help.
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