I have a 2013 Honda Accord with a keyless entry system and can't figure out how to charge the keyless entry fob. I've looked in the manual and can't find anything about it. I asked the dealer and they don't know either.
All the fobs I have had contain a small watch battery. They seem to last longer than I keep the car. I have had them last 7 or 8 years.
Older Honda keyfobs have a US quarter sized circle on the back with a groove in it. Stick a quarter (or your equivalently sized currency) in the groove and use it like a screwdriver. It will probably have an arrow on it, but if not anti-clockwise is the correct way to turn it. It will turn about ¼ of the way around and then you can pop it out (slap it against your palm if it's a bit stuck). The newer ones that are integrated with the key itself have a small screw on the back instead of a coin-sized slot (probably what you have if you've got a 2013 Accord).
Either way it probably contains a CR2025 (in US markets, at least) battery which you can then replace.