Originally Posted by Dmax
If you have the Comfort Access option, which let's you start the car without inserting the key and open the door with the key in your pocket, you can try replacing the battery, which is a simple CR2032 coin type cell behind a removable panel on the back of the key fob.
It sounds like your car may not have the the Comfort Access option. In that case your remote is not supposed to be "user serviceable". It uses a rechargeable battery that gets charged when the fob is in the ignition slot. This explains why your key fob works after it's been charged in the ignition for a while. Assuming you're giving the battery in the fob enough time to charge from the ignition, it may need to be replaced if it can't keep this charge for even a day or two.
If you're handy, you can try Googling for some videos and directions on hacking the key fob apart, replacing the battery and gluing it back together. If not, you may need to go to a dealer. I wouldn't be surprised if the dealer charges you $200-$300 for this.
These cars came with two keys btw.
The FOB for a 2007 (all) series I believe) have a non-replaceable Battery in the non-comfort access FOB. Take it to the dealer to get replaced.
Don't feel bad about the price even Honda and other lesser brands are $ 300.00 up.
Either that or just keep using the key untill you can get it replaced.