Just replaced my battery. Pretty easy process. It was sooner than I had expected though cause in 2 1/2 years both key FOBs ran out of battery.

 

I just replaced mine after 2 1/2 years. It's like a buck each on eBay, so whether it's a gauge or icon, just replace it. Why take a chance.

Strangely, my spare key, which has never been used, had to be replaced as well. May be it works like a cell phone, which it keeps trying to search for signal and depletes itself.

 

Just replaced mine after a year and a half - i think i had the same little icon

It's a #2032 battery, I got it for $4 at CVS and its really easy to replace

 

You have the part about similar to the cell phone correct:

'Since the Intelligent Key is continuously receiving
radio waves, if the key is left near equipment
which transmits strong radio waves, such
as signals from a TV and personal computer, the
battery life may become shorter.'

The above is from the 08 owners manual.

 

I am happy that its easy for others, but these were not all made the same. My case is about to crack as I try to pop it open with a screwdriver - sorry but this is a piece of crap and I hate that I have to break it to change the damn battery.

 

...just let the dealer change it for $7.50 then. I changed 5 batteries so far, 1-G '07, 2-Altima '07 and 2-G '08 and I didn't have a single problem.

FYI, I got my batteries from walmart, 2 Energizers for about $5.

 

Kept at it slowly till it finally opened. Sheesh!

Got my batteries from a local e-bay dealer. Energizer 5 batteries for $6.40 tax incl, delivered. Expiry Jan 2016. So that part worked out well!