How long do Sony batteries last?

I've got two NP-FG1 Sony batteries, InfoLithium, G type. One came with the Sony camera, and I purchased an extra battery (same), at the same time. The purchase was made in 2008. This being November 2012, both batteries are still very, very good. Sometimes I rarely use the camera, and wait for a total discharge to occur. I charge for the recommended time. More importantly, when one battery discharges, I place the second one in the camera. The second battery is always charged. Sometimes, a full charged battery, (NP-FG1) will last months just sitting out of the camera. The battery in the camera always discharges first. upon this aspect, it is unclear to me why the battery 'sitting' on the shelf will remain charged for months. The only reasonable explanation I can provide, is that the battery inserted in the camera uses a small amount of current for time, date, etc.