When I switched from a standard 510 to an eGo I bought 2 batteries and thought they were the best thing I'd ever find. I bought 3 more for backups and tested them when they arrived, all worked fine. I threw them in my stockpile, keeping the original 2 in rotation. I then decided to give mods a try and quickly found out that while eGo's are good pv's, mods simply outperform them. I quit using the eGo's and promptly forgot about all the batteries I'd stocked up on.
About 6 months later I ran across all those eGo batt's in my stash and decided to put them up for sale in the classifieds. When I tested them however, 2 of the brand new batt's wouldn't fire. I fiddled around with them to see if I could fix them and found that the buttons had quit working. Repeatedly pushing the button for about 10 minutes brought one of them back to life, but I couldn't get the other one revived. I think the contacts corroded or something. I don't live in a really high humidity environment.
Since you already have 3, I'd suggest you don't buy any more until you need them. They may not work when you need them anyway. And, if you decide to upgrade in the near future you won't be stuck trying to sell them like I was.