Cache Battery question

I recently changed out a cache battery on a system that I've been nursing
along (520) that doesn't get used for anything but I just keep it going
just in case....
Anyway, I had bought a battery for it and replaced it last year and 
assumed
I'd be good for the 1000+ days it showed.  I replaced that battery a few
days ago after it screamed at me about the few days it had to live before
dying.  WTH? So I noted the days on May 11 as 2 power on days and 1001 to
warning with the intent to check it again in a few days.  Today, May 15,
I looked and it says power on days 6 and 662 days to warning! 
So is there some kind of magic IBM has with their batteries that we 
can't replace a "like" battery with? 

Bill 


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