what caused it to die is it is old!! Don't waste your time chasing something if you didn't have any symptoms of an electrical drain before. If you are bored and good with a multimeter, by all means try to trace some parasitic drains, but I am confident that you just had an old battery. Have you had any weird electrical issues as of late? And, this is a good example of a battery having "voltage", but not "amperage" to start/run a car. But that is a whole different discussion for another time.

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Dave S.
Northeast
2003 4.6is
2005 E53 4.4
2003 E39 Msport(a REAL ONE!)
1959 Fiat 600