Battery Died Suddenly

Battery went from working fine day after day and then suddenly nearly dead the next day. Connections, wiring, etc. all checked out OK. Voltage dropped from 12V to under 6V while trying cranking.
I connected to a trickle charger and in 2 minutes it showed fully charged. Same symptoms on starting again.
I connected a battery from another car and it started fine. Alternator was putting out 15V steady.
Took the old battery to a specialty store and it was less than half capacity. Bought a replacement and installed it. They also checked alternator & regulator and were fine.
Group 65 battery with 3 yr full replacement warranty was $90, so I took a chance on that one over the $140+ versions at the chain shops.
No help needed here -- just passing on the info.
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