Q: Voltage drops from 12.5 to 3.0 when key is turnned to acc

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I just bought this 2001 lesaber from an older lady . Super nice car , though it won't crank over. She had gone to start it herd a loud pop from under the hood like a fire cracker. Stater would engage but it wouldn't crank over. Engine turns fine by hand . So we check the voltage at the jump point under the hood , it showed 12.5 . When I went to start it it dropped to 3.0 volts . So I'm thinking bad battery maybe, took the one from my truck and put it in the Buick . Now nothing works at all !! No accessories what so ever. Checked all the grounds and cleaned them , no evidence of bad ground cables or anything. Voltage still drops when I turn the key on , starter will no longer engage! Any advice would be appreciated. She had a starter put on it before I looked at it too, no difference, by the way

My car has 117000 miles.
My car has an automatic transmission.

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