You are saying something will drain the battery but you are also saying the battery has a resetting voltage of 12.4 which is fully charged. Numbers like 13.2 are only right after charging that's not a resting charge.
Your symptoms sound like either a loose connection somewhere can be power or ground side or a defective battery.
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2011 E70 • N55 (me)
2012 E70 • N63 (wife)
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