BMW Repair Questions? Ask a Mechanic for Answers ASAP
I'd suggest to double check the main battery connections as it sounds to me like they may not be fitting very well on your new battery or may just not be tight enough or they coudl be corroded
So I'd suggest to use a wire brush on both connectors and the battery post and then refit and tighten so they are snug fit
also check the condition of the main engine earth connection and if any of the high power (40A +) fuses are corroded or broken
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