Terri's point is probably the most important one. Get a quick disconnect so you can hard wire the one end to the battery and leave the quick connect terminal easily accesbile in the trunk. I think any decent CTEK, Battery Tender or NOCO unit will do the job well. The trick will be ease of connect/disconnect for you so you keep it hooked up.
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Josh (PA) -
'19 X5
'17 991.2 C2 Cab
'11 135i Convertible
'11 328xiT
'09 X3
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