I do the same Ron07X5.

Cigarette lighter volt meter, smart charger if the X sits for more then 3 days.

I also replace the battery when voltage drops to 12.2 after sitting for 2 days.

12 yrs/200k miles and still on OE alternator, OE battery lasted 8 yrs, Autozone battery 4 yrs, just installed a Bosch unit.

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'03 X5 4.4 Sport, last of the M62s (8-03 build date)
I believe in deadication to craftmanship in a world of mediocrity!