it's just all relying on how much electric the car need for its equipments, if yr car has been modified to have some more electricals, e.g. new Hi-Fi system, new headlight or even foglight ......... these may affect the life of the battery ...

Usually, I'll change the battery after 2 years, cuz you know it's quite anxious/ embrassment to get the car restart when your car is running out of battery in the road.

Good conventional wisdom is to get your new batt., put it on a slow
trickle charge for a few days to fully top it up, and then swap it out
when convenient for you; not when it goes toes up and leaves you
stranded. :D

Many brands are available. It's an easy DIY. Be sure the vent tube
is removed from old batt. and reinstalled on new batt.

Here's a thread from my swap out on our 6yr old oem batt.

http://www.xoutpost.com/x5-e53-forum/...apped-out.html

Hit the Search Tab and you will find a plethora of threads on the topic.
GL,mD

On my previous 3.0i 2003 battery died w/o any real warnings some weeks ago. I needed to tow car to dealer (good that I paid extended BMW roadside service - free tow). Even the strong original battery it does not really give any warnings before it dies. We were sitting on school car line when worst happend - X5 is pretty heavy car to push and stear w/o power. If you have any indications of weak battery, change it sooner than later. Dealer took $250+ to put original battery in.

:thumbup:

Sears, $120, replaced at 60K/4 yrs old.