Confirm before replacing. OBC menu on the dash. If the voltage reads considerably > 13.8 and no dips < 13 it's just the battery. Additionally, it's a good idea to evaluate off yet starter is going before you replace a battery, though that doesn't seem to be the case here.

I quite often see people incorrectly diagnose the situation and replace both a battery and alternator when it was the starter the whole time.

A starter when failing can take a year to completely fail but pull up to double the normal current. It will "feel like" a weak battery. Hence the need to measure voltage at rest and driving to help determine which of the three are actually failing.

My alternator wore out a couple years after I got my '01, but it's an air coolle Valeo and eazy to change the brushes and slip rings so I just refurbished mine. About two years later my battery went. It will charge up to full voltage and start the car fine every time unless even a little charge was taken off the top (iPhone charging for 30 minutes required a jump start, the final straw was 8 minutes or hazard flashers couldn't start), then half a year ago or so, starter finally went kaput so I've went thru all three. I have the 3.0 so the mechanics of changing are different but the diagnostics are the same.

First get a good charge on the battery with a charger. If you have a good volt meter with min/max or ironically a really crappy one with an analog dial, hook it up between the jump point under the hood and the cig lighter (POS side at cig).

When you crank the engine the voltage drop is approx. 1/180 the current to the starter. Eg 225/180=1.25v. (I haven't run that test in a while the ratio could be different I'm pulling from cobweb memory). I will do that test again soon to compare with my new starter. I posted my original test results on xo to compare.

If you noticed hard slow cranks before starting leans towards starter, if you noticed no problems then after a short bit of engine off power use eg listen to the radio for ten minutes and need a jump it's the battery. If the symptom of random dash lights like abs came on when you are coming to a red light it's the alternator.

(Most likely candidates).

Look at the date stamp on the battery. They usually last about five years.

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)