Originally Posted by jdstrickland
The red battery light is really a Charging System Light

Guys, This light will turn on if the charging system sees a "out of limits" (bounds) voltage in the charging circuit. It does not know anything other than volts. A shorted (collapsed cell walls) battery, a dead or dying alternator regulator, poor contact in the Alt brushes, a break in the charging circuit (Alt wiring, battery wiring grounds, etc) all can cause the light to turn on. The one I enjoyed in the past is the four way flasher indicator turned on but does not blink when a battery started to collapse and shorted out. Car started and ran fine. No codes.

Bredliner is correct as always, there is more than one way to test our vehicles leveraging the onboard monitoring. But be aware, that is monitored voltage not a live reading. All of these displays are processed by the CPU (DMU?) then displayed. In a vehicle that is normal it is good enough for 99% of users.

Under the heading of "So What?" I hold a Master SAE certificate. But I don't think that moves my random thoughts and postings to "Master Levels" as this is a DIY forum. I have seen many "masters" arguing about lots of fine points including my own two electrical engineering Master degree brothers who have ZERO auto experience. I have a lowly general technical engineering education background, so I don't count at home either.....

I think the bottom line here for all users is to double and triple check the starting /charging system voltage as everything is suspect on a 10+ year old system where you are not the only one who touched it first. Who knows what wire was left loose, or has gotten worn out or stressed out in the last few years? Batteries die every day some new some way older than 11 years. Check and test them before moving forward as batteries and wire connections and grounds will lead you way from the problem to the parts replacement process real quick. A quick jumper wire to provide a known good ground may display a weakness.

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2005 X5 4.8IS
The Blue ones are always FASTER....

Current Garage:
2005 X5 4.8is
2002 M5 TiSilver
2003 525iT
1998 528i
Former Garage Stable Highlights
2004 325XiT Sport
1973 De Tomaso Pantera, L Model
1970 Dodge Challenger T/A 4 sp Alpine White
1970 Dodge Challenger T/A 4 sp GoManGo Green
1971 Dart Sport, “Dart Light” package
1969 Road Runner 383
1968 Ply Barracuda 340S FB Sea-foam Green