[rocknexus]

I seem to be having some quick battery drain issues with my 08 335i e92. It's gotten pretty bad now as I came home from a 30 minute drive (after needing to jump start it) and I parked it and locked it as usual. Come back to it maybe 30 - 45 mins later it didn't have enough juice to start.
And now just recently I left it on the charger to charge, it charged, started up and I turned it off. Now I waited just maybe 2 mins before yellow "battery voltage low" light came on and turned it self off 15 seconds after. Tried to start it, it BARELY had enough juice to get going. It lit the dashboard up with all these airbag lights and DSC malfunction and everything and the navigation screen was black for a little bit too

Any leads? Will answer any clarifying questions.

[e91Owner]

Given that you charged it and it died pretty soon after doesn't sound as if it's a bad alternator or voltage regulator. Either way, the easy way to rule out bad batteries, alternators are with a multimeter.

Round number wise, somewhere around 12V with the car off and 14V with the car on. If you're significantly outside of that, you have a problem with one or the other. It's most likely a bad battery given your description, but you can test for parasitic drain by measuring the current drain with the amp clamp on your multimeter on an accessible battery cable in the trunk. If you have a multimeter without an amp clamp, you can connect your multimeter in series with the negative battery terminal. A drop like you are describing would need the A, not mA port.

If all of this sounds like Greek, take your car to autozone and have them test your battery.

[Steve Sas]

There is a menu in the COMBO display that shows battery voltage.

[Mark963]

Could be alternator diode. The car will run but will drain the battery if a diode is in the regulator has gone open. It'll show high ripple if that's the case and the DC voltage reading will tend to be a bit low when it is running/charging. Various testers will show if there's a blown diode. There are some cheap battery testers on amazon/ebay that will read ripple voltage for under $50. A regulator replacement is the cheap fix for that.

[bubarak]

+1 for voltage regulator relay

[Efthreeoh]

Your battery is probably near dead. How old is it? If it's near 7 years old it's probably on it's last legs. People tend to look to deep into problems with charging, when it really is just a dying battery. Once the battery begins to sulfate, it will no longer hold a charge for any amount of time. E90s need a decent reserve capacity, which is why the factory battery is 90 amp-hour. 90 AH is about 215 minutes of reserve. That's why BMW 90/720 batteries are worth the price they have nearly double the reserve capacity than batteries offered at the major auto part stores.

[charlie_m]

Originally Posted by Efthreeoh

People tend to look to deep into problems with charging, when it really is just a dying battery.

And running too long with a dying battery can over-tax the alternator, causing *it* to fail too. My SOP is to load test the battery first, replace battery (probably), then verify charging system.

[Efthreeoh]

Originally Posted by charlie_m

And running too long with a dying battery can over-tax the alternator, causing *it* to fail too. My SOP is to load test the battery first, replace battery (probably), then verify charging system.

Batteries are far less reliable than alternators and cheaper to replace. Age is a good indicator of battery health. Any battery over 3 years old should be the first suspect when energy problems arise. My E90 had a 3-year old battery and started to exhibit energy issues. It was a simply a bad battery. I replaced it a few weeks ago and the car's energy profile is back to normal.

[cliffschum]

Advance Auto tested my battery and charging circuit, see attached. Battery fine, only 4 years old, 92Ah AGM. CCA still respectable - 526 vs 570 (new). However, ripple voltage is very high, 258 mV, which is supposedly 10x normal...which indicates a diode short.

Car still starts fairly well, but could be better. (starter has been replaced)

Voltage reads 12.31, should be 12.6...

How do I determine if this is an (expensive) alternator, or, a (more reasonable) voltage regulator issue ?

I have Bavarian Tec diagnostic..

No codes or faults..

Thanks,

Cliff
2006 E90, 230K miles..

[opjose]

Rectification is done in the alternator before the regulation circuitry.

The affected diode is part of the rectification circuit.

[Steve Sas]

I don't think you would get 14v output if diode was shorted.

If car is not giving issues suspect workshop has misdiagnosed or making numbers up.

If you have a multi-meter, set to AC and measure across battery.

[cliffschum]

You could very well be correct. Dealer receipt says they installed 61-21-2-353-812 EP Battery, 92 Ah, which internet sources state at 850 CCA. I asked Dealer to install most powerful battery available.

Now, Advanced Auto report says my battery is only rated at 570 CCA. He never looked at battery in trunk, just hooked up his diagnostic to engine compartment posts.

Battery 3.75 years old. Now thinking I should be good for at least this winter.

[brenden1207]

following this as ive had a e90 in similar symptoms