Low Voltage after new alternator and battery

[174bpm]

So I'm not sure if I'm chasing ghosts or not but my M3's voltage after new alternator and battery seem low. The alternator and battery in question were both purchased brand new. Valeo 439317 and Costco Interstate H5 lead acid battery. The old alternator was making whining noises and after removal and had play in the bearing. The old battery was fine but was dying pretty fast after sitting for a week or so. After I finished install I decided to measure the voltage at the battery and under the hood at the jump point terminals. Voltages from the old battery and alternator to the new battery and alternator are pretty identical to each other with the car on and off. I don't currently have any lights on the dash, car starts up normal, drives normal. Just the readings are low. 12.5 when off and 12.9 with engine running. No hot cables or anything. A quick search on M3cutters found that more than a few guys said that it is normal for these cars to read low. At this point I don't wanna keep throwing parts at the car as I believe the alternator and battery are good. Guessing I could have a parasitic drain, so my next step is go through and pull some fuses and set up a test but any other help is greatly appreciated.

[sapote]

1. A quick search on M3cutters found that more than a few guys said that it is normal for these cars to read low..
2. Just the readings are low. 12.5 when off and 12.9 with engine running

1. it depends on the state of charge of the batt.
To be sure, the batt should be checked after fully charged. You have 2 choices: drive around for an hour, or connect to a charger for 4 to 6 hours.
With a full chargerd batt, engine off = 12.6v min, and engine running = 13.85v min and 14v typical.

2. 12.9v engine running is low, unless it was charging a low discharged batt. Also try to measure directly on the nut/cable at the rear of the alternator with the other probe (black wire) on the alternator body.

[174bpm]

The battery is brand new from Costco but I hooked it to a trickle charger anyways and it said it was fully charged and healthy. I’ll try the alternator while I get home from work. Appreciate the help 👍🏽

[ac427]

You can do dashboard test number 9 to see the charging output. As mentioned it shouldn't be 12v

I'd return the battery. No point in having one that's is suboptimal from the outset.

[sapote]

You can do dashboard test number 9 to see the charging output. As mentioned it shouldn't be 12v

I'd return the battery. No point in having one that's is suboptimal from the outset.

The dash hidden display is not acurate for this kind of alternator/batt check -- we're talking a few tenths of a voltage here. Use a voltmeter instead.

[ac427]

It's good enough to show the static and charging output and does display tenths of a volt.

It shouldn't come down to a difference of 0.4 volts regardless.

[174bpm]

Battery is brand new from Costco but I will swap it out just in case. It was tested as being in good working order. Today I measured voltage at the jump points under the hood. It was at 13.7. Back in the trunk next to the battery it was 12.6 however. Bad battery cable?

[sapote]

Today I measured voltage at the jump points under the hood. It was at 13.7. Back in the trunk next to the battery it was 12.6 however. Bad battery cable?

Should measure directly on the battery posts.

This seems to be a bad cable connection; it’s very hot somewhere due to the 1V drop.

[sapote]

It's good enough to show the static and charging output and does display tenths of a volt.

It shouldn't come down to a difference of 0.4 volts regardless.

It can show charging or not charging, but can't show precisely at what voltage because the DME measuremet is slightly different than the actual measured voltage with a voltmeter.

[174bpm]

Measurement was taken from dead center on battery posts. Last night I cleaned all contact points with a wire brush on the negative and positive cables in the trunk and jump points under hood. Re tightened everything back down snug. Don’t know what else it could be.

[sapote]

Measurement was taken from dead center on battery posts. Last night I cleaned all contact points with a wire brush on the negative and positive cables in the trunk and jump points under hood. Re tightened everything back down snug. Don’t know what else it could be.

Could be at the BST(triggered by airbag event to open circuit the starter red cable). Feel around if hot with engine running.

[sapote]

Check at these locations using your thin skin and voltmeter:

skin detect heat: around BST, fender jumper post
voltmeter to measure voltage difference: cable clamps to batt posts; chassis to batt negative post; engine GND harness cable to engine and to chassis.

[ac427]

Battery is brand new from Costco but I will swap it out just in case. It was tested as being in good working order. Today I measured voltage at the jump points under the hood. It was at 13.7. Back in the trunk next to the battery it was 12.6 however. Bad battery cable?

If you have a jump start cable you could try the following:

Before you start the car

Link one end to the earth point stud on the engine bulkhead and the other end to the engine. One of the lifting hooks will do.

Start the car. Was the engine more eager to start?

Now measure the voltage at the battery.

[sapote]

If you have a jump start cable you could try the following:

Before you start the car

Link one end to the earth point stud on the engine bulkhead and the other end to the engine. One of the lifting hooks will do.

Start the car. Was the engine more eager to start?

Now measure the voltage at the battery.

Or just measure the voltage from alternator body to the car frame -- should be less than 0.2v with engine running -- and this should reveal if the engine grounding cable issue.

[174bpm]

BST isn’t hot at all nor are the terminals. The wire itself is kinda hot but only an inch or so away from the BST. Obviously can’t do the skin test on the part that goes through the car but where it connects in the engine bay is completely cool. Have another alternator being delivered tomorrow so gonna do you’re guys test on both the old and new. See if it’s a ground issue or not.