scottalexander

Run an experiment.

Trickle charge your battery so the battery is at a full charge.

Now the question is:

a) can the battery 'hold' a charge ? How long can it hold a charge? Is it discharging itself?

b) is there a parasitic drain in the vehicle discharging the battery prematurely?

c) is the diode in the alternator discharging the battery while the car is off?

donpb

You half read my mind.

I have a battery charger that both charges and tests the battery as it charges. When I used it before, the battery charged fully. The pain about that is I am not in a house and so no where to plug in the charger.

I have to remove the battery from the car and lug it all the way to my place. Its just not urgent because I am driving my other car now.

scottalexander

Get a super long extension cord and run it out your window to charge the battery.

donpb

I'd need a 300 ft extension cord. I live on the 2nd floor and my car is located in the lower parking garage.

scottalexander

In that lower parking garage there is a ceiling light.

Tap into that ceiling light and install a 110v socket for your charger.

donpb

Yeah I'll get right on it. I'm sure my Leasing office will have no problem with that.

donpb

Battery is on charge now. I drove the car for about thirty minutes today.

Last voltage reading was 11.94v this morning. Tonight its at 12.17v. We'll see what happens on the charger.

FormerE30Owner

The alternator needs to put out more than 14.1 volts, typically around 14.3 when the battery is 100% charged and more when discharged, around 14.6 to 15 volts when charging. It seems that your battery is not charging fully. I would suspect the alternator.
Your conclusion that the alternator is putting out the correct voltage is wrong and one cannot change the current setting on it. The current into the battery is a function of the alternator output voltage and the battery internal resistance, i.e. state of charge - if the battery is fully charged, its internal resistance is high, thus more voltage is necessary to further charge it. At 11.94v at the battery terminals, the voltage from the alternator should be higher than the 14.1 you measured in order to charge the battery to a nominal 14.2 v. If fully charging the battery with a good charger restores the battery to full capacity, but later the battery is low again after some days in the car, then definitely I would check the alternator.

donpb

I guess you better tell Bentley that their e60 manual is wrong then...

Charging system quick-check
- Use a digital multimeter to measure voltage across the battery terminals with ignition OFF and then again with engine running. Battery voltage should be about 12.6 volts with key OFF and between 13.2 and 14.5 volts with engine running.
- Run engine at about 2000 rpm and switch ON electrical loads (fans, lights, rear window defroster, and wipers). With all accessories on, battery voltage should be above 12.6 volts.
- The regulated voltage (engine running, battery charged, accessories and light OFF) is usually between 13.2 and 14.5 depending on temperature and operating conditions. If the voltage is higher than 14.8, the voltage regulator is most likely faulty.

FormerE30Owner

Those figures are for a good battery and a good alternator. You measured 11.94 with engine off, which indicates a low charge on the battery, therefore the voltage when the engine is running should be higher than 14.1 to charge the battery. I have seen 14.8 on my battery with engine running and usually the voltage is at 14.3 when the battery is fully charged. The battery with engine off is at 12.6. When my alternator did go bad some time back, I did see less than 14 v on the battery when engine running and a less than fully charged good battery.

Perhaps all it is for your situation is a parasite current draw and not the battery or alternator. We'll see I guess.