charging voltage

[TiesTorN]

alternator rebuilt with genuine bosch parts (by myself) last year so there is no problem with it. was having the same symptom before rebuilding it anyway (before it went complete kaput)

charging voltage is 14.4v when my car is cold started. voltage gradually drops as it warms up. output drops to 13.5V when fully warmed up, and drops to as low as 11.9v at idle (@ 500 rpm) it should be staying at 14+ (@1500+ rpm), why the drop?

I'm suspecting corroded cables & terminals somewhere with increased resistance (which increases further with heat)

what can it be? maybe the ground wire?

[StephenVA]

Charging system testing is NOT just volt readings to determine function. Alternators put out VOLT and Amps (see side of the alternator case for level of output). You will need a carbon pile tester (example image below) with an AMP clamp reader to full test the charging system and battery in any vehicle.

Starting/Charging Testing instructions are listed below (for like the 24th time)
Starting & charging systems testing techniques
Without spending too much time on starting & charging systems testing techniques, here are a few quick tests to determine a battery issue, a charging system issue, or a primary wiring issue.

Skill Level: 2 + (read and follow instructions)
Tools required: Digital Volt Meter (DVM), battery terminal cleaning tool, and 8mm, 10 mm sockets and ratchet with extensions.

You need to read battery voltage before starting any test on an electrical system as it will influence the results of Starting, Charging, and electrical tests of any kind. (Note: battery must be fully charged, see chart below).

Step One: Get out a digital volt meter, Read battery voltage (Key off). Read voltage (Key on) at the following points Battery, under hood jump point, and at the alternator. You only need to determine if you have 12 volts at the alternator with the key on (BIG FAT WIRE). A simple handheld digital wave form tester will get you in the ball park when it comes to batteries. (Further discussion on these testers is for another engineering forum).
Step Two: You need to read the voltage at the BATTERY at idle, 1500, and 2500 RPM to determine voltage regulator function. Better still would be a Volt Amp tester (VAT-40 Image below) to induce a correct AMP load to read actual AMP/Volt output from the alternator and battery, as that is the only true testing method. Having a DVM that has min/max capability will make your testing easy.
Step Three: Remove and clean each and every ground in the trunk, jump point, and at the frame rail for the block (bad grounds = lots of electrical issues). Clean the starter/alternator cable connection under the car, just under the dead pedal.
Step Four: Failure to get correct output on running test means DEAD alternator (volt regulator, brushes, windings, etc.) Remove and bench test the unit or drag it to your local auto parts store for a second opinion (which will be worth exactly what you pay for it....)

Base line numbers
Battery Voltage should be 12.6+V (Key off) See chart below.
Battery voltage should be 13.5v - 14.5v on running tests (note large fluctuations at running RPMs is a SURE sign that the Volt Regulator is not doing its job. Smack with rubber hammer and retest. Rebuilt units have large variance of quality control from good 85% of the time to approx 50/50%.

NOTE: The exact charging voltage will vary according to the battery's state of charge, the load on the vehicle's electrical system, and temperature. The lower the temperature the higher the charging voltage, and the higher the temperature the lower the charging voltage. The "normal" charging voltage on a typical application might be 13.8 to 14.3 volts at 77 degrees F. But at 20 degrees F. below zero, the charging voltage might be 14.9 to 15.3 volts. On a hot engine on a hot day, the normal charging voltage might drop to 13.5 to 14.3 volts.

Note 2: TIS specifies alternator voltage as 14.3 +/-0.1 V. But that is measured at the alternator output post. The values seen using the cluster will be 0.5 V or more below the alternator voltage output.


Batteries surface charge.

What the heck is a surface charge?
When testing wet cell auto batteries, some will show a 12.xx Volt result, but as soon as a 100-150 amp load is applied, they drop straight down to 9.8V or lower. Classic example of a collapsed cell wall (internal short). This is why too many DIY and techs get stumped when they pull out a meter and say "It shows 12.xxV so it must be good”. NOPE, batteries are tested for voltage holds and AMP draw capacity as well as the ability to recover and recharge over and over again. Batteries are nothing more than an energy storage device. Their usage back in the 1920's allowed for a "self-starter" option, now days we take it for granted. In the future we will use the engine ignition system to fire off a cyl while disabling all other cyls (opening the exhaust valves) allowing for a start that requires no electrical/mechanical starter. A topic for a SAE publication but an interesting future none the less. The challenge is for the average DIY to approx. the load test with KEY OFF.

Solution: Set a volt meter to DC volts 0-25 scale or auto ranging if so equipped. Read and record voltage
Turn on everything in the car for 15 seconds:
Headlamps to high beam
Heater blower motor to high w/ A/C on
Seat Heaters
Rear Defroster (and rear seat blower if so equipped)
Interior lights
Radio
Fog lamps
Cigar lighter
etc, etc, etc

Read voltage after testing. You should see a drop off of .5-1.5Volts during the test and a slow rise back to 12.5V+ on a completely healthy and fully charged battery in 30sec to 1 min.
A battery that is DEAD, will show 9.8V or less and never recover until charged.
Battery Registration: There is NO battery registration required for proper operations in a E39 or E53 application.

Alternator testing tips
To quote another poster...

How to load the alternator at engine idle for alternator output tests:


Note: Alternator output should be tested with the engine idling under the following four loads after at least 15 seconds of idling (for the alternator to ramp up):
Turn the high beams on
And, turn the rear defroster on
And, turn the fan on (full force, I guess)
And, turn the wipers on (to the regular setting, I guess)

These loads should be left on while the alternator output tests are being performed.

Once access to the alternator is gained (access for the I6 is vastly easier than the V8):

• Check the voltage from the alternator #30 post and ground
  
  • It should be the same as the battery voltage
  
  
• Now disconnect the harness connector to the alternator:
• Check voltage from terminal 15 and ground (field voltage)
  
  • It should be the same as the battery voltage

More info: Another way to check alternator output is with an oscilloscope. Observing the "ripple voltage" pattern will tell you at a glance whether or not all the alternator windings are functioning. A "good" pattern should look like the top of a picket fence. If any of the humps are missing, it means one or more of the windings is grounded or open, or there's a bad diode. Most battery/charging system testers also have a test function that can detect bad diodes.
Ripple testing with a DVM:

ANOTHER QUICK CHECK FOR BOSCH ALTERNATORS

One way to check the integrity of the alternator and diodes on Bosch alternators is to check the voltage readings at the D+ (blue wire) terminal and B+ terminal. The voltage reading should be the same at both terminals. A difference of more than one volt would indicate faulty diodes and the need to replace the alternator.
Tips link: https://www.diyauto.com/manufacturer...ion-by-bluebee

Volt Drop Testing (Image below) will show you issues with grounds and wiring problems. Read the text and test a few spots to determine if your car has clean connections.
https://www.engine-light-help.com/voltage-drop.html

Are you having other electrical issues? Alternative issues: Ignition Key switch failure, wiring junction box rusted (see water leaks under passenger seat (E39), Water standing in trunk under the battery (E53) and the normal wiring broken connectors, grounds), etc....

Tip Sites for more info:
https://www.aa1car.com/library/2002/cm10220.htm
https://www.bimmerfest.com/forums/sh...d.php?t=510579 (Everything you wanted to know about BMW starting and charging systems for DIY)
Need a Video? https://video.search.yahoo.com/searc...d&action=click

Helpful?

[JimLev]

No it shouldn’t stay at 14+ volts, that will over charge your battery.
While you may have some corrosion (negative ground strap between chassis and the pass side motor mount) sounds like it’s working fine.
At 500 RPM the alternator isn’t putting out enough current, the car is running off the battery which is lowering the battery voltage.
I’ve seen mine below 12.6 volts plenty of times at idle after the battery has been charged up from starting it, stereo on, etc, heat or AC, etc.
My typical reading at the cluster while driving is 13.6-13.7 volts.

[StephenVA]

Under FULL load below 80 degrees F your charging system should display voltage over 14.3V.

Without any load on the system, the readings (Alt Volt and Amp output) will start high, then drop as the battery no long requires charging to hold steady a 12.5V surface charge. The output is based on running requirement demands. As Jim mentioned above, driving voltage will depending on alt load and ambient temps. Use a handheld voltmeter on the battery directly as The values seen using the cluster will be 0.5 V or more below the alternator voltage output. This is referred to as "Display Voltage" as it is a process read not live voltmeter reading.

If I had to guess, I think this poster is using his cluster to read battery voltage

[TiesTorN]

volt is needed to charge the battery, not amps. you can push a lot of amps onto a battery but it won't properly charge unless the volt is high enough. 14v will never overcharge a flooded battery. above 14.5v (and a loooong chargng time) is needed for that.

I measured 13.5v with the multimeter. cluster usually shows 0.3v lower than the real value (so mine shows 13.2v on cluster) and there is like 0.05v difference between different terminals which is normal. the values I wrote are all measured with the multimeter. I don't trust the cluster reading.

(@jimlev) 13.6-13.7 reading on the cluster actually means 13.9-14v in real which is very good.

there is definitely something wrong with my car and I'm suspecting the ground wire and the connector terminals under the car. probably corroded. I'll also check starter connections as stephenva suggested. any other suggestions?

[StephenVA]

The opposite is also true. You can get an alternator output of 13V and never charge a battery if it can not produce amps to overcome resistance. https://science.howstuffworks.com/en...uestion501.htm So if you have lots of corroded connectors, it will take more to overcome those poor connections.

Remove and Clean the "jump point" cable ends under the hood, as they tend to get real corroded. Double check (unbolt and clean each ground in the battery area) and do a volt drop test before and after to see if your efforts resolve anything. There is also multiple grounds for the engine under the car on the frame rails. The difference between cars maybe nothing more than demand required hence higher voltage required.

If you really want to test the output of your alternator, you will need a 150 Amp carbon pile load tester, as you can't get a known and measured high load on the charging system by turning on all the accessories.
You need to read the voltage at the BATTERY at idle, 1500, and 2500 RPM to determine voltage regulator function. Better still would be a Volt Amp tester (VAT-40 Image below) to induce a correct AMP load to read actual AMP/Volt output from the alternator and battery, as that is the only true testing method approved by SAE.

Testing is simple: Hold at these RPM for 20 secs and read voltage. If you use a volt/amp tester it forces the alt to push out max while testing. You are looking for fluctuations during the test.

[BimmrMeUpSnotty]

I love it..... One comes on here stumped, then starts telling you guys that you are all wrong.
You don’t need to charge a battery with 14.4 volts, it’s the amps that rams in the juice. You do a constant 14.4 charge on any 12volt battery and you will cook it. What do you think the regulator is for? Initial voltage starts high, to push the current(amps) as the battery gets up in voltage, the alternator, if the regulator is in proper order, or, the charger, if it is smart, will ramp down the voltage, just as your alternator is properly doing so. Otherwise, it will cook the battery. As the charger/alternator see’s the battery getting up to it’s proper voltage, it does not need the 14/13 volts any longer to keep ramming the amperage/ current into the battery. So it ramps on down. Your alternator is doing it’s job. You have had constant issues with this, You said that you got a new battery the last time you had issues, which if my forgetful memory is correct, was not too long ago. What condition is your battery in? Clean every connection to the battery, the battery grounds, the connection to the starter, and to the alternator, grounds on the engine. Do a load test on the battery, I don’t care if it is new, it could be bad with a bad cell.
Here’s another question, aside from the corroded connections you may, or may not have, what are the condition of your belts and tension pulleys in?
Seems like your car is always in dire straits, yet you always try to correct everyone.... instead of telling everyone they are wrong, let others help you out, it’s why you started the thread in the first place.

[StephenVA]

Serves me right for posting......

[JimLev]

Guess my 40 years in electronics means I don’t know sheeet from shineola.

Might need a few boxes of this.

[ross1]

Why is everyone trying to educate the ineducable ?
I don't see anything wrong with your charging system in spite of you having repaired it.
Perhaps you should alter the voltage regulator to deliver 14.5v all the time. You could then watch your battery boil.

[TiesTorN]

I give up. you guys are incapable of reading my posts thoroughly and understand my question. you are always assuming I am a noob user when I'm not. I always search google and try possible solutions before writing here.

[Mike WW]

So when did charging specs drop down so low? Granted E39s are far more complex beasts than E12s, but the alternator/battery relationship hasn't changed much. Shown is a scan from the big blue book the official BMW service manual.

[StephenVA]

One last post of useless insight, then I am done.

On the drop off of voltage after starting and warm. NORMAL function of a regulated charging system. This process has been the same that I know of since the 1960's and before with the introduction of ALTERNATORS. At idle your car is drawing more than the alternator is putting out so it draws from the battery which is why you see battery voltage. How much is that draw? Are accessories turned on that create a high demand? Don't know as it is YOUR car not mine. Do you have a super duper amp in the back seat sucking up all the energy of the low charge at idle?

Have you performed any drop tests to confirm you DO NOT have bad grounds or high resistance in your cables? Have you removed, cleaned and reattached all of the negative cable ground points and the positive cable connectors, including the harness pins to the alternator? Have you inspected the wiring jacketing at the rub point along the way. A poor ground will give your car all types of challenges. I can not tell you how many DIY's can't figure out simple resistance testing processes. It is not rocket science. A simple drop test from the battery to the alternator will confirm functionality. Without a base line everyone is just doing a SWAG to your posts.

If you are seeking constant voltage output from your alternator charging system that has multiple regulators built into the car's wiring system (Read "controllers"), I am at a loss to understand why and How to accomplish that feat without a re-engineering a 20+ year old design. Note: Once you get spikes of 15V or higher your electrical system will be toast.

I am now done. Please report the battery fire results....

I am now off to Costco to get more popcorn and a few marshmallows to enjoy on the car roast.

[JimLev]

I am now off to Costco to get more popcorn and a few marshmallows to enjoy on the car roast.

LOL, for what its worth, I just got back from a 75 mile popcorn run too, StephenVA must have horded all the popcorn everywhere.

Ties, before I started my car this morning it hadn’t been started for 4-5 days and not on a smart charger, the battery read 12.55 volts.
I started it up and the cluster read 13.7 volts, the battery read 14.01 volts, this was at 550 RPM.
The cluster stayed at 13.6 to 13.7 for my whole drive except what I had to stop. It would drop down to 12.7 but then quickly go back to 13.6 - 13.7.
I do have a 400w JL Audio amp, a 100w sub, and an electric fan that runs at maybe 1/3 speed all the time.

Maybe your readings are lower because of the parts you used to rebuild yours?? Or maybe corroded connections. The ground strap to the pass side ALUMINUM engine mount and the steel bolt/washer will cause galvanic corrosion.

[TiesTorN]

One last post of useless insight, then I am done.

On the drop off of voltage after starting and warm. NORMAL function of a regulated charging system. This process has been the same that I know of since the 1960's and before with the introduction of ALTERNATORS. At idle your car is drawing more than the alternator is putting out so it draws from the battery which is why you see battery voltage. How much is that draw? Are accessories turned on that create a high demand? Don't know as it is YOUR car not mine. Do you have a super duper amp in the back seat sucking up all the energy of the low charge at idle?

Have you performed any drop tests to confirm you DO NOT have bad grounds or high resistance in your cables? Have you removed, cleaned and reattached all of the negative cable ground points and the positive cable connectors, including the harness pins to the alternator? Have you inspected the wiring jacketing at the rub point along the way. A poor ground will give your car all types of challenges. I can not tell you how many DIY's can't figure out simple resistance testing processes. It is not rocket science. A simple drop test from the battery to the alternator will confirm functionality. Without a base line everyone is just doing a SWAG to your posts.

If you are seeking constant voltage output from your alternator charging system that has multiple regulators built into the car's wiring system (Read "controllers"), I am at a loss to understand why and How to accomplish that feat without a re-engineering a 20+ year old design. Note: Once you get spikes of 15V or higher your electrical system will be toast.

I am now done. Please report the battery fire results....

I am now off to Costco to get more popcorn and a few marshmallows to enjoy on the car roast.

well thank you, this is the actual answer I was waiting for.

yes I did drop tests. 13.5v is actually the value when everything is turned off except for the headlight and radio (close to 13.6 with everything turned off). when I turn on heated seats and the rear window heater, voltage drops to around 13.2v @ 1000+ rpm and 11.9v @ idle. I don't think these are anywhere near normal ranges. (ps: battery indicator wont light up even at 11.9v so the ecu still thinks it's charging)

I have only removed and cleaned all the negative and positive connectors that are in battery bay. it fixed my voltage fluctuation back then but obviously wasn't enough to maintain a proper voltage. so according to your drop test I have heavy resistance on my cables somewhere and I need to find it. just as I thought. thanks.

to be clear; these are multimeter values. I don't use cluster. it shows 0.3v lower. (substract 0.3 from all the values I wrote if you want cluster values)

as a side note: 15v is not enough to fry car electronics. that would be 16+ volts.
note 2: voltage regulators are there to maintain a stable voltage throughout the alternators rpm range, it's not there to reduce voltage after warming up (at least on older models like these) I recommend watching educational youtube videos about alternators.

[JimLev]

Ties, we posted at the same time, see post 14.

Also did you know the battery cable under the car is spliced to the cable that runs to the alternator? It’s by the rear pass side of the engine subframe.

[TiesTorN]

LOL, for what its worth, I just got back from a 75 mile popcorn run too, StephenVA must have horded all the popcorn everywhere.

Ties, before I started my car this morning it hadn’t been started for 4-5 days and not on a smart charger, the battery read 12.55 volts.
I started it up and the cluster read 13.7 volts, the battery read 14.01 volts, this was at 550 RPM.
The cluster stayed at 13.6 to 13.7 for my whole drive except what I had to stop. It would drop down to 12.7 but then quickly go back to 13.6 - 13.7.
I do have a 400w JL Audio amp, a 100w sub, and an electric fan that runs at maybe 1/3 speed all the time.

Maybe your readings are lower because of the parts you used to rebuild yours?? Or maybe corroded connections. The ground strap to the pass side ALUMINUM engine mount and the steel bolt/washer will cause galvanic corrosion.

yours is completely ok. you would see 13.9-14v if you measure it with a multimeter. 12.7 in your cluster probably means 13v with a multimeter and it's still ok.

parts can't be the problem because as I said before I used genuine bosch parts to rebuild mine and it did push out 14v constantly in my friend's e38 when I swapped it with his. there is a high probability that I have a bad connection or a cable somewhere in my car.

Ties, we posted at the same time, see post 14.

Also did you know the battery cable under the car is spliced to the cable that runs to the alternator? It’s by the rear pass side of the engine subframe.

didn't know, I need to check these asap. thanks.

[BimmrMeUpSnotty]

....

[BimmrMeUpSnotty]

I give up. you guys are incapable of reading my posts thoroughly and understand my question. you are always assuming I am a noob user when I'm not. I always search google and try possible solutions before writing here. I even told u about my alternator swapping test but still getting useless answers.

1,There was no assuming going on.
2.You didn’t need to point out how you always search the googlymoogly before you post anything here.

[DennisG01]

alternator rebuilt with genuine bosch parts (by myself) last year so there is no problem with it. was having the same symptom before rebuilding it anyway (before it went complete kaput)

charging voltage is 14.4v when my car is cold started. voltage gradually drops as it warms up. output drops to 13.5V when fully warmed up, and drops to as low as 12.3v at idle (@ 500 rpm) it should be staying at 14+ (@1500+ rpm), why the drop?

I'm suspecting corroded cables & terminals somewhere with increased resistance (which increases further with heat)

what can it be? maybe the ground wire?

Ties, you mentioned that you thought you weren't getting help. I just want to point out, that while what you're asking might make sense in your head, from the viewpoint of a bystander reading through this thread, it's not very clear what you're asking. The guys responding are doing their best to help. For example... "charging voltage is 14.4v when my car is cold started. voltage gradually drops as it warms up. output drops to 13.5V when fully warmed up, and drops to as low as 12.3v at idle (@ 500 rpm) it should be staying at 14+ (@1500+ rpm), why the drop? This sentence doesn't make sense. You're stating that V drops as RPM drops (true), but then state that V should stay higher at higher RPM', but then ask why it drops. It doesn't drop at higher RPM's, it rises. What I'm trying to get at is that maybe you aren't being very clear with what you're asking. It may make sense in your head, but we can only respond based on what you've written. Before spouting off, take a minute to look back at what you've written and see if possibly it's a simple case of miscommunication, instead putting the guys down.

Resistance decreases with heat.

[ross1]

I give up. you guys are incapable of reading my posts thoroughly and understand my question. you are always assuming I am a noob user when I'm not. I always search google and try possible solutions before writing here. I even told u about my alternator swapping test but still getting useless answers.

# SaCl

[JimLev]

Resistance decreases with heat.

No, you got that backwards. The hotter the wire gets the more the electrons collide with each other which increase the resistance.
In conducting liquids it will decrease.

[StephenVA]

+1^
when the jacketing melts off it is hot and has high resistance. The opposite is true of girl friends

[DennisG01]

No, you got that backwards. The hotter the wire gets the more the electrons collide with each other which increase the resistance.
In conducting liquids it will decrease.

Hmmm. Maybe I've been wrong all these years!

Is this true even when we would compare something like 30*F to 80*F? Or is this one of those things where resistance decreases as temperature rises, to a point, then it starts to decrease? Or is it more of a linear type line graph?

I do understand the hot wire thing you're referencing, Stephen... too much amps through too small of a wire, for example... hence the reason for fuses.


EDIT: Nevermind. I did some googling and found wire resistance calculators and the formula yields a linear result. Now that I'm thinking about it, I guess I always thought that since you lose power (a battery, for example) in cold weather it meant lower resistance without really thinking it through and realizing that it's the chemical reaction that slows down, not the e's running through the wire.

Ties, sorry for the misdirect.

[TiesTorN]

Ties, you mentioned that you thought you weren't getting help. I just want to point out, that while what you're asking might make sense in your head, from the viewpoint of a bystander reading through this thread, it's not very clear what you're asking. The guys responding are doing their best to help. For example... "charging voltage is 14.4v when my car is cold started. voltage gradually drops as it warms up. output drops to 13.5V when fully warmed up, and drops to as low as 12.3v at idle (@ 500 rpm) it should be staying at 14+ (@1500+ rpm), why the drop? This sentence doesn't make sense. You're stating that V drops as RPM drops (true), but then state that V should stay higher at higher RPM', but then ask why it drops. It doesn't drop at higher RPM's, it rises. What I'm trying to get at is that maybe you aren't being very clear with what you're asking. It may make sense in your head, but we can only respond based on what you've written. Before spouting off, take a minute to look back at what you've written and see if possibly it's a simple case of miscommunication, instead putting the guys down.

yeah maybe. I actually meant it's 14.4v at higher rpm when cold compared to 13.5v at higher rpm when warm. so it's 14.4v vs 13.5v.

I always tell you guys english is not my native language. I may make mistakes or can't explain myself enough or seem very harsh sometimes and I'm sorry about that.

what I really get angry is when some people here try to teach me about basic things like charging fla batteries or what the rest button do on my other topic. I'm not here to ask you about these I already know about them. I ask a spesific question about my voltage drop and people start saying I'd fry my battery at 14v lol... no I won't and that's not what I'm asking.

anyway, jimlev and stephenva helped a lot so I thank them.

ross1 and snotty are always there to put me down regardless of topic.