Audi A6 4F 3,0 tdi weak attery charging

[janek555]

Hi guys, i try to solve charging problem on my A6 longer time. The alternator charges the battery, but with weak power. In VCDS the specified voltage for ALT is 14,3V, but charging voltage at iddle only with lights ON is 13,8V. I have new AGM Exide 105Ah battery (correctly coded), i have chcecked al connections from generator to battery. At 100A generator current only 200mV voltage drop = 2mohm all circuit. Also cable between engine and car is new, all connections brushed. I tryed 3 different voltage regulators with no effects (all regulators had 61 channel 10.... OK, OK communication/ generator). Also i tryed new alternator, with no effect. Next test... 3 different J644 modules, all with the same behavior (14,3V specified, 13,88V charging). Last test was uploading software version of J644 from 4xx to 5xx... no change. Today i measured 5A fuse of J644 module.. it is OK. At this moment i really dont know where can be problem, another A6 have 14,7V specified voltage. During winter the car had problems with cold starts due to wek battery.

My opinion is, that in J644 module is some exciter current limiter, or another type of limiter? Really, i dont know. In VCDS, the DF signal never exceeds 50%, so from this information i think it can be some software issue.

thank you for all suggestions

[swalker]

thats a well detailed post.. does the voltage vary with engine speed? have you measured the voltage with a second source calibrated DMM? your 0.2V drop sounds good - your 0.2Ohm sounds way off
as the alternator is controlled - doesn't just charge for the craic, does the voltage change between no electrics on to fully loaded?
try taking a voltage reading at the battery in these 4 different ways
1 - at idle no electrics turned on
2 - at idle as much as possible electrics on - main beam, fog lights, blower full tilt should do it.
3 - at 2K RPM no electrics
4 - at 2K RPM all electrics on

[janek555]

hi, electronics is my job, so i use professional measuring devices
but with the vary of the RPM (from iddle to 3000 i tested) it is change in voltage in range 100mV, no more. The resistance i calculated is 2 miliohm (0,2V/100A) and it is perfect value.
With the increasing load for alternator (rear defrost, heating seats...) the voltage decrease to 13.3V. Tomorrow i will measure all values, that you requested

[swalker]

as you have changed everything it sounds like it is a connection problem somewhere, have you cleaned the jump start points?
access to a thermal camera? lol if the alternator is putting out the current and its not getting to the battery then the power has to go somewhere?!?!?
ohms law s great but i think you have made a miss calculation - if there was 100A you wouldn't need the thermal camera - just follow the smoke...
ohms-law-illustrated.jpg

[janek555]

usual connection problem junction in engine place is cold. iahve measurements...
specified voltage from VCDS 14.3V
iddle no load ..... Vbat = 13.85V
iddle with load ..... Vbat = 13.6V
2000rmp ..... Vbat = 13.9V
2000rpm ..... Vbat = 13.65V

iddle voltage at startup terminals 14.05V (only lights on)
at full load (seat heaters, front rear defroster, audio on,) the DF signal never exceeds 45%
"swwalker"... the current is flowing to the battery without problems, but the charging power is affected with somethink. All the power connections are in good condition for 100%. I will chceck the LIN bus communication with the scoope and decode it

[swalker]

is it possible to measure the output directly on the alternator?
the charging power is the current - remember v= IR so if you are missing the current the voltage will compensate, which is what you are seeing.

when loaded the R goes up, power (I2R) from the alternator should compensate, but you seem to be missing it - question is is the alternator dumping it out or is it getting lost going to the battery?

[janek555]

ah sorry my mistake... the 200mV voltage drop was measured at 100A load from alternator (13,8V at alternator and 13,6V at battery) from these data i calculated 2mohm resistance of circuit between alternator and battery. I would ask you.. do you still have allroad A6? Do you have VCDS? can we compare some data between our cars? (DF signal, currents and voltages) thank you

[swalker]

ok so i got it wrong in my first post you where saying 0.002 Ohms - this is impossible the resistance of 8 gauge cable is 0.002 per meter never mind the connections. - refer back to my pic, volts is always there and amps is dependent of ohms (ohms is load) - car sitting idle aint drawing much power lets go with half an amp. so plug that into V=IR we get a resistance of 0.4 ohm which is still small - but again it is all relative if you alternator where pushing out 100A the battery would boil and there would be a fire hence the BCM J644.

if you can get access to the back of the alternator and measure it loaded then measure the battery would give us a better idea. it is looking like a control thing but you have already swapped out enough so lets get some facts down.
I have a saloon and OBDeleven, i should be bale to get the live data if you throw some up i will see if i get a chance to compare at the weekend.

[janek555]

hi, i did measurement during drive, seat heating 100%, front and rear defrost ON, audio on... DF signal 35%, voltage 13,6V directly on battery, battery current dance about 10A. (If i compare VCDS and battery voltage measured by ammeter it is the same). can you try the same conditions please? i have Valeo 180A TG 17C020 alternator installed in car

[swalker]

I haven't had a chance to scan my car as yet - work is flat out and DIY has taken over. if it wasn't for keeping my dongle in my car i probably wouldn't be able to get at it in my garage at the minute, i am sure there is a mini in there somewhere.... when i get a chance i will have a look. how are you getting the ammeter in circuit?