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my car coming to 3yrs old this August, the battery suddenly died. i had to send the car to services center for battery replacement. free. as under warranty. but the car stranded for 3 days in the service center.

the SA told me that the battery voltage fluctuate (with old and new battery) i really wonder why.

i have ME adapter installed. it have car battery voltage display. the apps only connect to ME adapter while engine running.

today i just realise, the battery fluctuate between 12.5v to 14.6v within minutes.

is this correct? or something wrong with my car.


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The alternator should put out a steady ~14V. A unit putting out 12V, even part of the time, is not enough to properly recharge the battery and run the vehicle's ancillaries under operation.
 

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12.5 is correct for a static battery (ignition on but alternator not operating). If the 12.5 reading was at start up you are probably OK. If you see that value in the midst of driving I would them be concerned about the charging system.
 

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12.5 is correct for a static battery (ignition on but alternator not operating). If the 12.5 reading was at start up you are probably OK. If you see that value in the midst of driving I would them be concerned about the charging system.
this is what i thought too, common sense.
i believe the SA is refusing to change it under warranty.

what i should do?

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First, does the voltage drop to mid 12 while you are driving? With all the draws on the battery like air conditioning and power steering let alone all of the electronics the alternator may not be able to reach charge rates at idle. If you are cruising at 2000 rpm and the voltage still does not reach charging levels, then I would be concerned.
 

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This is what was happening with mine too. I don’t have the voltage readings but it got so bad that the car stopped my transmission in the middle of the street while I was driving!
 

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First, does the voltage drop to mid 12 while you are driving? With all the draws on the battery like air conditioning and power steering let alone all of the electronics the alternator may not be able to reach charge rates at idle. If you are cruising at 2000 rpm and the voltage still does not reach charging levels, then I would be concerned.
it just flactuate between 12.5 to 14. on both of my merz. this reading from ME adaptor

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This sounds reasonable. If it was a constant 14 the battery would be at risk of over charging and boiling off fluids. The alternator output is regulated to avoid that scenario. If it was dropping below 12.5 I might be worried. But there are a lot of variables based upon motor speed and loads.

Also with Covid and shelter in place a lot of people are driving less. Parasitic drains caused by the computers in your car can cause batteries to get weak wherein you can start the car but while driving the computers might get confused as the critical power suppliers fluctuate and sensors give errant data.

And after 3 years of discharges and recharges as the car sits and then is driven the battery may be getting toward the end of its life. To avoid this I keep my cars on battery tenders. Cheap tenders can work but I've found the more expensive ones work better. No affiliation and limited testing but from the advice of others I've settled on CTek.
 

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I would have the alternator check that it is working as it should. The alternator should put out 14 volts which should supply power to run the car. The extra power is used to keep the battery charged. If the alternator is not performing as it should then the battery is not getting full power and will gradually be drained.
 

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From a Porsche technical journal ...
26154


26155


Also ...


And ...


Over the years maintaining battery life by reducing sulphation has become a science. This has an affect on the operation of the alternator. Also a digital output is easier to control than an analog (stator current on the alternator) so the alternator is on (14 volts) or off (battery voltage ... 12.5 +-).

If in doubt give a call to your MB SA (service advisor) and ask for an explanation of how MB engineers designed the charging system on your car.
 

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Slept on this and woke up thinking I might have left some holes ...

In the old days (my trickle charger is over 50 years old) we had transformers that put out a bit over 14 volts with no load. Transformers are current limited based on their size. With a load on the current limited transformer the output voltage will drop. So when you hook the charger to a dead battery you might only see 4 or 5 volts. Then that voltage would increase as the battery charged and required less current from the charger. In a day you might be seeing 12 volts. In another day you might be seeing 14. But by then if you pulled a cap to look inside the battery you would see bubbles escaping the acid inside. The battery is boiling. This is not good.

When an alternator kicks in it needs extra voltage (beyond 1.2 volts per cell × 6 cells = 13.2 volts) so we expect to see 14 when a good alternator is spinning fast enough to charge the battery while supplying current for all of the loads. A note to the side ... Alternators develop this power at fairly low speeds and that advantage is the reason we don't see generators anymore. But this power still may not exist when the motor is at idling speed so at a stop sign the battery may discharge even if the alternator is enabled.

Now if the alternator supplied that power all the time the battery would see 14 volts like with a trickle charger and it would eventually boil as in the case of leaving a trickle charger on too long. By now it should become obvious why battery tenders (that can be attached to a battery over a long winter cycling on and off based upon now the battery is holding charge) are different than simple trickle chargers.

But if the full charge of a battery is 13.2 volts why are we seeing 12.5 when the alternator shuts off? The battery has internal resistance and a maximum current output. Climate control and spark plugs and heated seats and audio and computers all require power. Without the alternator this power is supplied by the battery. An ideal battery without resistance and this with infinite current capability would not drop voltage but in the real world a voltage drop is natural.

Reference Ohm's Law ... V = I × R. In this case you are looking at voltage drop within the battery based on battery resistance and current through the battery equaling the voltage drop within the battery. This is subtracted from the 13.2 volts that a fully charged battery would otherwise indicate.

Theoretically you could measure the current vs the voltage drop and assess the quality (internal resistance) of the battery. But that's for another day ... Brand of battery, connections to battery, temperature of battery, type of battery (lead acid vs AGM vs Optima have different characteristics), et cetera leave too many variables for an OEM dash indicated application IMHO.

Anyway, hopefully the above helps clarify things rather than muddying the situation.
 

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Slept on this and woke up thinking I might have left some holes ...

In the old days (my trickle charger is over 50 years old) we had transformers that put out a bit over 14 volts with no load. Transformers are current limited based on their size. With a load on the current limited transformer the output voltage will drop. So when you hook the charger to a dead battery you might only see 4 or 5 volts. Then that voltage would increase as the battery charged and required less current from the charger. In a day you might be seeing 12 volts. In another day you might be seeing 14. But by then if you pulled a cap to look inside the battery you would see bubbles escaping the acid inside. The battery is boiling. This is not good.

When an alternator kicks in it needs extra voltage (beyond 1.2 volts per cell × 6 cells = 13.2 volts) so we expect to see 14 when a good alternator is spinning fast enough to charge the battery while supplying current for all of the loads. A note to the side ... Alternators develop this power at fairly low speeds and that advantage is the reason we don't see generators anymore. But this power still may not exist when the motor is at idling speed so at a stop sign the battery may discharge even if the alternator is enabled.

Now if the alternator supplied that power all the time the battery would see 14 volts like with a trickle charger and it would eventually boil as in the case of leaving a trickle charger on too long. By now it should become obvious why battery tenders (that can be attached to a battery over a long winter cycling on and off based upon now the battery is holding charge) are different than simple trickle chargers.

But if the full charge of a battery is 13.2 volts why are we seeing 12.5 when the alternator shuts off? The battery has internal resistance and a maximum current output. Climate control and spark plugs and heated seats and audio and computers all require power. Without the alternator this power is supplied by the battery. An ideal battery without resistance and this with infinite current capability would not drop voltage but in the real world a voltage drop is natural.

Reference Ohm's Law ... V = I × R. In this case you are looking at voltage drop within the battery based on battery resistance and current through the battery equaling the voltage drop within the battery. This is subtracted from the 13.2 volts that a fully charged battery would otherwise indicate.

Theoretically you could measure the current vs the voltage drop and assess the quality (internal resistance) of the battery. But that's for another day ... Brand of battery, connections to battery, temperature of battery, type of battery (lead acid vs AGM vs Optima have different characteristics), et cetera leave too many variables for an OEM dash indicated application IMHO.

Anyway, hopefully the above helps clarify things rather than muddying the situation.
Good info but an alternator has a voltage regulator that would control the volts going to the battery based on the battery needs to be fully charged and not cook the battery.
 

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I did simplify some stuff!!! And your statement ... Based on the battery needs is enlightening. There's the old way (basic charging) and the new (smart alternator charging).

The voltage regulator is actually a control to adjust the current to the rotor which then affects the rotor's magnetic field and thus the transfer current / voltage to the battery. To my knowledge these days that is more of an on off control. I have links to smart alternators regarding this a bit later. But for thoroughness, back to dumb alternators ... In fact the voltage regulator had basically two states but in the low state it still allows some bleed current to the rotor so there will still be some current from the alternator even in its "off" state.


But the more you look into modern systems the more murky things get. From another MB model Forum that I suspect holds for the GLA ...

I believe the W212 is equipped with a "smart alternator" that only charges the battery when the engine load is low or when you are decelerating to increase fuel economy by recapturing kinetic energy (similar to a hybrid). The battery voltage should be 13.6 - 13.8 volts during "charge" mode with excursions permitted as high as 14.5 volts. In a car with a "dumb" alternator, the test voltages you obtained would signal a dead alternator (or voltage regulator). With the "smart" alternator, you may or may not be measuring when the alternator is "charging" so the dealer is somewhat correct in that the computer (Star Diagnose) is the most reliable way to evaluate the performance of the alternator. The dealer is full of crap with the emissions cycle thing as the alternator behavior is all about fuel economy rather then emissions (although increased fuel economy does result in fewer emissions).

Add to that ...

.

And the above brings to question the secondary battery charging problems seen in the GLA line.

If you study battery tenders you will find patented algorithms that adjust charging currents based upon time and measurements. Once a battery reaches charge the tender will wait for a discharge and then recharge in a cyclical way. Thus they will not damage the battery.

Add to that environmental and CAFE requirements of cars and automotive engineers have created a very muddy picture of what is going on in the charging circuit.

What we do know is that a battery at rest or under light load will read about 12.5 volts. A battery being charged will show around 14 volts. Side note ... Flooded batteries are not identical to AGM batteries and require different charge voltages which is why you should not replace an AGM OEM battery with a lead acid battery.

It appears, and would be reasonable to assume, that MB has incorporated a smart alternator charging system into the GLA. For MPGs this means there will be times that they shut the alternator off to reduce loads on the motor. That is true for performance (removing load for acceleration). In these cases a shift between 12.5 and 14 volts and back while driving would make sense. The dealer approving this pattern of voltage changes implies the GLA uses smart technology.

Some other reading ...


The above talks about capturing breaking energy into the battery via the alternator.

Or a quick video explanation ...

 
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