Hello,

I have purchased a 2017 Octavia MK3 1.6TDI. Right from the start the car displayed a "Low 12v battery, start the car in order to use the infotainment" error.
I did that, I got a Varta AGM 70Ah 760ah and coded using a friend's VCDS. I was away for work and used the car about 1 time a week with no errors (I even did a 200km trip, no problem).
As I changed job i started using it more often and it started telling me the error above. With a multimeter I checked and the error appeared even with the battery at 12.4v or 12.76v before turning it on.
It's not an alternator problem as it charges (12.9v-13.3v-13.4v). I even removed the negative to see if the car stayed on and it did.

The previous owner mounted canbus LEDs in the foot area, above the head and in the trunk. But those should not be a problem.

Has anyone encountered a similar problem? Is there a firmware update that I can do? (does the BCM need one? i dont know).

Edited 13 October, 202513 Oct by Portocala84

First Batterie from Factory?

No, it was a EFB Varta

I would not assume that 'Owner mounted leds' would not be a possible issue either.

You say "It's not an alternator problem as it charges (12.9v-13.3v-13.4v)."" That doesn't seem right to me. I have the same car (2017 1.6 TDi). I changed battery from original EFB to Yuasa AGM, Charging voltage most of the time is 14.8 volts. Did you change battery type to "FLEECE" when you recoded ?

Disconnect those extra LED lights at least until the problem is solved.

16 minutes ago, BazzY said:

most of the time is 14.8 volts

At 14.8V, I'd be concerned about the control units. At 100% demand from the alternator, it shouldn't be more than 14.6V.

I asked in another of your posts on a similar subject if you are in the habit of listening to music/radio while the engine is not running - are you?

You mean me?

1 hour ago, Cairus said:

You mean me?

No, I mean OP @Portocala84

On 13/10/2025 at 21:04, Portocala84 said:

Hello,

I have purchased a 2017 Octavia MK3 1.6TDI. Right from the start the car displayed a "Low 12v battery, start the car in order to use the infotainment" error.
I did that, I got a Varta AGM 70Ah 760ah and coded using a friend's VCDS. I was away for work and used the car about 1 time a week with no errors (I even did a 200km trip, no problem).
As I changed job i started using it more often and it started telling me the error above. With a multimeter I checked and the error appeared even with the battery at 12.4v or 12.76v before turning it on.
It's not an alternator problem as it charges (12.9v-13.3v-13.4v). I even removed the negative to see if the car stayed on and it did.

The previous owner mounted canbus LEDs in the foot area, above the head and in the trunk. But those should not be a problem.

Has anyone encountered a similar problem? Is there a firmware update that I can do? (does the BCM need one? i dont know).

Even with a "fully" (85%) charged battery, these BMS systems will always charge at 14.6/14.8V immediately after starting but normally only for 20/30 seconds or less and will then charge at 13.3/13.6V, but on overrun will always charge at this higer voltage. I have a DVM (digital volt meter) permanently plugged into the auxiliary power socket so quite easy to see the charging behaviour, they only cost less than a fiver.

I would monitor the battery terminal voltage with a multimeter while someone starts the engine, you should then see the voltage rising to ~ 14.8V for a brief time.

Even so, that Low 12V battery alarm should not be up at anything above say 12.4V.

Borrow your friend's VCDS and take a set of readings as below, will give some idea of what's what, these are mine for a 6.8 year old Polo with its original 59AH EFB battery, these were taken with ignition switch only on hence the negative battery current, due to the daylight running lights etc., but readings can also be taken with engine running.

  IDE01834   Battery voltage 12.313 V

  IDE01836   Battery current -4.420 A

  ID01837   Temperature of battery sensor 21 °C

  IDE01839   Battery charge level 82 %

  IDE01841   Battery internal resistance 7.4 mOhm

  IDE01842   Usable battery charge 33 Ah

  IDE01843   Battery voltage at rest 12.5 V

  IDE01948   Battery internal resistance not normed 5.8 mOhm

Edited 15 October, 202515 Oct by Johngerard

4 hours ago, Cairus said:

At 14.8V, I'd be concerned about the control units. At 100% demand from the alternator, it shouldn't be more than 14.6V.

Wrong. 14.8 is normal for AGM if the BCM sees battery needs topping up.

And I can tell you, even without a data set for start-stop or with normal battery settings in the gateway or AGM, my mqb vehicles at home, there are 3 of them, not over 14.6V... you can also drive completely without start-stop parameters, you can also remove the fake recuperation and the Maximum is 14,6V

3 hours ago, Cairus said:

And I can tell you, even without a data set for start-stop or with normal battery settings in the gateway or AGM, my mqb vehicles at home, there are 3 of them, not over 14.6V... you can also drive completely without start-stop parameters, you can also remove the fake recuperation and the Maximum is 14,6V

What is fake recuperation and how is it switched off ?

Temperature is probably a factor (in maximum voltage levels).

Edited 16 October, 202516 Oct by Breezy_Pete

Probably, the DVM is excellent for showing the charging pattern but does read 0.2V ish lower than what VCDS or the actual measured terminal volts show, I've certainly seen DVM readings of 14.8V so would think that occasionally the charging votage goes as high as 15V. I will log a few VCDS readings on the run soon.

Edited 16 October, 202516 Oct by Johngerard

12 minutes ago, Johngerard said:

Probably, the DVM is excellent for showing the charging pattern but does read 0.2V ish lower than what VCDS or the actual measured terminal volts show, I've certainly seen DVM readings of 14.8V so would think that occasionally the charging votage goes as high as 15V. I will log a few VCDS readings on the run soon.

Unless your DVM is one of the cheap ones, I'd go with its reading over the VCDS every day, especially if it is either a Fluke or one of the professional Bryman meters which have an accuracy of around 1 or 2 mV.

2 hours ago, Graham Butcher said:

Unless your DVM is one of the cheap ones, I'd go with its reading over the VCDS every day, especially if it is either a Fluke or one of the professional Bryman meters which have an accuracy of around 1 or 2 mV.

Johngerard made reference to 'DVM costing less than a fiver' so I wouldn't necessarily assume it to be as accurate as a Fluke DMM.

Having said that, there are plenty of relatively inexpensive and accurate DMMs available without going to the expense of a Fluke.

Edited 16 October, 202516 Oct by Warrior193
typo

6 minutes ago, Warrior193 said:

Johngerard made reference to 'DVM costing less than a fiver' so I wouldn't necessarily assume it to be as accurate as a Fluke DMM.

Having said that, there are plenty of relatively inexpensive and accurate DMMs available without going to the expense of a Fluke.

True, but I was just highlighting a few well known and respected make that some people might know about.

Let's not get carried away debating 14.6 Volts vs 14.8 Volts. That's not helping the guy solve his problem.

So - to address the actual question .

It is normal for a low battery warning to pop up at switch on if the car has been idle for a while, such as 2 or 3 weeks in the airport carpark when you go on holiday. This may also occur if the car is only used for very short trips as the battery has never had the opportunity to fully charge. There is actually a considerable dip in voltage as soon as the door is opened, and more when ignition is switched on. This is what triggers the annoying low voltage warning. However, as Johngerard said above this will result in charging at the higher rate (14.6 or 14.8 i.e. something above 14.4 ) immediately after start up. How long it charges at the higher rate depends on what the BMS/BCM thinks is the battery state of charge - could be a few seconds or several miles. If this is not happening then there is some problem with the battery or charging circuit and controls.
New battery was fitted and coded - Did you enter your VIN in your friends VCDS in order to do the coding - and did you enter a new serial number for the battery as well as new parameters ? If not the new code may not have been saved -so BMS doesn't know it has new battery.
If the previous owner fitted mods (those LEDs) did he/she do any other changes ? Has the battery monitoring module on the battery negative terminal been removed? bypassed? disconnected? Has coding been changed to try to permanently disable Stop-Start?
Don't forget the basics - a poor connection anywhere in the charging/battery circuit will upset the system. Does the voltage measured at the alternator match the measurement at the battery when charging? In one case we had similar problems to this with a Fabia TDi which turned out to be a poor connection of the engine earth strap which had burned/corroded after not being properly cleaned and tightened after the Skoda dealer did the cambelt and water pump.
Hope this at least helps you to know where to look !

Goog luck!

Please report back if you find the solution so others can find help when they need it.

3 hours ago, BazzY said:

depends on what the BMS/BCM thinks is the battery state of charge - could be a few seconds

BMS and Gateway.

The BCM has nothing to do with the car's energy management in the direct sense. It is a command receiver for the gateway and deactivates comfort features when necessary. Therefore, the battery sensor and alternator are sub-participants of the gateway.

If the battery sensor fails or delivers implausible values, a permanent charge should be carried out and a message should appear in the vehicle

Edited 17 October, 202517 Oct by Cairus