hello all, and HNY ! 

I've got an oldish diesel beetle that looks to have much of the same instrumentation stuff inside it as the fabia 1.  At the moment the car needs jumping every morning.  It's on the parky side (-1C).  battery is taking a charge and alternator is working.  but voltage drops from nominal to 12.4 over a few hours, and to 12.2 overnight.  turning on the ignition tanks the battery to lower than 8v (picks up back to 12.2 quickly.- perhaps the glow plugs?).   

 

I can't see any heavy battery draw on the DC clamp meter I'm using.  so haven't started the fuse by fuse search.  

 

the battery is less than a year old and, as I said, charging normally.  

any suggestions for what to look for, please?  

 

thanks as ever

Justin

Happy New Year. I'd suggest first checking the battery terminals for tightness, then having the battery load tested - dropping to 8 volts strongly suggests a faulty cell in the battery.

thanks @Warrior193.  wouldn't a faulty cell show in reading static voltage?  i.e. there would be 1/6 of the potential missing from the overall battery size?  

Not necessarily, the bad cell will still be in circuit (series), but drop its voltage when its on load.  

What voltage are you reading on the meter at the terminals when the alternator is operating?

1 hour ago, jpadie said:

wouldn't a faulty cell show in reading static voltage?

Not always. A good DIY test is to open the engine bay, look at the battery and have someone else operate the starter. A bad cell may well look like it is "boiling" when under load.

40 minutes ago, Paws4Thot said:

Not always. A good DIY test is to open the engine bay, look at the battery and have someone else operate the starter. A bad cell may well look like it is "boiling" when under load.

^^^ True for batteries with removeable cell plugs - pretty rare now, mostly sealed type.

Parasitic current drain.

15 hours ago, Warrior193 said:

Not necessarily, the bad cell will still be in circuit (series), but drop its voltage when its on load.  

I agree had same in my sons fabia. Starts first time ok then stopped at garage and battery flat. His is petrol but diesel would take more current. Read 12v when charged up again then after starting pretty flat again. Seemed to be charging ok on car with no warning lights etc and took full charge with charger. New battery and its fine now.

Battery no good.

When its being jump started does it turn over at normal speed or is it slow. It could be a faulty starter motor. I had one that the coils were failing and it was only turning engine over about 1/2 speed which was too slow for my old diesel merc van. It would just manage if there was an extra battery involved but wouldnt start when cold unless it was jumped. If your battery is only about a year old I would take it back or get it checked as its probably still under waranty. Check earth connections and battery terminals as well.

thanks all. 

it's not every time so diagnosis is a challenge.  my thoughts:

1. parasitic drain:  it sometimes shows low voltage after only a few hours/overnight.  sometimes not.  I think this is a distinct possibility but wonder whether the current drain is caused by moisture incursion/freezing.  the current drain tests I have done when the engine electrics have been off for ten minutes show a drain of only 90mA.  

2. faulty battery:  my gut feeling is that the battery remains ok.  it's a sealed lead acid one so whilst I could get into the cells I haven't done it yet.  given that the battery voltage tends to be around 12.4 - 12.2 I am thinking it may be that one cell has become sulphated.  my next step is to try applying a current limited overvoltage to desulphate.  

3. faulty starter: the starter feels normal.  the battery tanks when the ignition is turned on but before cranking.  i think this is likely to be the glow plugs causing a load which is consistent with the battery having a load fault. 

4. warranty: the battery was an expensive one bought in Feb 2024 when the car had a very similar problem (yes, I know this points elsewhere).  Sadly I don't have the receipt.  

It's not unheard-of for new batteries to fail prematurely, definitely the first thing you need to do is carry out a load test on the battery. If you do not have a load tester available, the easiest way to do this is with a fast-response DMM on the battery terminals while someone cranks the engine.

The near-fully charged battery dropping to around 8.0 VDC while only loaded with ancillaries and glow-plugs strongly suggests your battery is faulty. 

1 hour ago, jpadie said:

thanks all. 

it's not every time so diagnosis is a challenge.  my thoughts:

1. parasitic drain:  it sometimes shows low voltage after only a few hours/overnight.  sometimes not.  I think this is a distinct possibility but wonder whether the current drain is caused by moisture incursion/freezing.  the current drain tests I have done when the engine electrics have been off for ten minutes show a drain of only 90mA.  

 

90mA is a bit high but could be quite normal depending on whats fitted to car. probably caused by alarm or something thats on even when ignition is off like clock etc. Have you tried reading for fault codes? If its the heater plugs it may show up something. I have had one short out on me in the past. I would as said check earths. If ones coroded or poorly connected then it could cause low voltage when under load and vary depending on how wet/frozen it is. Electrical problems are a nightmare. Other option is to start removing fuses and try and see whats causing the drain. Had an old volvo with a bad backup battery for alarm that was drawing 150mA until it decided to burn itself out. Took me a while to work that one out. It was the same fuse as the clock on the dash.

90ma is massive.

 

That said if the way you are measuring prevents the canbus modules from going into sleep mode then its plausible.

 

The following is for Octavia platform vehicles, it should be the same for Fabias but I frequently get things wrong where they are simpler, others will correct me if I am wrong.

 

10 minutes is probably long enough but 20 will be for sure, disregard those who say an hour or several hours.

 

All the doors and bonnet must be locked for the Can network to shut down (after a while) leaving the bonnet open or reopening it to take a voltage reading at the battery will prevent the Can network from shutting down or re-awaken it resulting in a false high reading for parasitic consumption.

 

Disconnecting a battery terminal to take a current reading (I dont trust low current DC readings from a clamp meter) will also wake things up.

 

The best way is to connect the meter in series on a 10 amp current range using crocodile clips or something else to maintain the contact, force closed the bonnet latch mechanism so the BCM thinks the bonnet is closed.

 

Lock the car and come back to take readings at 1 minute, 5 minutes, 10 and 20 minutes, you should see the current drop down to the region of 15-30ma.

 

Have you by any chance fitted an aftermarket radio or dashcam etc?

ok.  time for an update ... 

1. was measuring with a DC clamp meter.  i don't know what it's precision is like at low current.  I will shift to a proper electronics multi-meter for the next tests.

2. my other half got stuck in the car when at a supermarket and got mugged into buying a 180 eur AGM battery.  which is now installed.  I fitted it last night and today it is measuring 12.2v.   Worrying

3.  This is a "New Beetle" rather than skoda. But the instrumentation and clusters look v similar to my fabia.  less damaged perhaps!   

4.  everything on the car is OEM.  Additions are an Echo Auto which is plugged into the cigarette lighter.  On the beetle the ciggy is connected to the ignition (not the "until the key is removed" wire).  

so it looks like something in the car is draining the battery.  I'm wondering how to fool the BCM so that I can check the draw fuse by fuse with someone in the car.  I should have sprung for the extra tenner and the bluetooth meter!  

Does the car have start/stop?

If not, an AGM battery isn't the right type.

Have you come across measuring current by measuring mV across fuses?

Just now, Breezy_Pete said:

Does the car have start/stop?

If not, an AGM battery isn't the right type.

Have you come across measuring current by measuring mV across fuses?

the car does not have the start/stop.  i know it's not the right thing but it's bought now and that's the disposable budget for Jan gone!  

yes - I can measure voltage across fuses.  without knowing the resistance of the blades all it tells me, though, is whether there is an unexpected current draw.  It's a good start though.  Question is: how do I do that whilst keeping the BMS uninterested?  Wouldn't triggering the seat sensors stop the ECU going into shut down?   

This is a method worth trying

 

2 hours ago, Breezy_Pete said:

This is a method worth trying

 

thank you Pete.  the transfer table definitely makes it easier!  

initial tests show no pd across any of the larger fuses.  and the battery has floated back to 12.4.  i'm giving it a full charge using a (supposedly) pukkah AGM charger then will leave the car for 18 hours to see what the voltage looks like.  

7 hours ago, Breezy_Pete said:

Does the car have start/stop?

If not, an AGM battery isn't the right type.

Have you come across measuring current by measuring mV across fuses?

But no harm in fitting one - apart from to the pocket. The AGM should generally be far superior in life and performance.

One thing that should be checked is if the alternator is capable of correct voltage output to properly charge it.

Edited 4 January, 20251 yr by Warrior193
added information