I've been diagnosing with the Carista app and Carista OBD2 on my 2014 Yeti Greenline 1.6TDI.  I may ask another question on that, but right now is it normal for the OBD to read the battery voltage as lower than a multimeter across the battery teminals? It's typically about 0.3V lower, sometimes even more.

 

I replaced the original battery this week, thinking it wasn't holding charge well enough to be confident of starting over the winter, and suspecting that a low voltage may contribute to the half dozen yellow warning lights I get. With the old battery the ODB had read 11.8V (before starting the engine) after leaving the car overnight - with the new battery that's now about 12.2V in the morning. With the engine running it reads from about 13V to 14.5V when driving along, which seems good for charging.

 

The "096 Enduroline AGM Stop Start Car Battery 12V 70Ah" from Tanya came at 12.85V by my multimeter. Yesterday evening I drove 20 miles to a meeting then 20 miles back home, so it should be well charged still. The multimeter read 12.85V this morning. When I connected the ODB2 and turned on the ignition the ODB read 12.3V and the multimeter 12.53V.  After turning the ignition off the ODB read 12.2V and the battery terminals read 12.8V.

 

Is it just getting reasonable voltage drops, or is there a fault? I did get ABS/EBrake light. Error 00668 - Supply Voltage Terminal 30   for a  while after changing the battery and recoding it, but it disappered after a few miles.

I should add that I hope all the warning lights are related to only the left hand front wheel rotation sensor. The sensor and hub magnet are being replaced next Thursday by my local garage. 

Your multimeter - and whatever the ECU is using to measure volts - are not calibrated devices. In addition, you have no idea at what point the ECU is looking to give out a voltage reading. It may even be a compound of four or five different sensors.  The main thing is you have a new battery and the charging system appears to be functioning. Cross fingers for the hub magnet / sensor change fixing your warning lights.

On 07/11/2024 at 10:31, dutyhog said:

I may ask another question on that, but right now is it normal for the OBD to read the battery voltage as lower than a multimeter across the battery teminals? It's typically about 0.3V lower, sometimes even more.

As inspectorman has put plus your Carista OBD2 is drawing power from the car to operate.  Multimeters will also vary in readings depending on how accurate they might be, cheap digital ones can vary a lot.

 

 

On 07/11/2024 at 10:31, dutyhog said:

The "096 Enduroline AGM Stop Start Car Battery 12V 70Ah" from Tanya came at 12.85V by my multimeter. Yesterday evening I drove 20 miles to a meeting then 20 miles back home, so it should be well charged still. The multimeter read 12.85V this morning. When I connected the ODB2 and turned on the ignition the ODB read 12.3V and the multimeter 12.53V.  After turning the ignition off the ODB read 12.2V and the battery terminals read 12.8V.

12.85v is good on new AGM battery, subject to how accurate your multimeter is, but I would still fully charge (any) the battery with an appropriate charger maintainer before fitting it to the car, at 12.85v it wouldn't take long.  The VW (over) complex battery management computer program will not also fully charge the battery and 20 mile runs mean little as you could be using more electric than goes to the battery depending on your journey(s), driving style and electric use.   

 

In case you don't already know these VW self-studies tells you about the batteries, battery and start/stop systems. -

 

SSP-234-Vehicle-batteries.pdf

 

SSP-426-Start-stop-system-2009.pdf

 

SSP-504_Vehicle_Batteries.pdf

 

Edited 8 November, 20241 yr by nta16
typos

Thanks Inspectorman and Nigel. I hadn't previously read all of  these VW self-studies, so it was helpful to go through them all.

I did charge the new battery overnight before installing to make sure it was full, and again on the car a few days later after spending time at home running it down a bit looking at the system.  Our usual journey is the 20 mile drive to the town,  It's mostly on a winding, hilly single track road and not above 4th gear and 30mph until we get to a main road for the last 2 miles. There are frequent stop/starts to let cars pass the other way, and to avoid sheep, deer etc. Maybe the charging schedule is similar to driving in a busy town?  (The particle filter stays clear. So the engine must run suitably fast/hot for long enough.)

My multimeter is fairly upmarket, and DC voltage is specified as within 0.5%. But I suppose it could have drifted - I haven't  calibrated it for several years. I used to work in a university electronics department , with good access to useful kit. (I once needed accurate voltage to challenge our home electricity supplier.) It does still agree well with another good meter that I keep on my boat.  I know how instrument that display voltage aren't necessarily particularly accurate - one on my boat reads  0.2V lower than the meter across it and another 0.25V higher.

I may seem like looking for too much in this, but it's my nature to wonder if something a bit unexpected is a clue to a problem. The MOT is on 6th December, and I don't want tot risk being without a car. There is a bus twice a week from a bus stop 9 miles from home - not much use to us.  So I'll wait until the sensor is replaced before doing anything else.

 

I've seen various figures as to how much the start/stop battery is charged to usually 70%, 80% to allow for topping up by the car's system.  If you was to plug-in/connect some sort of monitoring device you'll see how things vary in use of the vehicle or have a look in the Fabia Mk3 forum (and others) for where other members have posted charts and readings.

 

Obviously if you have any concerns then checking the car's charging system with your multimeter(s) would be the next logical step as with a Carista you are using one set of computers program(s) to check another that is the car's computer program(s).  I don't know the Carista but with scan tools generally you need to ensure the program in them is specific to your model and year and is fully up to date otherwise errors can occur.

 

I think you are correct to check things, 10 years for a start/stop battery isn't bad but it's demise might have been hastened so it's worth checking if that's the case or not.  I fitted a (non-start/stop) Enduroline battery to my car and it gave good service, still on the car now AFAIK.

 

Personally as my wife does many short journeys I very occasionally do preventative recharges fully recharging the the AGM battery using an appropriate battery charger maintainer following the instructions in the car's 'Owner's Manual' and charger maintainer's booklet.

 

Unless it's different in Scotland you can MoT anytime from now early and keep your original renewal date "up to a month (minus a day) before it runs out".

 

Good luck.