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Discussion Starter #1
Pre lockdown I was having issues with my VW Passat (67 plate TDI 150 GT) stalling at low speeds. Since lockdown battery seems be discharging although used for two 30 minute journeys once a week. I had to call out breakdown service as after driving 30 mins to go food shopping the car would not start and the battery was flat! After charging with battery pack I made it home. This was the only time I saw the battery warning light.

Since then I have been charging the battery to top it up on a weekly basis but quite often the engine will take 3-4 seconds to start. This weekend the car was driven for 1 hour and 30 minutes and now seems much happier starting.

Just been out to check the battery with a multimeter which shows 12.1 volts with engine off and 14.5 volts with it on. Would I be right to conclude that the battery voltage is low and that the alternator is fine?

The battery is a Moll EFB and from doing a bit of research there was a dodgy batch of these in 2017 across the VAG group.

The car is going for service on monday so I will ask them to look at it.

Has anyone had any similar battery issues in any of their other VW's and does anyone have any suggestions? Photo of battery.
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Hi @Percy247,
If you can hear a high-pitched squealing or squeaking from near the front of the engine (I'm assuming the Passat's alternator is in the same location as the Up!), the alternator may be starting to die, but if you're getting 14.5 volts when the engine is running, then it's doing its job on some level. It is possible you've got a problem in the wiring somewhere which has created a new drain on the battery (are you sure all accessory switches are off when you leave the car?), but a failing battery seems more likely. Do you still get ~14V as soon as the engine is turned off, or does it drop back to ~12V immediately? It's worth noting that some start/stop batteries (I believe just the AGM ones) are especially picky about what voltage is applied to them when charging; if you haven't already, I'd check whether your charger has a specific setting for EFB or start/stop batteries.
A battery can provide the right voltage without providing enough current (juice) to start the engine if the cells inside are damaged. That will happen slowly over time in any battery due to the charge/discharge cycle, which is why start/stop engines have special batteries designed to cope with many more of these cycles than a standard engine. Even amongst our family's VWs, the Up!s seem to be particularly sensitive to the amps a battery is able to provide - they'll be fine-fine-fine-dead - and many batteries are failing faster now because they aren't getting regular top-ups to compensate for the slight loss of charge between drives in 'worn' batteries. Although the start/stop batteries are bigger to cope with the bigger starting load and constant recharging, that bigger load means the failure-to-start threshold is also higher. You may well have been unlucky and got a dud one from the batch you mentioned, but it's also possible it was on the way out through use anyway and lockdown has just tipped it over the edge (there have been several passionate debates on here about how long a battery should last, so without wishing to set that off again I will simply point out that most replacement batteries come with 3 year warranties).
Either way, it's still a good idea you're taking it in - stalling at low speeds is dangerous, especially if you can't restart reliably. It's possible there's a fault in your start/stop system as well, which could damage the battery as well as affect its ability to start. Simply changing out the battery wouldn't fix that problem.
 
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Discussion Starter #3
Thanks for detailed reply as ever @plancast. No squealing from the alternator to date. Fairly sure that all accesories are turned off. I might have not locked the car once or twice overnight but I have been double checking since trying to get to the bottom of this problem. I will check the battery voltage again.

I agree the problem lies somewhere between the battery, alternator, wiring and the stop start system with the battery being the most likely culprit. I have checked my ctek smart charger and it charges EFM batteries in standard mode and there is a separate mode for AGM ones.

I agree I have been lucky with stalling at low speeds. Has been a very intermittent problem and maybe happened only 6 times both with stop/start on and deactivated. On all occasions it was entering roundabouts or going through junctions slowly in second gear. Each time I had to press the ignition button to start the engine.

The car is in still in warranty till november so I think I will leave it with VW for as long as they need to hopefully get to the bottom of it. Then next round I will fit a decent AGM battery instead.
 

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Thanks for the update. Intermittent problems are the most infuriating and hardest to diagnose. Keep being firm with VW about getting things resolved whilst it's under warranty. Mine still has a transient fault that wasn't resolved before the warranty expired (just before lockdown), but as it only seems to happen on long drives it's less of an issue at the moment! Owners with start/stop Up!s are pretty active here, so anything that might be useful to them when you do resolve the issue would hopefully be welcome.
 
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