I only do maybe 70 miles per week and it seems this is not sufficient to charge the battery enough for Stop/Start to work. The battery SOC is 50%, it used to be 60% or more so it seems the battery maybe slowly deteriorating.
Does this problem sound familiar to anyone ?
pcr: Forgive me if you know this already, but there's a lot going-on in SS decision making process.
For SS to actually stop the engine;
1. the car has to be stationary,
2. the engine revs must be below a threshold (I think it's around 1200rpm),
3. the coolant temperature has to be within a certain range (25 -100 degrees C, I think),
4. the brake vacuum has to be more than a threshold value
5. if air-conditioner is on, the difference between the cabin temp (SS actually measures the air vent temperature) and the target temperature is no greater than about 5degrees C (hmm... not sure about this number),
6. the diesel particulate filter is not in re-gen mode (not sure if yours is a TDI) ,and
7. the remaining energy in the battery is sufficient to enable SS to restart the car again.
So lot's of things that need to be correct for SS to work! In my car, none of these things are important, because I've disable SS entirely! My own personal view (which I acknowledge may not be shared by others - which is entirely OK) is the SS is the most dangerous thing in the car! But I digress!
In respect of your battery SOC @ 50%, the way that the mk7 checks if there is enough charge in the battery is via a small control module that sits in the negative battery lead (it's fitted in-line just next-to where the battery cable fits over the battery post). The battery sensor's job is to measure stuff like temperature, voltage and current which is needed to assess whether there is sufficient energy remaining in the battery to re-start the car. How the battery sensor has calculated the State of Charge of 50% is another consideration. The way that this control module normally does this is via a look-up-table in its memory and through record-keeping of past usage. But sometimes the information stored in the battery sensor is a lie!
I don't mean to imply that anyone has erred, but if the car has been jump-started, or charged incorrectly, the numbers on which this calculation is based can be messed-up because the battery sensor is bridged. The correct way to jump-start/charge the car is to first connect the positive lead and then to connect the negative lead
to the car body. The second bit is important because if the negative lead of the 2nd battery/charger is connected directly to the car's native battery, the battery sensor module will be by-passed. This means that the battery data is not recorded by the sensor during the charging process and the "battery condition" that is used in the SS decision making process no longer coincides with the actual battery condition.
The second important piece-of-kit from a battery charge perspective is the voltage stabiliser (which sits under the wheel arch). As its name implies, this device stabilises the power supply under heavy load conditions like the SS operation. Sometimes this unit is faulty and typical symptom is that the car's consumables (radio, SATNAV, telephone links) will reset. If any of these things are happening in your car, then this control module might be the offending item!
Anyhow, just thought that you might like to have an alternative view of the possible causes for your problem. My apology for the long winded explanation!
PS:Of course, you may well be correct that your current battery has gone pear-shaped and it needs to be replaced!
