Start/Stop

[Guy]

I'm pretty sure that start/stop systems from all manufacturers disable the system if the battery charge gets too low. Both my parents cars have stop/start, and they've never had any trouble with them (BMW and VW/Skoda systems)

What I'd like to know is if this disablement occurs before the charge gets so low that the battery has been damaged. If not, then it's a useful way of being reminded that the battery could do with a top-up charge.

With the older non AGM batteries, just letting the charge drop below about 50% can result in some sulphation and loss of capacity. AGM technology (as used in start/stop) is supposed to be better but even so can have its life shortened by being allowed to discharge to too low a voltage.

[vc-10]

Cars with stop/start also have regenerative braking. You shouldn't have to worry, the regen keeps the battery topped up*. However, the batteries in cars with stop/start don't tend to last as long as older ones apparently, due to the rapid cycling of charge/discharge, but not enough to worry about IMO.

*Assuming you don't do 'American' driving. BMW have had issues with the 7-series in the US, because the battery runs low on long journeys as they are all 'highway' miles and it never gets a chance to regen, running really low, which then affects the battery life. In the UK, and driving a Polo, I don't think you'll have that issue! This is a quirk of the different way that official fuel economy is calculated here and in the US- our 'extra-urban' cycle still includes enough deceleration in it to charge it back up. Therefore, there's no penalty for having the alternator kick in, as it would never do so during the test. In the US however, their 'highway' measurement doesn't have enough deceleration, particularly to charge all the gizmos in a 7-series, and having the alternator kick in would negatively effect the official fuel economy. So instead the battery becomes a 'consumable', like the oil and pollen filters

[Dink]

the battery would never be left to discharge enough to damage it as that would probably struggle to start the car.

i didn't know that they actually put a clutch on the alternator?

[Guy]

Cars with stop/start also have regenerative braking. You shouldn't have to worry, the regen keeps the battery topped up*. However, the batteries in cars with stop/start don't tend to last as long as older ones apparently, due to the rapid cycling of charge/discharge, but not enough to worry about IMO.

*Assuming you don't do 'American' driving. BMW have had issues with the 7-series in the US, because the battery runs low on long journeys as they are all 'highway' miles and it never gets a chance to regen, running really low, which then affects the battery life. In the UK, and driving a Polo, I don't think you'll have that issue! This is a quirk of the different way that official fuel economy is calculated here and in the US- our 'extra-urban' cycle still includes enough deceleration in it to charge it back up. Therefore, there's no penalty for having the alternator kick in, as it would never do so during the test. In the US however, their 'highway' measurement doesn't have enough deceleration, particularly to charge all the gizmos in a 7-series, and having the alternator kick in would negatively effect the official fuel economy. So instead the battery becomes a 'consumable', like the oil and pollen filters

That's a very interesting reply - thanks. For the first US I've read UK. I keep an eye on my digital voltmeter and I've noticed that when the alternator is providing a charge in normal running the voltage rarely exceeds 14.7V (and that only for short periods). Whereas on deceleration it frequently is 14.9V. My instinct is that disabling stop/start will be kinder to the battery and as I don't like stop/start in critical traffic situations anyway that's no great hardship. Batteries have always been "quasi-consumables" to some extent but it pays to look after them in order to get a longer life. And Stop/Start AGM batteries are significantly more expensive (double the cost?) of wet lead acid batteries, plus a replacement needs your friendly VW dealer to code them into the car's computer which is a further expense.

I have a friend with a large BMW and his battery was playing up. He couldn't find where it was located so anyway took it to his BMW dealer who diagnosed a replacement battery was required. On collecting his vehicle (and paying a not inconsiderable sum) he asked to be shown where the battery was located for future reference. The reply was polite, apologetic, but firm. "I'm sorry sir but I am not permitted to disclose that information because the battery is not a user serviceable item".

[Guy]

the battery would never be left to discharge enough to damage it as that would probably struggle to start the car.

i didn't know that they actually put a clutch on the alternator?

Does it definitely have a clutch on the alternator? I'd have thought the ECU could control the alternator directly and "tell" it to produce maximum voltage/current on overrun and, depending on battery charge level, less or none on acceleration. But there would be no need to stop the alternator spinning as it's presumably not that much of a friction drag when it's not generating electricity.

[Dink]

99% of alternators are self regulating, the field winding is controlled internally by a regulator.

at full load the alternator could output best part of 2hp but that would never happen.

i'd be more worried about the voltage getting to 15v than it going low

[Guy]

I've done a bit of Googling on AGM batteries. If you are charging one using an intelligent charger it seems that for the initial charging (bulk) phase when a constant current has been maintained while the voltage is raised to a maximum and then held as the current is allowed to slowly decrease (absorption phase), then a maximum of 2.5V per cell - ie 15.0V for a 12V battery - is acceptable. However it is temperature sensitive and higher temperatures require slightly lower voltages.

I'd originally thought that the location in the boot under the spare wheel for the Polo GTI's battery was inconvenient. But it could be advantageous in that the battery will likely stay cooler there than if it were under the bonnet.

[h5djr]

I have an 2014 Audi A3 DSG and I have set the Start/Stop to be always off as I find it does not work that well with the DSG. My wife has just purchased a new 1.2 TSI Polo and she would like me to do the same for her Polo. It needs VCDS to do this and I don't think the cable I have is suitable. It's OK for the Golf and A3 but not for the Polo. Does anyone know which cable is required?

[RUM4MO]

Do you really want/need to switch it off on your wife's new Polo? Remember if you are observed in stationary traffic with the engine running for longer than ?? minutes and have a Stop/Start function in your car, you are going to get fined - as anyone without Stop/Start will also get fined, but the car should look after the situation most of the time if it has Stop/Start.

Stop/Start will improve your MPG as well as reduce emissions - my wife's new Polo obviously has Stop/Start and I wondered if/how we would get on with it - but in reality if should be operating seamlessly. The comments about it taking too long to restart the engine seems weird, with a properly working car, as soon as the clutch is depressed, well before the pedal is down to the floor on a man trans car, the engine will be running in a stable manner, so there should not be a problem.

As for cars with Stop/Start having AGM batteries, not true for small petrol engine cars, the battery technology used there is EFB, which is better than standard batteries but not as "durable" as AGM in terms of how many "start cycles" they will tolerate in a life time. These EFB can handle the higher charging voltage that will be handed out by the alternator on the over run, I too wondered about that after monitoring the charging voltage during differing conditions.

Anything that is considered desirable will have been thought of within VCDS, the only current issue concerns the BCM which is a 6R0 version and not yet figured out by Rosstech.

[h5djr]

As far as I'm aware there is no legal requirement for a Start/Stop system. In my case if I can see I'm going to be stopped for more than a few minutes I turn off the engine myself in the same way as I've done for the last 40 years. Personally I found too many instances where the Start/Stop cut the engine just at the wrong moment and would therefore much prefer to be the one in charge. My wife had the Start/Stop working for the past four years on her Audi A1 with a manual gearbox but it does not seem to be as good with the DSG, hence the wish to switch it to always off as my A3.

It's very simply to do by just changing one of the parameters using VCDS but I'm sure I need a different cable to the one I use for my A3.

[alexperkins]

Same cable as your A3 - VCDS is universal across the VW/Audi/Seat/Skoda range.

The start/stop on the DSG is a bit random. Friend of mine has it on his Golf Estate and I cant say i like the way it works either.

[RUM4MO]

Same cable as your A3 - VCDS is universal across the VW/Audi/Seat/Skoda range.

The start/stop on the DSG is a bit random. Friend of mine has it on his Golf Estate and I cant say i like the way it works either.

There is an outside chance that h5djr has the VCDS CAN (only) version, as opposed to the HEX - CAN - ??? version, I saw that on a posting maybe in another VAG forum recently and the "CAN (only)" version was cheaper and very restrictive wrt models it could be used on. So, maybe h5djr needs to see if he can trade that version in against the "all singing and dancing" version, I did that years ago with my VAG-COM dongle.

Edit:- it was this forum and this section, the thread with " yet another VCDS question" in the title.

[h5djr]

The cable I have is the CAN only version which suits the A3 and Golf but not the Polo.

[alexperkins]

Ah I always forget the can only cable.

Ross tech will do a trade up to the hex can version

[KiwiME]

I disabled mine in VCDS only because when I get out of my car to open or close the garage door it shuts off and requires a key restart. It should be smart enough to stay "on" in park (DSG.)

The system predicts the voltage required for the next restart and that can be adjusted (in VCDS) upwards enough so that it never shuts off.

Seeing as the DSG effectively "holds the clutch down" at intersections and is essentially the same technology as the manual clutch, I don't see any issue doing the same with the latter to avoid shutoff in those moments when you don't need the distraction of a stopped engine.