Stop/start not stopping/starting

[demand79]

Please help. I’m desperate! Had my 2017 S max from near enough brand new for 3 years. Stop/start worked fine until about 6 months ago when it stopped working. Decided to trickle charge my battery (which is a frigging nightmare to get to), and lo and behold, it worked fine again. Two days later thou, it stopped working again. So took it in to Ford. They called me a few hours later to say they’d sorted it by recharging the battery. I said that’s what I’ve already done, but they said the battery is showing fine on their computer so good to go. Took it home, stop/start worked fine for two days…then stopped working again. Thought ok, I’ve only done short journeys past two days, it’s fine. Anyway, day three I drive 1.5 hours straight to the Peak District, and stop start didn’t kick in once. Took it back to Ford again. Exactly the same response. Still I gave them the benefit of the doubt, and two days later, again it stopped working! 
Back to Ford. This time they said they removed the battery, tightened up some wires, charged it and its fine again. They said this is just how the stop start thing is, it doesn’t always work. I said that’s utter nonsense. I’ve had it three years and this is not how it’s always been. They said they could replace the battery for £300 but their computer is showing the battery is fine. I said will this solve it? They said it will probably do but not for long, the stop start will again only work for a few weeks with a new battery. (They haven’t charged me anything for the last three visits, I should add). 
Anyway, today is day two so I’ll see if it is still working. But I’m at my wits end. They’re adamant this is just how stop/start is! Isn’t this like saying the electric windows will work for a couple of years but after that you have to open them manually?? Why would they have stop start/electric windows in a car if it only works for the first two years??

I’m not that fussed about the stop start feature, it’s just the fact I know it’s not working correctly that’s bothering me. To me it sounds like the battery is not recharging or it’s being drained or something. But they disagree. I’m an anxious driver as it is, and having the thought at the back of my mind that something is not right and it might let me down on a long journey is making me even more anxious. 
 

please help. Thank you. 

[DaveG1975]

That’s exactly how my 2019 Focus is I’m the same I’ve even tried resetting bms works for a few days then stops if it’s fitted is should work I would like an answer to the problem as well 

[Paulius55]

I am afraid this is the way it is with the "smart" charging system on the recent Fords. It is all to do with the "State Of Charge" that the battery Monitoring System (BMS) detects. The BMS will never allow a full charging current to the battery, in an effort to reduce alternator drag, which increases emissions, and it will inhibit heavy current devices frequently. You will find that, in the autumn and winter, just when you need it, the heated front screen will not operate as the BMS decides that the state of charge is not enough. You can temporarily get round the problem by charging the battery independently and resetting the BMS and the screen and stop/start will work again for a while. 

[DaveG1975]

What percentage of charge does the battery need to be before stop start kicks in 

[Eric Bloodaxe]

2 hours ago, DaveG1975 said:

What percentage of charge does the battery need to be before stop start kicks in 

Don't know if anyone has a definitive answer to that, but I have seen about 75% suggested in one place, minimum 11.3 volts in another.

Every car forum (for ICE cars anyway) you look at has got threads on this topic (several active on this site alone at the moment). When you search around, it seems there are more reasons for it not to operate than to function correctly, eg:

https://batteryworld.varta-automotive.com/en-be/start-stop-does-not-work

I always disable it when I start the car anyway, but forgot the other day and was amazed to find it actually worked when I stopped at the first traffic light I came to!😀 (Though I had travelled about 50 miles by then so the engine was good and warm and the battery had been topped up, I guess.)

[AlCLE]

The start/stop system depends on many signals to enable it, including:

Coolant temp 

battery SOC

Air temp

oil temp

Aircon demand

Driver door switch

Current draw for other systems 

Etc, etc. 

 

If your battery is original then it could be that it won't get to the minimum required SOC anymore. The car is still perfectly fine, though.