I had my stop start stop activating about 2 weeks ago.  Not really a problem it's irritating, but I was concerned why.

This morning I drove maybe 10 minutes from home most of it was stationary parked up with engine running idle and fans on and lights.  Then I stopped the car dropped something off got back in and drove 5-6 minutes with full lights, and the heater fan on full.  It was also one of the first mornings it was cold.

I popped into a shop for about 40 mins and when I left the shop the car turned over but didn't start.  It does this about once every couple of months, tried again it turned over maybe once, then it clicked. I realised the battery was almost completely discharged.  

I got it jumped and drove 10 minutes home.  Left it a few hours and tried starting it, it started fine but struggled when I restarted it and barely caught on the third go.

The battery is now discharged again.   I have ordered one from a local motorfactors, and of course they are not in stock.   

The car is almost three years old with 42k on it.   Is it likely to be anything other than the battery, I don't get any warning lights the battery is discharging / not being charged.

I was curious how long these batteries last.

4 hours ago, genty1980 said:

The car is almost three years old with 42k on it.   Is it likely to be anything other than the battery, I don't get any warning lights the battery is discharging / not being charged.

The battery should last a lot more than 3 years.

The Ford Stop-Start system is very sensitive to battery condition, and may not Stop once the battery ages a bit, but it should still start ok, and not go flat.

Have you checked, by lights flashing off or going dim, or by measuring the battery terminal voltage with a meter, that it is not a faulty starter motor or a wiring connection?

The Ford smart charge system is complex on these Mk3 cars, the BMS (battery monitoring sensor, mounted on the -ve terminal of the battery) has been known to fail. If it gives the wrong signals, the car could fail to charge the battery.

I suggest double checking that the battery is duff. Then, either with the existing battery fully charged, or a new battery, monitor the battery voltage. This may be possible via the dash display, or get a cheap cigar lighter volt meter.

But in either case, compare the battery voltage measured at the terminals with a meter, under varying loads & conditions, against the in-car display before trusting it. There are a lot of wires and connections between the battery and the in car display. Even if all connections are good there will be differences due to wiring voltage drops.

Check that the car can put out well over 14v (as at the battery), ideally 14.4v to 14.8v, at times. All car batteries need this voltage to charge properly, though this system will cut back the voltage much of the time into the 13v to 14v range, if it thinks the battery has sufficient charge. I believe it aims for 80% of full charge, rather than 100%.

After standing overnight, battery voltage (no loads on) should be over 12.2v, ideally 12.4 to 12.6v.

 

My 2016  Focus Ecobost battery has just failed after 3 years 5 months. It has stop start system that I do not use.Car is regularly used with few short journeys and has covered 34250 miles from new and regularly serviced. AA who helped me to Ford main dealer said he would expect modern day battery to last 6 to 8 years. What length of battery life for Focus Ecobost cars are owners actually getting?

i cant really say for stop start. on my 2007 fiesta 1.6tdci the battery lasted just over 10 years 

My 2012 Focus Ecoboost is still on its original battery. The stop start hasn't worked for more than a year but the battery still starts the car so I'm not buying a new one.

With the Varta make, the best quality for Ford non-start/stop batteries was the CCA, which I have on my Ford Puma (Varta Silver Dynamic), and comes with a 5 year warranty. When I checked the original battery on my ST250 (2016), it was an EFB (enhanced flooded battery). As far as I can see, the best replacements of any make, carry a maximum 4 year warranty. So it seems that manufacturers acknowledge a shorter lifespan for start/stop.

On the subject of start/stop batteries, as previous, my Focus ST250 original battery was an EFB, listed for Entry-Level start/stop systems. My wife's Citroen C4 Cactus (2019) only has a 1.2 ltr, 3 cylinder petrol engine, but is fitted as standard with the more advanced AGM (absorbent glass mat). This I found strange, because the ST is fitted with heated front and rear screens, heated seats and a heated steering wheel, which can require heavy electrical loads. My ST is not driven regularly, but the start/stop system has worked most of the time. Although the original battery was functioning fine, I had the chance to purchase a YUASA AGM battery, for a very good price. So the original Ford EFB battery 60Ah 590A is now replaced with a YUASA AGM 70Ah 760A.

My 1.0 ECOboost requires a new EFB battery every 2 Years. This is mainly because of relative short trips and a lot of power consuming options.

The advanced battery charging system with Battery monitoring System of the Focus MK3/MK3.5 is specifically developed for an EFB battery. The battery charging and monitoring characteristics of this system are not suitible for an AGM battery. Installing an AGM battery in a Focus MK3/MK3.5 with Battery Monitoring System will result in a drastically reduced life expectancy of the battery. 

2 hours ago, JW1982 said:

The advanced battery charging system with Battery monitoring System of the Focus MK3/MK3.5 is specifically developed for an EFB battery. The battery charging and monitoring characteristics of this system are not suitible for an AGM battery.

That is bad news if the MK4 is the same. I was hoping that by the time my car needs a new battery that FORScan would have an option to reprogram the BMS for different battery types/sizes.

I know it can be done via As Built Data for some cars, so presumably they had an AGM option when they were built. 

I have a 2019 focus with 4200 miles on it now it need a new battery it’s never been right from when I can the car back in March this year so is going to have to go back to Ford 

My 2017 Focus ecoboost 150 battery died at 3,1/2 years & 10k miles. It often set off the alarm when I opened the door 'cos it was low charge when it was only 2 year old, thought it just needed a charge up but dropped to 12.2v a day after charge. With the lockdown & little use it dropped to 12.1v but never failed to start. I got a replacement before it let me down.

Most battery sellers specify a battery that doesn't fit. The oem is 175mm x 175mm x 278mm long. nothing else fits yet they come up with some at 315mm long or 190mm high.

The oem Varta was a pita to get out 'cos the handles don't come back, trying to grip it on the sides is difficult, I replaced it with a Yuasa, the handles on that open back so easy fitment. The Yuasa on my bike is 7 yr old & still good, hoping for similar.