I have a 2017 Fiesta 1.0l ecoboost, with 16400 miles. It is used regularly, mainly short trips, with the occasional 100mile round trip. It is garaged at night. Recently I noticed the auto stop/start was not working. This didn't bother me as I usually turn it off when I get in the car. But I have read that this can happen when the battery is <85% charged. (not confirmed). Shortly after, I noticed, on one occasion, that the engine only just turned over but started straight away. I just put this down to a hiccup. The car seemed to start fine after that for a week or two, then although the engine was warm, it failed to turn over. I charged the battery and drove to my local Ford dealer the next day. The engine would not turn over for the return home. I booked the car in for a diagnostic test, and in the mean time, bought a new battery (though not a EFB type). The new battery was fitted by the dealer and diagnostic tests passed. However then the dealer noticed that the output voltage was varying from the alternator and said that a new alternator was required. After fitting the new alternator the output voltage was still seen to be varying. They contacted Ford Technical department who advised that this is normal for a Smart Charge system. This left me thinking "was a new alternator required after all ?" which cost me £440 on top of the £95 diagnostic test ! The dealer didn't seem to be too clued up on the Smart Charge system.

So beware that these modern batteries can suddenly fail (not like they used to be when then slowly ran down) and although I have read that the battery needs to be an EFB type, this does not seem to be so. I have also read that it is not advisable to jump start a Smart Charge system from another car because if the battery is very low as the alternator can put out 18v, this can fry some modules on either car! Best to use a booster pack. This is not mentioned in the user manual. Also not mentioned is that the Battery Monitor Sensor needs to be reset after fitting a new battery, and there is no mention of the auto stop/start not working if <85% charged.

If you haven't fitted a EFB or AGM Battery to a vehicle with "stop/start" and you make use of the stop/start function you can expect a traditional battery to fail pretty quickly.

I know that if you move from EFB to AGM to need to inform the car via diagnostics that a different type of battery has been fitted so the charge profile is adjusted. This is probably true if you have moved from an EFB battery to conventional lead acid to ensure that the battery isn't "fried".

1 hour ago, BDP said:

. I have also read that it is not advisable to jump start a Smart Charge system from another car because if the battery is very low as the alternator can put out 18v,

That is kind of true, but rare. The safest way is to jump start off the other vehicles battery but with the engine not running. If after 4 tries to start the dead vehicle stop before you kill the other battery.