Hi!
If could anyone explain me this (unusual in my opinion) situation with AGM battery (that small accesory batt) on 2015 XF 2.0 petrol. Lately I've noticed that ECO start/stop doesn't pop in. Maybe due to weather, maybe due to bad battery. Anyway, I've checked the voltage of AGM "small battery" while it was connected to he terminals and it schowed ca. 11V on multimeter. So I thought it was dead, it wasn't relpaced since 2015. I've disconnected it form terminals and put it out the car, and then I've checked the voltage again and it showed... 12.6 V. Connected it back to terminals, and it shows 11V again. Anyone knows what is it all about? Is it normal? Does somehow car electronics affect voltage measurement?

 

The small battery isn’t AGM iirc. It’s only function is to power the auto start switch / solenoid on and off. The big battery is AGM and does all the heavy lifting. Try charging the small battery, or run a charge across both. You should read 12V plus on both.

 

When charging make sure to connect the neg lead to the car body, not to the battery's neg terminal. Otherwise the battery module will get confused.

What you describe sounds like a dead battery though. 11V whilst under load (in the car) and 12.6V when free points towards a high internal resistor. That's what happens when ageing.
You can further test by adding a small load (i.e. a 55W 12V light bulb) over the battery whilst disconnected from the car. The voltage of a good battery should go down only a bit (i.e. from 12.6 to 12.4V). A bad battery will drop significantly, typically under 12V.
You also want to test if you get charge voltage (above 13.3V) when the engine is running.

Chris

 

The standard small stop start battery is an Exide EK151 this OEM battery is AGM. I recently replaced one. It had an SOH of 40% SOC of 60% and a voltage of 11 volts. The new battery after charging had an SOH of 100% a SOC of 100% and a voltage of 13.2 (2.2 volt per cell average) All readings the same in and out of the car. It is possible your stop start battery requires replacement. I agree with Chris. If after replacement you still have problems the battery control module is the next suspect. The car message centre will let you know if your main battery is failing. It does not indicate issues with the stop start battery.