I am obviously doing something wrong as I can't seem to find anything about alternators and batteries, anyway, yesterday travelled to visit relatives approx 30 miles away on mostly motorway, lights off, heater fan at 2 and, about 25 miles in, I get a low battery warning.

Borrowed multimeter and checked when we arrived. Ignition off battery voltage 12.32V, Engine running 14.9V, headlights and fans running 14.78V. Are these acceptable figures? and if so why do I get battery low warning especially at high speed on the motorway?

Hello Bill, looking at that 12.32V battery at rest figure it's low especially after such a long journey. Also the charging voltage is towards the top end at 14.9V which would indicate the battery is not holding it's charge and the alternator is holding the voltage at a higher level in order to get the battery to draw a charging current from it.

In short, you either have poor battery connections, or more likely the battery is very near the end of it's life.

Thank you for your prompt reply. What should a fully charged new battery read at rest?

Depends on the actual type of battery but typically 12.6 to 12.7 Volts after it's been standing for a couple of hours.

At 12.32V your battery is only about 60% charged

 

I'm about to have a new battery fitted tomorrow by Halfords. Will they make sure that the ECU is informed of the new battery and what happens if they don't?

15 minutes ago, Grampabill said:

Will they make sure that the ECU is informed of the new battery

I can absolutely guarantee with out any hesitation they won't.

This does not apply to your Ford Focus and to almost every Ford vehicle. 

Sorry didn't quite understand your reply. Did you mean that there is no need to inform ECU?

Ford do not use coded batteries. The only thing on your car that you could reset is the battery monitor sensor (BMS) but it is generally not required as within one day the system will correct itself.

Once again thank you for your prompt reply and clear information.

New battery fitted today ay Halfords by a nice man who new exactly what he was doing. Also plugged in a thingy to the cars brain and fiddled for about 2 mins then stood back and wished me well. Apart from resetting the windows no problems. Excellent service!