in the mondeo (mk3 2.0 petrol auto)

the battery in the car is showing 12.3v with everything off

not enough to start

give it a jump and it starts on the button, then it's showing 14.19v and with a little bit of throttle it's showing 14.26v

the fiesta i jumped it with is showing 14.46v when running

is this a small enough issue to make you think it's not the battery?

the replacement unused battery is showing 12.56v sitting on the shelf