P38A 12.5 volts and not enough to start engine.

Hi, If I leave a trickle charge on all night, the car starts first turn of the key. If I leave it for a couple of day's it will not turn over as though the battery is flat, but checking the battery which is fairly new, it is showing 12.5 volts. Any Ideas ? I can't grumble it's been a while since anything went wrong.

How new is "fairly new" and what brand ?

What's the voltage with engine running & things like lights on and air-con fans running ?

Petrol or diesel? Measure the voltage while you turn the key to pos. 3 and see how low it drops. Ideally should not drop lower than 11v, 10.5 as an absolute minimum

It's nothing to do with voltage, it's current, although 12.5V would indicate that the battery has a reasonable charge it may not be proving enough current to the starter motor.

Is the car parked near a strong transmitter or wifi ,This can stop the car going to sleep and after a few days wont start ,,,could try disconnecting the blue wire on the receiver in the boot ..