Battery not charging (but battery light not on)

[mcfee03]

Hi,

I did my first mechanical task of removing my intake, replacing my ccv and disa.

• The car was not started/driven for 3 months.
• negative cable disconnected from the battery (in the boot)
• positive cable disconnected in the engine area.
• When start the car (has to be on jumper leads) volt meter shows it at 12.x.
• I bought a new battery, and it also would not charge (IE not charging?)
• When car starts (with new battery)- voltmeter reads 14.xx, then quickly drops down to 12.xx

In taking off my intake- is there something that could be disconnected to cause this?

Is there a way to test if the alternator is the definite cause? I can remove the alternator easily.

Could it be the alternator regulator or some other part related to the alternator?