This is a reverse polarity circuit as taken from Afrotechmods channel on Youtube.


The 12V battery is charged via a generator charging coil and has a peak voltage of about 45V. I know there should be a regulator in there but on this particular one it doesn't.
The coil sends a pulse rather than constant voltage as it is just a magnet sweeping a stationary coil on every revolution. (every 20milliseconds)
The Load is a microcontroller and associated peripherals all fed from a buck converter, MP1584 rated for 28v input which seems to tolerate the 45v from the charge coil.

Now the battery fully charged is 14.5V so I want a zener above that but is it wise to approach the VGS or reserve some wiggle room? Or is it better to use a zener voltage just above battery?

 

Zener is to protect the PMOS from not exceeding the GS voltage limit you find from datasheet, but allow it to be enough to PMOS fully conduct. Resistor is to allow G to go low and Zener to conduct. Zener doesn't do much of anything for the battery voltage, it just consumes little current.
Example pic where you need to limit GS voltage because else it would be 30V = MOSFET killer.
Low voltage applications where it is sure that GS limit never exceeds, it is unnecessary.

 

I understand the GS voltage can not exceed the limit, but I was asking the choice of zener voltage, do I keep it as low as possible (just over the battery voltage to avoid wasting power when in sleep mode.). Or is it better to keep it just under Vgs? As I understand it a zener doide will actually leak current as it approaches it's zener voltage? is this true or does it actually matter?

 

I understand the GS voltage can not exceed the limit, but I was asking the choice of zener voltage, do I keep it as low as possible (just over the battery voltage to avoid wasting power when in sleep mode.). Or is it better to keep it just under Vgs? As I understand it a zener doide will actually leak current as it approaches it's zener voltage? is this true or does it actually matter?

As i see, your system voltage never reaches the voltage that is a dangerous to PMOS GS(if 20V) which makes the Zener useless.
If Zener is needed, it should always be lower voltage than GS (with some margin).
The current consumption is up to you and depends the value of R only if zener is in use and voltage above its voltage.