Battery power back-uped circuit

[flotulopex]

Hi All,

I'm now (finally...) building my project and its about time to order the missing components to have something to play with (without any reference to naughty thoughts ).

So, here's the circuit I'm building:


Reading the (very interesting) MicroChip's AN1149 about battery charge and power backup, I can see that a particular attention is to be paid to the choice of the Q1's (the P-Mosfet) where a "proper" Vgs(th) has to be chosen.

Unfortunately, I'm not an electronician, just an electrician creating himself (lots of) problems with this PIC stuff = I don't know how to calculate or determine with exactitude how to dimension the needed P-chanel Mosfet

Can anyone help me out and suggest a component (according to the DS, a combined S-diode and Mosfet is nice) or the Vgs(th) value I will have to find to make my circuit work? Instead, I would appreciate someone explain me how to "calculate" this Vgs(th) I'll need....