Random no-start/no-crank

[RickHaleParker]

You could lug the battery out of the car and trickle charge it inside...

I know, not palatable for this low-med. level of annoyance, but there it is either way.

He could install a solar powered battery maintainer. A small panel that fits on the rear deck and a PWM ( Pluse Wave Modulated ) Charge Controller would work best. PWM charge controllers where invented to protect batteries during charging.

Think of installing a flexible solar panel on top of my XC90 and a PWM CC in the battery area. Run the cable down the channel for the Satellite radio antenna cable, which I don't have and don't want.

[erikv11]

Ignition switch is the same for all 850s, that's what you want.

No V70 ignition switch will work in your 850. The key chip argument is irrelevant here, not sure why that is being discussed.

[foggydogg]

I would open the glove box and watch the light in there as I'm turning the start key. If the light dims way down in response to turning the key to start, you need a new battery. If the light stays bright while you turn the key to start (and no starter motor action), you need a new ignition switch.

that is one heck of a sharp answer and as soon as the sun goes down tonight, I know what I will be doing -

Thanks a million, I love "old school" diagnostic like this

Corrosion on the signal wire on the solenoid won't show a diagnostic power draw, because since the solenoid doesn't make, nothing happens downstream of the ignition switch. I went down the immobiliser ring rabbit hole before I figured it out.
(You don't have one of those, though, so one less hoop for you.)