Help! New guy here. Having issues sidelining my beloved Volvo. Need advice, please...
 

'05 S60
My beloved Volvo began running funny awhile back. I cannot currently bring her to the dealer and drop thousands like I used to. 1) She stalls during startup but mostly smooths out after warming up. 2) Decrease in hp & acceleration. 3) Only one headlight will light up and is so dim you can hardly notice. (Hi beams work fine). She's been an excellent car with virtually no trouble over the years. I parked her after these issues came, virtually all at once. Noticed the battery died. With a jump start, she seems to be running worse, and a jump is needed seemingly every time now.
Clearly my battery needs replacing but I'm in no hurry to do so, with the other issues awaiting me.
UNLESS the battery could have been the cause of some of my issues.
What's the likelihood of that? I know these sweet rides can be temperamental in such ways. But maybe I'm reaching here by hoping they're related.
Thoughts???
-Thanks in advance!

I don't think the battery could cause that, but....
It's easy to find out. Borrow another battery from another vehicle, swap them out and attempt to start the car. If it works fine you have found the problem (you spousal unit and friends will think you are an automotive god!) , if not (most likely) you are going to need to get a scan tool and see the list of codes that come up. Post the list here and see if someone can ID the root cause.

As a side note, I try never, ever to jump modern cars, the electronics are too sensitive. I have personally cooked my wife's MDX jumping an old Tahoe of mine (just shy of a grand to fix it) and a neighbor jumped a 3 Series at a mall with his S60 and replaced his alternator the next week.
I would suggest you get a battery charger- I purchased one for around 100 bucks that work fine for when my cars die. The 'instant start" feature is not really very good for a totally dead battery and it needs a little time before you can crank it (hence my impatience with the Tahoe- see "Haste Makes Waste") . In a pinch, hook the cables up, start the good car and go in and have a beer. After a while the dead battery will hopefully have some charge and your car won't go berserk trying to charge it and the good car.