Any issues with refilling a partially full keg?

[neomantra]

I have a 5g batch of beer that's done fermenting in the carboy. Tomorrow we're hosting a picnic and I have a 2.5g keg that's empty. I thought it might be nice to keg it and add to the variety of stuff available for our guests. Obviously I can only put half the batch in to this smaller keg, so I have a few questions about that.

1) I really doubt it, but does anyone think that it would risk over exposure to oxygen if I only siphon out 2.5g and just leave the rest in the carboy for now? I don't really have time to bottle it at the moment.

2) Saturday we have another picnic to go to. If this keg isn't kicked and I wanted to top it off with some more beer from the carboy would it foam all over the place when I was siphoning the additional beer into the keg? I'm thinking not, but just figured I'd ask in case anyone's done this previously.

 

[johngaltsmotor]

The biggest problem will be carbonating. You'll definitely have to force carbonate with LOTS of agitation.
1) the problem will be that when you siphon off you would be best to replace that volume in your carboy with CO2 to avoid oxygen taking up that space.
2) the beer more than likely won't foam excessively when you add the flat beer. But back to the original comment, you'll then have to shake the tar out of it.
I'm not sure you'll have enough settling time for the CO2 to fully dissolve and not leave you with a tap that only shoots foam.

 

[wilserbrewer]

Even though you can force carb a keg in 5-10 minutes, it tastes a helluva lot better after cold conditiongg in the keg of 7-10 days. Sounds like it would be too much of a rush job for you to keg and drink tomorrow?