Should I store ammunition sealed in battlepacks, or should I open the pack and store it in a can with dessicant?

Your choice...  I do both.  I have the battlepacks stored, then 500 or 1000 rounds loosely packed in ammo cans ready to be used.

Stuff such as the South African 5.56 ammo that comes sealed in 300 round battlepacks does not need to be opened - just stash it away in a good quality ammo tin (you don't need dessicant under these circumstances). If the battlepack is already opened, I just take what's not used and put it inside a suitably sized zip-lok bag and stash it away until my next range session happens.

(corrected for spelling)

I leave the portuguese 7.62 ammo that comes in battle packs sealed until I am ready to shoot it.  Since its air tight already and packaged for to sit on a shelf for 50 or so years I feel I should leave it as is.  Now for loose boxes of xm193 or q313a i definetly throw them on stripper clips into bandoliers inside an ammo can with a dessicant or two.  I see no need to waste ammocans by breaking down battle packs and filling them with the ammo unless you have no other "loose" ammo to shoot.

Exactly what he said.

Thanks all, it seems the concensus is to leave the battlepacks sealed until ready to use.

The only problem I could see is that sometimes battlepacks will have small holes in the pvc due to the process of shipping and/or handling.  I had this problem on some SA where the packs were shippped in a cardboard box.  When the box was delivered to my door, it looked like someone had taken a bat to it.  Anyway, some of the packs had been compromised as a result of UPS beating the sh*t out of my package.

As far as SA battlepacks go, if you are worried about moisture, 7 of them will fit perfectly into a ammo can for linked 40mm grenades (and I mean perfectly, you would swear the can was designed for this purpose).  The can looks like a smaller version of the big 20mm ammo cans, with two latches and diagonal ribs on the side of the can for reinforcement.  The cans are hard to find though, wish I had bought more of them when I had the chance.