Having experienced 4 failed batteries, yes the decharge is normal and is all part of the learn cycle. What Symantec never seem to warn about is the time it takes for this process to complete (while its running, for obvious reasons it keeps disabled the writeback policy and runs like crap). I've had it take 15 hours to complete before and its really impacted our backup window! Basically as soon as the policy shows as Writeback decent performance should soon return.
We too experienced poor performance when we first got our 5220's (although we only have a single disk tray so cannot comment on your issue there..). Our issues were caused by a number of factors. See my previous post which has come great suggestions from the guys on this forum.
SLP tuning - If you are running AIR or tape duplications this helps. I suspended all SLP processing for the 1st period of my window to get a bulk of the backups through and then start via the task scheduler. I also use the Lifecycle_parameters files in db\config to batch things up. This really helped and keeps the noise in the activity monitor to a minimum..
Limit I/O on disk pool. This was the biggest help for performance and I have tuned it to 35 streams.
Use query based client selection and use Vmware resource limits in the master properties. We limit to 2 concurrent backups per datastore. The query selection balances load accross the virtual environment and despite annoying features like being unable to rerun failed jobs from the monitor this has been worth doing in our environment.
External factors. We had a i/o issue on the LUN that had out VC database on it. When the backups were hitting the virtual enviroment all the VC logging etc was creating a lot of noise on the DB which created performance issues all round..
Of course you may just have issues with your appliance(s) but hope this helps. I went through a whole heap of pain but happy to say all is finally running well. Definitely a good idea to do some manual queue processing as mentioned above..
Just waiting for a battery to fail at 6pm on a Friday night now..
Cheers
Ed