The capacitors need to be reformed before they're powered up for any period of time.
I just responded to the same question on photo.net with the following recommendation:
I suggest the following, which will take a week altogether:
At some convenient time of day (noon perhaps), power the pack up for ONE minute, then turn it off.
The next day at the same time turn it on for TWO minutes, then off.
The next day four minutes, next day eight, then 16, 32, and an hour.
If the pack holds without failure, it should work reliably.
If it fails it will be quite obvious, usually with a loud bang, possibly accompanied by smoke.