Like Andrew said, you can fit 4 BBs in the tray if you stand them on end. They're a pain to wire up that way, but that's the only problem. You can get the replacement converter section for the power center, and it will shorten your charge time plus probably (?) give you a more ideal charge curve, but before that module was available, BB used to say that it was no problem using the standard PD charger.
Think hard before investing in them. For those who need them, they're awesome; but in most cases for most owners, I'd argue that your money is more wisely spent elsewhere.