There's a refresh button on the charger. It's pretty self explanatory. You hook the battery to the charger and press the button. It drains the battery completely and then recharges it from scratch to hopefully cure the "memory effect" that can plaque NiMH batteries. It's worth a go.
If that doesn't work, I recommend generic batteries from mydigitaldiscount.com. It's a somewhat hit-or-miss situation on getting a really great one, but they're dirt cheap compared to OEMs. The one I have left in service holds a really nice charge - I've used it for all-day weddings on a 1D II and the first bar hasn't dropped after a thousand shots or so and quite a bit of reviewing and deleting.