If four-player Wii Tennis is costing you loads in battery money, check out this Thanko USB cradle and Wiimote charger. The cradle plugs into the back of the Wii in the USB port and charges one Wiimote at a time. The batteries charge in 3.5 hours and give you 25 hours of gametime, which should be fine as long as you remember to place your Wiimote on the stand every few days.
It's a little more expensive at $53 for 2 and $30 for one than the Wii Powerstation, but blends in a little better with your Wii thanks to its all-white finish. Not a big deal unless you're anal about your Wii.