Vronko says the battery-replacement demand curve starts with a 10-month-old iPhone. "That's when we get the first run of customers," he says. These customers are often heavy iPhone users who may have lost up to 30 percent of the original battery life--and 70 percent of the remaining battery is not good enough for them.

Next, Vronko sees a pick-up in demand for battery replacement with 12-to-15-month-old iPhones. The peak age for battery replacements is 18 months.

Rapid Repair charges $20 for just the iPhone battery, although you'll have to be brave enough to put in the new battery yourself. Or it'll cost $50 for the battery and Rapid Repair to do it.

For do-it-yourself folks, swapping in a new battery on an iPhone 3G or 3GS isn't terribly difficult. The units have tiny screws for removing the outer shell, and the battery has a pluggable module. This wasn't the case with the original iPhone, which had a battery that was soldered to the unit.

Vronko hasn't seen the iPad yet, although he figures battery replacement won't be easy given the iPad's unibody design. Another sign is the fact that Apple itself plans to replace whole iPads rather than batteries.