It turns out the transmitters work fine with two AA batteries rather than three. This means it would be possible to put an ipod battery in the next version. An ipod battery has 3.7 v and 850 mAh, meaning it has 3.145 watt hours of energy. Two AA batteries have a total of 3.2 v and 1500 mAh, meaning they have 4.8 watt hours of energy.
There are also other lithium-ion batteries out there worth looking into. It would be nice if the next version of the door minder had at least as much battery life as the current one.