One possible solution.
Use 4 rechargeable AAA batteries, 2 in the sensor and 2 outside in a battery holder wired in parallel.
Wouldn't that make the batteries charge each other - Like the old expression, "water seeks its own level" - In other words, the 2 batteries inside are down to 2 volts, the ones outside are a fresh 2.4 volts - The electricity flows and makes the 2 pairs equal, or 2.2 volts all around. Then, when the voltage drops below the "threshhold", the motion sensor loses the codes ![Head Banging B:(](http://forums.x10.com/Smileys/default/headbanging.gif)
I think the batteries, if mounted in parallel, would act like a charger for each other unless you had the proper diodes to block the current from the other pair. Like a diode on each positive leg (1 from each pair) so that the current didn't flow into the other pair. That might work. ![Grin ;D](http://forums.x10.com/Smileys/default/grin.gif)
Not quite, if you have one set of batteries drawn down to 2V instead of 3V, they would do as you indicate.
Do AAA batteries last only 1 month? No, they are still fairly up there.
Assume 2.9 V.
Adding fresh 3.0 V batteries would make the combination 2.95V which is perfect!
If that is not perfect enough and you really want to go crazy
Keep 4 AAA batteries charged for each MS16a.
Change 2 and then change the other 2 that were in parallel.
No loss in power.
And finally: Bring a 3 Volt source with you with alligator clips.
Clip on the 3 Volts, remove and replace the batteries.
Remove clips.
Simple, tiny, cheap, and cosmetically exactly as original!
No loss in power.