well, if the "thing" (actually called a carriage) goes down when the door is disconnected, then it's not your photocells, assuming you even have them. it's also not a battery issue. what is happening is the opener is sensing a safety reverse issue whith the door. the weight of the door is slowing down the motor enough to trip an rpm sensor inside the motor case. one of a few thing can cause this. first is the door, when you move it manually, does it roll smoothly?? cold weather exposes any weakness in the door. lubricate all the rollers, pulleys, and hinges. DO NOT use wd40, use any other spray lubricant silicone or oil base, both types work the same, however i prefer oil based. if you have stretch springs, the pulleys attached to them and the track (4 total) are a wearable part. if they are wobbly or don't roll smoothly, this will cause all kinds of problems. oil will help them temporarily, but they should be replaced.
if the door works smoothly, it could just need a simple adjustment to the down pressure (sensitivity). without knowing what brand you have, i can't offer much help there, but almost all manufacturers have a website with instructions on how to adjust this.
if you have a sears, liftmaster, or chamberlain opener (same manufacturer) a 3rd possible cause could be the circuit board. as they start to go bad, the sensitivity has to be decreased, sometimes all the way, just to get them to move. no cure for that other than replacing that board. most of my similar calls involve the first 2 things i mentioned.
garage door tech 18 yrs