I go through about six of the taller bottles for a handheld propane torch a year. My BIL cooks mostly on a small outdoor propane grill using the short, squat green bottles.
I bought a fitting through Harbor Freight which said it would refill these small bottles from a 20- or 30-gal propane tank. Dubious, but it does work.
The connector goes on the propane tank and then the small bottle screws on the other end. You first freeze the small bottle, connect it to the tank, turn the tank over and open the valve. You can hear a very slight hiss if you hold your ear to the tank. Close the value to the tank and then remove the bottle.
I suspect it would work at maximum efficiency if the tank were at room temperature and the small bottle right out of the freezer.
Used my postal scales and it seems to refill very close to original weight.
Saves me a fair bit of money over a year not having to keep buying replacement bottles.
I have a propane tank of the type used on a forklift. I can get liquid or vapor from the valves. Leaving the main tank upright I connect to the small tank. No freezing is involved. With the main tank connected to the small tank, I place the small tank horizontally and with the small bleeder valve that is in the top of the small tank at 12 o'clock position I open the liquid valve slightly and with a pair of long nose pliers I PULL the small tank bleeder valve stem and hold it open until liquid propane starts to exit and then I shut off the main liquid tank while releasing the grip on the bleeder valve on the small tank. The small tank is now filled to the amount engineered into the design. Note...I made my adapter from an old propane torch kit and propane tank adapter from a junk BBQ.
I also have one of the Mac Couplers as I remember them to be called. I gave $14 for mine in 2007.
I don't bother to chill or freeze the bottles before filling but do make sure the bottle is entirely empty, i.e. no more pressure in it before I attempt to fill on a hot summer day when greater pressure is in the 20# bottle. I also weigh each to make sure I haven't overfilled it.
I got a lot of empty bottles from a fellow that uses a propane lantern. He will often toss a bottle before it is empty because he knows it won't make it through the next use without running out and doesn't want to change them while guests are around the campfire. My brother uses a propane lantern and has also saved me some that I need to pick up the next time I visit.
One BIL was in the H&A/C business for about 40 years. I asked him about the freezing thing. He said propane flows better between temperation differentation. If the say 20-lb tank was at 50 degrees, and the small bottle at 30 degrees, then it would flow towards the colder one. If reversed, the 20-lb might not fill the small bottle at all.
I do a bit of copper pipe soldering in the shop. I'm currently using one of the tall bottles. When it runs out I'm going to try switching to one of those squat green ones intended for a small cooking stove. More fuel and more stable base.
Do you folks not realize what the bleeder valve adjacent to the main attachment port on the small tanks is used for? It is the vent. It is used to make certain that the small tank is not over filled yet filled adequately. It is a reverse acting valve and the small needle portion is made to be pulled NOT pushed in order to vent the bottle. With the bottle horizontal and the vent valve at 12 o'clock open the main fill valve on the fill tank and permit the small bottle to fill. When you think the bottle has taken all the propane it will accept open the vent using a pair of needle nose pliers until liquid propane starts to escape. At that time the bottle is properly filled and has enough room for expansion of the gas safely. Obviously, this exercise needs to be done outdoors and away from a flame.
My experience was that you don't need to freeze them or bleed vent them, just fill a 100% pressure free bottle on a hot summer day when the larger bottle has max pressure. I weighed the ones I filled to make sure they hadn't been overfilled and all weighed out as being full.