I don't know who first invented the "light bulb in a cookie tin" water warmer, but what a great idea, and life saver I'm sure, for so many of us who raise chickens. When we built our coop/run in the Fall of 2014 one thing I worried about was how to keep the girls' water from freezing in the Winter. Then I found the water warmer idea, I think right here on BYC. So I made one, put it in the coop, and ran an extension cord from our deck over to the coop so I could plug it in when needed. Then we had a pretty good wind/snowstorm and lost power for, I think about a day, last Winter. Long enough that we had to change the water a number of times so the girls wouldn't be trying to chip ice with their beaks. So I started thinking, why not design a warmer that could run off of a deep cycle (or marine) battery? So I did. Being a car guy all my life, I've got parts... lots of old parts. So I took an old interior light socket, mounted it inside an 8 inch cookie tin (right in the middle so it would distribute the heat evenly, soldered a lamp cord to the socket, and on the other end, a couple of 30 amp style alligator clips. Ones that are big enough to clip onto the battery terminals. Now in case you're not sure what a deep cycle/marine battery is, it's pretty much the same thing as a car battery, only they're made to deliver a slow, steady flow of electricity, unlike a car battery that's built to deliver a quick heavy duty jolt to turn your engine over and start it. Naturally, (again, car guy) I have some of these batteries for our camping trailer (RV batteries are deep cycle batteries) and other toys that guys like me play with. So I hooked it up and left it on to see how long the battery would last before it needed to be recharged. Three days straight, (more or less) before the bulb dimmed and finally went out. Next test - would it generate enough heat to keep the water from freezing? We use the plastic waterers with the opaque bottle and the red plastic bottom tray in our coop and run. Well, I can tell you this much, it got down to -2 last year, and the only ice that formed was a very thin layer on the top of the water that stayed at the top of the bottle as the girls drank the level down. The great thing for us is, we hook up the 12 volt model to the battery when the girls are out in the run in the daytime, and use the 120 volt house current one in the coop for the night.
This way I can go four or five days easily before I swap batteries and recharge the one that's getting low. So I've built a few of them now, and even gave one to a friend of ours who also raises chickens. He loves it because his coop is pretty far from his house and doesn't have house current to it, so he just uses it until he has to swap batteries and doesn't have to worry about freezing water anymore.
I've included some pics showing the parts I use, and the finished product, and will be happy to answer any questions for anyone who wants to make their own, or if anybody is interested in buying one from me I would sell them for around $30 plus shipping, which I'm guessing would probably be about $8 to $10 in the US. I don't have exact prices, since some of these parts I had hanging around the garage, but if I had to guess, I'd say you're probably looking at $15 to $20 dollars, not including the battery, which, if you didn't have, a new one would probably run anywhere from $75 to upwards of $100. A car battery would work just fine (please don't go taking the battery out of your car, killing it, and ending up with no transportation) and you would also need a charger, if you don't have one. You could charge it from your car, but that's just wasting way too much gasoline, even at today's prices.
So like I said, any questions, please feel free to contact me.