Keeping lager fermentation temps without refrigeration?

I want to make a lager sooner rather than later. It being as hot as it has been, I of course need to control the temps, but the only thing I have is a large insulated cooler I made from garbage cans, 2 cans, one inside the other, with as much fiberglass as I could stuff between them. It is very efficient at keeping things cold, especially if I stuff a very thick old down coat around the top.

So, other than using trial and error, what would be the appropriate amount of ice in how much water needed to maintain 54F in the 6.5gal fermenter itself? I probably learned this in High School or College Chem, but that was a very long time ago. And if I use ice bottles, how does that impact it?

I'm sure I'd have to consider heat generated by the yeast, heat loss of the cooler, ambient temps... and in the end I'd probably end up using trial and error.

Holding at 32 is easy, holding at 54 is hard.

I do this for all of my lagers, it's not hard at all. I use an Igloo Ice Cube in the garage. With an insulated container like you have, the trick isn't calculating how much heat loss, etc., you just have to catch it at 50 (or 54 in your case) and try to hold it there. After brewing, I chill in the pot to get it as cool as I can, then rack to a 6 or 6.5 gallon carboy. I put this in my cooler and essentially chill it down the rest of the way in an ice bath (I usually buy a 20 lb bag of ice and use all of it). This drops it below my fermentation temp. If you can take the temp of the water bath on a regular basis (at least every 12 hours) and adjust accordingly it's best. I use a 60 qt Ice Cube. The water bath matches the level of wort in the fermenter. If I swap out 2 liters of ice every 12 hours, it usually holds 50 very well. I estimate I get a temp swing of 48-52.

I keep Gatorade bottles (they seem to freeze/thaw well without cracking) in a variety of sizes, the 16 oz ones work the best IMO, since you can use 3-4, whatever you need. I guess this is trial and error like you say, but with a large thermal mass in an insulated container, the temp swings aren't very large at all. As long as you're there on a regular basis to monitor it, it's no big deal. Once primary fermentation is over (7-12 days), you don't have to do it anymore. Since your water bath volume is different than mine, your method may be a little different, but once you get your temp, it's not too hard to hold it.

Thanks. That's the kind of advice I was looking for. I guess I'll play it by ear after I get it started.

I do something similar. What helps is I fill the water bath up to the level of the beer, and then float a thermometer in the water bath. If the water bath is 50 degrees, the beer is darn close.

I also use a 60 quart igloo (pics in my gallery) and it's nice because it has wheels and I can move it to the coolest area in my basement. Once I'm at 50 degrees, I only need to change the frozen water bottles once a day in my 65 degree basement.