Partial mashing, here I come

I scored a used 5-gallon round Igloo water cooler at the local homebrew club raffle last night. It has a false bottom, but I'm thinking I'd rather make a ss braid for it. The false bottom fits so tight it doesn't go all the way to the bottom. I'm also going to replace the bulkhead assembly with one described in How to Brew as I don't like the way the one already in there is made.

It came with a sparge arm, but I believe I'd need another cooler with spigot to act as a hot liquor tank to use that, correct?

General question - how much grain can be comfortably mashed in a 5-gallon cooler? I'm only intending to do partial mashes with about 3 gallons of boil volume, most likely adding extract at the end of the boil and topping up to 5 gallons. (I'm limited to an electric stove inside, so partial boils are about as good as I can get.)

Thus far I've done three batches total, all using grain, but these were just steeped specialty grains. I've never mashed before.

You can mash upwards to 12-13 pounds in one of those coolers, but that gets pretty tight. If you can't boil more than 3 gallons, you won't want to do nearly that much grain, because there's no way you could handle the amount of runnings you would be collecting. I would guess that you might be mashing around six or so pounds, just so that you aren't either collecting way too much in runnings or not sparging enough and having terrible efficiency. But, that's enough where you'll be able to start using grains like Munich and Victory for some additional character.

I'd recommend that you just batch sparge for now, especially doing partial mashes; put away the sparge arm for if you decide to go AG in the future (even then, you may not need it).

...Or you could just do all-grain half-batches it you are worried about mash capacity. That might cause you to get a few smaller-than-usual carboys but i've found that starting out at half-batch all-grain was a much more manageable experience than worrying about 5+ gallons! In time, though, I'm sure you will want to go full-batch.....it's just too hard to resist.

I'm still missing one brass coupler piece (Home Depot and Lowes were both sold out of it), but the mash tun is almost ready to go with my upgrades. (Ball valve on drain, stainless steel braid in a circle config for a manifold.)

Man, making that braid manifold out of a hot water heater hose was a real PITA. I don't have a garage or any kind of workshop so I was sawing away at it on a piece of plyboard on the kitchen counter with a hacksaw for what seemed to be ages. What made it particularly fun is I don't have a vise, so I had to hold the dang wiggly thing as I tried to cut it