For pete sake. According to the Battery FAQ posted just before your post, that 25% SWAG I used as an example was right.
I pretty much totally pulled that out of thin air, after watching the voltage on my first attempt to boil water - I could see the power delivery to the hotpot drop significantly about that point.
At 75% of capacity, sulfation begins. Lots of information here...
“For pete sake. According to the Battery FAQ posted just before your post, that 25% SWAG I used as an example was right.”
The way your post started, I thought I pissed you off. Anyway, I follow now - you made a guess and nailed it - good job.
You can see my point about voltage on that same FAQ page. One thing to keep in is that lead acid batteries for cars, or other engine start applications, are optimized for putting out a lot of power, very fast - which leads to a lot of plate area, which leads to thin plates, which leads to plates that fall apart with deep cycling.
If you don’t need high power levels, then you’re looking at deep-cycle batteries, which have much stronger (thicker) plates and can be run down lots of times (maybe hundreds, not sure), but they are meant to be run down completely. They’re typically used to power electric motors in boats, where the motor is small and the power levels are pretty constant (i.e., nothing like trying to start a V8 engine when it’s 10 below zero). The only deep-cycle batteries that I’m familiar with are about the size of car batteries, which is a lot larger than you’re talking about. However, if you want to get 18 Amp-hrs out of a conventional lead-acid, you’ll need a 72 Amp-hr battery to start with (i.e., use 25% of it). A 72 Amp-hr battery is about the size a car battery (I believe), so at that point, you may want to shift over to a deep-cycle and not have to worry about destroying it (and who knows, there may be smaller ones out there).
After reading some other stuff here...even with a deep cycle, you probably shouldn’t pull it down more than about 50% (rather than all the way down)...but it will take a lot of cycles at 50%. Anyway, you can research it, I don’t remember the sweet spot on those, and you may not be cycling much, so you may be able to go lower.