Originally Posted by TFrevold
My intentions prior to the offer of free sulfuric acid was to buy 2 gallons of 98% and mix it with 12 gallons of distilled water. that would give me about a 16% mixture
Don't plan on storing left-over 98%; it can creep out of the container and isn't worth the hazard of keeping on the shelf. It is also hygroscopic, so if the lid is not perfectly sealed, water gets in and the volume increases and can overfill your container.
The cheapo battery testers can measure down to a density of about 1.100, which is about 18% ircc. That would get you into the ball park. You might be able to "titrate" by looking for bubbles while adding baking soda (hmm). It seems to be fairly flexible in the process, but when you have trouble it is nice to know for sure that your acid is right.