Here is how to make 99% pure concentrated sulfuric acid from an old car battery, or if you have some 30% or so sulfuric acid you want to concentrate.

Put rubber gloves and eye protection on always when dealing with acids! And do not work with the acid inside!

Put your safety gear on, now take an old car battery and drain the acid into glass jars(with plastic lids) or an acid rated container. There is approximately 1.5liters of 30% sulfuric acid in a car battery! Now you have some 30% sulfuric to concentrate =)

Materials:
125ml conical flask (Borosilicate or Pyrex)
Battery acid or 30% Sulfuric acid
Portable gas stove, hot plate etc as a heat source

Put your safety gear on.

Place 90ml of sulfuric acid into the flask, and then place on the heat source on a very hot setting as sulfuric boils at a high temperature.

After a while it will start boiling and making water vapour come out the flask. It is now slowly becoming more and more concentrated as it boils the water out of the sulfuric.

When it has boiled down to the 30ml mark it is now very close to being pure sulfuric acid.

You will know when you have concentrated sulfuric acid is when white smoke of sulfuric acid vapours starts coming out the flask. The white vapours look like smoke not water vapour. You would almost think that you had lit something on fire. Whatever you do-DO NOT BREATH IN THESE VAPOURS- they are highly dangerous!!!

And now all you have to do is take the flask off the heat(be careful with the flask as it will be very hot) and pour the acid into an air tight, acid proof container. If the container cant take heat let the acid cool down in the flask first before you put it into the container.

A simple test is to put a few drops of the concentrated sulfuric on to a paper towel. Where the acid has been absorbed will soon go black and go all mushy!

Hope this has helped you. Be safe and have fun!