According to
http://wiki.diyfaq.org.uk/index.php/...y_construction a home-brew lead-acid battery can have a capacity of the order of 1 amp-hour per square inch of submerged anode and cathode.
You're going to have to source some 4-molar sulphuric acid (probably not available from your local Chemist Warehouse!), and make sure the whole assembly is rigid enough to not short out if bumped, made entirely of non-corrodible components, located in a secure facility which is well ventilated, and resistant to acid fumes and spray.
As for whether it's advisable (or legal) - I'll let you do your own research on that! If you don't know how to handle concentrated acid safely (and in particular, the hazards of adding water to concentrated acid) - don't even think about it!
(Personally, I'd feel much happier just buying commercial batteries - but where would we be if Volta and Faraday et al had never played with these things?!)