You have the right to sue the seller from fraud, and your remedies would be either "rescission" (give back the car and get back the money) or "monetary damages".
However, in a used car situation, it may be difficult to win because you had a duty to inspect the vehicle before you purchased it. Thus, at trial for oral misrepresentation, it becomes your word against his.
Perhaps you can reach a compromise whereby he pays you some portion, such as half, of the $5,000 it will cost to replace the IMA batter and catalytic converters.
I'm sorry to hear about the problems you are having with this car. In order to be able to advise you on your options, I would need to know whether the car came with any warranty, or if it was sold "as-is". It would also be important to find out from the Honda dealer whether this battery failure was something that would have been obvious to the seller. What is Honda's warranty on this battery and on the catalytic convertors? I know Federal and State law require long warranties (I believe 100,000 miles) on the latter. You may very well have remedies under both a fraud theory and the California Consumer Legal Remedies Act (CLRA), but I would need more facts in order to be able to give you an opinion.