What condition must be met for a battery to be rechargeable? either its anode or its cathode must generate a gas as a result of the electrochemical reaction. it must generate electricity via an acid-base reaction rather than via an oxidation-reduction reaction. the battery must be open to the outside so that it can vent any internal pressure that builds up from gases within it. the electrochemical reaction of the battery must be reversible.

The electrochemical reaction of the battery must be reversible.

Explanation:

An electrochemical cell is device in which chemical reactions produce electricity. There are two main types of electrochemical cells:  

Primary cells: In primary cells, the chemical reaction through which electric current is generated is irreversible and hence cannot be re-charged.  

Secondary cells: are those in which chemical reactions through which electricity is generated is reversible. Hence they can be re-charged.

if you mean hydrogen and oxygen then water is the product