Sir,
According to Section 2 (74) of CGST Act, 2017 "“mixed supply” means two or more individual supplies of goods or services, or any combination thereof, made in conjunction with each other by a taxable person for a single price where such supply does not constitute a composite supply;
Illustration: A supply of a package consisting of canned foods, sweets, chocolates, cakes, dry fruits, aerated drinks and fruit juices when supplied for a single price is a mixed supply. Each of these items can be supplied separately and is not dependent on any other. It shall not be a mixed supply if these items are supplied separately."
Therefore if you supply inverter and battery separately you can raise separate invoices and pay gst accordingly. If you supply inverter along with battery for a single price then you can issue a single invoice and pay gst at the rate prescribed for inverter or battery which ever is highest.