The Modern Bajaj Chetak Rides On
Battery for the post 2000 Chetak is woefully described in the owners manual as 12V-9AH.  This is too generic a description to locate the proper battery.  Most riders recommend the Yuasa brand.  The more specific descriptors are provided below:

Generic battery: 12N9-4B-1 9Ah
Yuasa: Yb9b
Parts Unlimited: rcb-9b
Exide: 12MX9-B

Note: these are regular lead acid batteries priced at $30-$60.  Many riders have tried to use AGM (sealed) batteries only to find the dimensions do not fit the battery box on the Chetak.

Recommendation: Use the former  bajajusa owner's trick of petroleum jelly on the battery terminals and you will never have battery post corrosion problem.

The Modern Bajaj Chetak Rides On The Modern Bajaj Chetak Rides On