Can i suggest you buy one of these battery testers, it may also indicate what is happening with the charging system as well. It worked really well on an issue i had on my car.
AUTOOL 12V Battery System Tester Health Checker, Charging Analyzer for Cars, BT360 Amazon product ASIN B071NS74YK
AUTOOL 12V Battery System Tester Health Checker, Charging Analyzer for Cars, BT360 Amazon product ASIN B071NS74YK