1. Goto battery shop ask them to check for generator charging & battery health with hydro meter.
2. check for corrosion / cracks over starter cable lugs & any corrosion / deposit over battery terminals. wash & clean,
3. make new lug connector for starter motor [ self / sulf whatever ] if cracked or corroded.
4. also check for battery negative connection cables to body shell & chassis.
replace everything above which looks bad, cracked, corroded damaged. Change battery only after everything above is in order and battery is not retaining charge from alternator / generator