Since you didn't mention what type of vehicle your talking about. If your battery is bad it will kill your alternator. Stay away from Walmart batteries, the reason is their advertise cold cranking amps is way off! Your think your getting a 650 CC amp battery and in real life it's only 500 cc amps! I've seen that way too many times at my brothers shop! Brand new battery! Now a simple dome light left on will drain a weak battery. A alternator only maintains a fully charged battery.