Hi,i would get a cheap voltmeter, sears for sround $20. And set the dial to read dc volts, then touch the neg. post w/ the black end and the pos w/ the red end. With the car not running you should read at least 12v to 12.5v, Then start the car and repeat test,should read 13v to 14.5v . Then while running turn on lights and fan to high and read again to see if the alternator can still keep the output of dc volts up to atleast 13v. Also i recommend a sears diehard gold battery , they cost a lil more upfront but repay you tenfold in the longrun.. as always make sure battery post has no loose connections and spray with a protector to keep out corrosion. good luck
