Test the alternator under load. Headlights (brights), AC, fan etc. The stock alternators are regulated @ 13.8 volts. That will go down under load as you are drawing more amps. Anything under 13.1 is unacceptable in my opinion. Others will say 12.8. A fully charged lead acid battery should read 13.2 volts. (2.2 per cell) Max charge rate is 2.35 per cell. (14.1 volts) Modern cars are regulated at 14.1 volts. The voltage reading is only telling you half the story. Where are the amps (flow) going? Into the battery? (good). Out of the battery? (bad). With an older car the wires have built up a resistance over the years. Couple that with an alternator that is only putting out 13.8 volts and you're always on the edge of acceptability IMHO. Good thing we have such large batteries in our cars. It's going to start getting cold so the electrical system will be working harder to get the car started in the morning. Glow plugs, cold cranking amps, etc.. The alternator is rated in amps. Stock is 55/ 65. You should have a 50% safety margin to make sure you have enough amps to charge the battery while you are using the other 50% for your lights, fan, sub woofer, blah blah, etc...... That means on a 55 amp alternator you should not draw more than a 27.5 amp continuous load in order to safely insure that your battery will get charged. Have Fun

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'70 F100 shortbed
'82 Diesel Westy
'83 Euro 300TD
Curtlo Viper
Yeti ARC
Surly long haul trucker