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My son has a 1984 Ford Ranger that keeps draining the battery.  A couple of days ago it wouldn't start.  We charged up the battery and it ran fine for about 1/2 hour and then quit on him in the middle of traffic.  We just replaced the alternator in it this morning and recharged the battery and it again it started up fine and we got it home, drove about 1/2 hour no problem.  It sat for about 2 hours and then it wouldn't start again.  It's not doing anything.  Sounds like the battery is totally dead again.  It had been running fine before.  

Thanks,