Check the charging system. With the engine running the battery voltage must be between 13.75 and 14.75 volts. If it is low, suspect the generator. If it is okay, measure the battery voltage with the engine off. It should be 12.6 volts if it is fully charged. If it's around 12.2 volts, it's good but discharged. If you find closer to 11 volts, it has a shorted cell and must be replaced.