need to CHECK the CHARGING CIRCUIT charge the battery if possible(may be scrap due sulphated plates) start the engine ,with the engine running put a digital mutli meter(DMM) set on 20v DC Across the battery terminals should see 14.5v on a 12volt system 8.5V ON A 6VOLT System with ALL electrical things turned ON anything less than the supply voltage be it a 12v or a 6v system then you have an ALTERNATOR problem can be with the regulator faulty, or may have slip ring brushes that are worn but this is not a major problem.
wirecutterman
September 2008