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Ok we have a 6l monster lorry, well 7.5 GVW Iveco horsebox on an S plate. Yesterday the battery light came on (very dimmly lit) then after 10 mins 5 miles or so it went bright. Looked up on Google which told me how to test the alternator so today I went and tested the voltage. Now on this lorry it runs 24Volts, and has two new batteries which crank the engine effortlessly. Anyway it registered 26.8V, turned the engine over turned all lights heater on etc and set the revs to about 1500 the volts then went up to 28/29 volts however no battery warning light came on so guessed the alternator was doing its job. I turned the lorry of then took it for a drive for about 8 miles and quickly the light came on again but only dim. When I parked it (engine turned off) the volts were then 31 at the battery. Would/could this be an alternator issue still, or is this voltage acceptable (seems high to me, but have no idea), or could the light be faulty and if so how do I test it. All help greatfully appreciated.