The battery is dead and by the info given it may need to be replaced. You may be able to get the charger to turn on and charge if you hook up a 12v car charger to six cells at a time and charge them for a little while and keep doing that until all 24 cells have bee done and you get at least 48.5 volts. It sounds like the charger is an automatic turn on and is doing an evaluation of the battery and it doesn't have enough power for the charger to recognize it and turn on. As for the codes they could be any number of things but I am willing to bet that it has a main card issue, especially if it was left outside for 2 years. It may not be worth it to get it going.