When you turn on the key the ignition will draw some electricity from the battery and this is usually shown by a slight dischrage of the ammeter ( just off line from zero). If the ammeter shows full discharge then it means just that. ie you have full discharge. If everything is switched on, lights main beam etc even this will not normally give full discharge. It sounds more like a short to earth which will give this reading. Finding a short is very time consuming as you have to go through the entire wiring system with a meter to isolate and eventually find the cause. I once spent weeks trying to find a short on a BSA C15. A strand of wire had fallen inside the distributor and was shorting to earth through the distributer body. All the best
