I once had an almost new battery that got totally flattened by an electrical fault, and wouldn't accept a charge despite being connected to a charger (4 amp-ish) for several days. Thinking the battery itself was faulty, I took it back to the shop from which it had been recently bought. They connected it to their high-output charger, and again nothing happened. They suggested leaving it on the charger overnight 'on the offchance', and the next day it was charging normally - evidentally the current from the powerful charger had eventually kicked the battery back into life, and it was fine ever since - but I'm sure my small domestic charger would never have managed it.
Jeremy