How to safely discharge a lead-acid battery?
Hook it up to a 60W headlamp bulb, that will take 5A. Car batteries usually have a capacity of around 45 to 60 AHr, if you assume the battery is fully charged and in good condition then it should take around 10 hours to discharge it.

I think that answers your question. But you should not fully discharge a lead-acid battery and leave it standing, you will permanently damage it.