Charging dead battery on Ford E350

I've used a battery maintainer to trickle-charge a really dead battery (under 9 volts) up to a level where an old resonance charger could start working on it. This is sort of a desperation approach, with carries some risk (e.g. if the voltage is low because 1-2 cells are shorted out, a battery could overheat).

Most modern "smart" chargers have protections for some (but not all) potentially dangerous situations, they will tell you "bad battery" for a variety of causes.