See my post 24. Some chargers will NOT charge a battery exhibiting very low voltage. I’ve not rigorously tested that on the 2 car chargers I mentioned, but, on the one the threshold seems to be 5 volts or so. The charger “thinks” the battery is a short, and stays off. Hook the low battery to a good strong battery through a “light” jumper cable (built in resistance to limit the current a bit) for a moment, remove that connection and hook the charger back up, and now the charger charges the low battery.
(Of course, severe depletion is also not good for car batteries, and severe depletion and a little time left that way is really deadly to them...)
From what I’ve seen, pretty much any “smart” car battery charger will refuse to charge a completely dead battery. (I think this is because they’re designed to verify that you haven’t connected it backwards, and it needs to see some voltage from the battery to verify before it turns on the power).
The battery chargers with the heavy transformer inside a metal case, and old-school analog ammeter don’t have this problem.