Perceived wisdom is that a car battery will be fubared if it has discharged below 10 volts. This is because of sulphation which means that the plates will be covered with lead sulphate which will reduce the battery’s capacity to hold a charge even if you can persuade it to accept one.
In a deeply discharged state you will either need a very dumb (old) charger or a very clever new one featuring multiple cycles to try to regenerate the battery. One way or another you will need to include a period of controlled over-charging to try to shift some of the sulphate. You will not remove it all and the battery will be permanently diminished.
If the deep discharge took place over a short period of time then the battery has almost certainly overheated internally and has probably developed an internal short because the plates will have bent and touched each other. If you suspect this to be the case (your charger fusing is a clue, but not definitive evidence) don’t try to charge it. Consign it to your local recycling centre and buy a new one.
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