Discharging a car battery below 6V - what are the consequences?

Due to carelessness, I allowed the battery to over-discharge. I left the key in the ignition overnight with the internal blower fan on, and in the morning it turned out that the battery had about 7V. The car started without any problems (old T4 1.9TD), but since then the battery seems to have completely lost its capacity. I charged it for over 10 hours with a charger, at the end the charging current dropped from the initial 8 to 2A, then it stood overnight and the car wouldn`t start anymore. The car`s quiescent current consumption is 50mA. The car installation is efficient.
Is there any hope that he will come back to life?

No, the battery is damaged and needs to be replaced.

There are rectifiers with a regeneration function, but there is little hope and it is more profitable to buy a new battery. What you have will only need to be disposed of, unfortunately.

Quote:

The car started right up without any problems

This was a mistake, if you charged it with a low current for a long time, maybe something would come out of it, and the alternator could even suffer.

GREAT WORDS!!
OK, I`ll tell you the whole truth.
There are 2 batteries in the VW T4 Multivan, min. 80Ah each, in my case the engine one was BOSCH self-service and the rear one (under the driver`s seat for the passenger compartment) was an unbranded one of traditional design. After turning the ignition, a special relay shorts the positives of both batteries for charging while driving. When you turn off the ignition, the batteries are disconnected and can be used separately for the engine and for lighting, the refrigerator and other devices inside the car. In newer models there is apparently an electronic system that disconnects them automatically when the voltage is reached. installation drops below voltage. charging. I have a simple relay that responds to the key in the ignition.
So, by leaving the ignition switch turned on, the fans, of which there are 3 in this car (front air vent, upper passenger air vent, lower passenger air vent from the passenger section heater - I don`t remember which ones were turning) and the radio, drained both batteries to "0". The front one had 7 V, the rear one discharged until the relay coil "fell off", which is probably to about 8 V. After starting the car with the ignition on and the discharged batteries shorted the alternator took such a hit that it simply fell apart . I managed to find a kind soul, I charged one battery, unplugged the second one and drove on the highway without charging, with the lights, radio and turn signals off, avoiding pressing the stop button for 300 km. The car was running and the engine was running at 6.8V!!
Since then, neither of the 2 batteries has held power. in Bosch the green light went out and despite charging it does not want to turn on. In the latter, I filled the electrolyte with water to the appropriate level, I charge and charge, the charging voltage is OK, the current does not drop below 2A, the electrolyte gases, the hydrometer shows that the density of the electrolyte does not want to leave the red field and the electrolyte itself is brown in color.
I`m writing this as a warning , because I`m already done with birds. Alternator repair PLN 250 + 2 batteries 80Ah each, inadvertently

If you consider leaving your car with the keys in the ignition (and turned on) as a nuisance, the cost of training is not high.

I agree with my friend here. Losing the car would be more painful.

The WV Multivan version has a fold-out couch, I slept soundly inside.

Well, you would lose your car and they would demand a ransom from your family

decided to buy 2 new batteries
Now everything works as before.