Charging my car battery

[HighVoltage]

Just today a friend came with his 12V GEL battery for a mobil home and it was sitting at 4V for a few month probably.

I guess your friend is not aware that allowing a LA battery to get below 1.75V per cell damages it severely.  For the best longevity LA batteries should be stored fully charged with a maintenance charge every few months.  An LA battery should not be discharged below 1.75V per cell with 1.85V per cell ideal to minimize wear.  Interestingly Li-Ion batteries provide the best longevity when stored under half charge, but they too suffer damage when discharged too deeply.

Well, once it is charged up completely, I will discharge it with a DC load and graph the voltage.
Will see how bad it is and how much capacity is left.