I have a 2014 Volvo XC60. A mechanic recently replaced the (main) battery with a (non-Volvo) Century battery (AGM-type 12v lead acid, 70Ah). I parked the car in the garage for 2 days and the battery went dead flat. Battery voltage was reading under 6 volts. I charged the battery with a charger, then took it back to the mechanic. He charged and tested the battery, and tested the current draw. All normal, no problems found. At my wife's insistence, he replaced the battery anyway. New battery has been working well for a few days, but once again I parked the car for 48 hours in the garage and found it dead flat again. Once again, the voltage was 6V. I have charged it with the charger for about 16 hours, which brought the voltage up to about 11.5V. I was able to start the car, and went for a half-hour drive and the battery is now charged to 12.7V.
I am totally baffled. What is going on here? How can a lead acid battery go down to 6V? How can it recharge and not be totally buggered? Although with the second battery I measured the voltage with the battery still connected to the car, with the first battery I disconnected the positive terminal and was still getting around 6V. I have a good quality Fluke multimeter that is working fine (I think?!).
Keys are removed from car (however car unlocked), lights all off, radio off. Never had this problem with the previous battery.