Car battery dies within two days of a full charge

What we told them

Two batteries behaving identically means the battery is not the problem — something is drawing current while the car sleeps. Put a multimeter in series with the negative terminal and wait forty minutes for the modules to go to sleep. A healthy parasitic draw is under 50 milliamps. Anything above that, pull fuses one at a time until the reading drops; the circuit you just interrupted is the culprit. Glovebox and boot lights that never switch off are the usual answer.