I have a new five-month old (yes months) battery that is not keeping charge. I disconnected and charged the battery overnight using a trickle charger. I got the bad battery indicator on the charger. So, I disconnected and put it back on the trickle charger again. Overnight, the battery charged to 13.3 V. I let it sit another day and 22 hours later, the charge is 12.91 V. 48 hours later, the charge is 12.83 V. I guess I will check it every day for a week, but how much is a disconnected car battery expected to lose over a day?
Update: Ten days later, yesterday, it was at 10.39 V.