Originally Posted By: wrcsixeight
Battery's self discharge varies greatly, and increases as batteries age.
the Saab has some sort of parasitic load from engine computer, door lock fob, ect.
The % displayed by any battery charger is entirely voltage based, and is at best a guess, and not to be takes as truth.
A healthy flooded battery can self discharge 12 to 15% per month at an average 75F ambient. Self discharge increases with battery temperature, decreases with decreasing temperature.
Once they self discharge below 80% the battery capacity starts shrinking faster never to be returned.
A trickle charger on a battery slowly depleted to 50% via parasitic and self discharge is an absolute joke. it has virtually no chance of returning the battery to 100% charged, despite that soothing green light it flashes at you. The battery needs to be brought upto 14.4v for a while, and most trickle chargers will never do this, but the word trickle is apparently very soothing to the human ear as it is repeated so often regarding batteries.
+1 to all this.
Alternators do not fully and properly charge batteries to begin with. Dumb trickle chargers will never allow a balancing charge and proper float. Smart ones with temperature compensation can, but still may be conservative in their logic.
AGM batteries self-discharge slower than regular flooded batteries, so if you are seeing this sort of depletion, I think higher than normal parasitics should also be looked into.