They do tend to short internally and they have been known to be bad from factory. Age plays a factor on the oxidation process of the internal lead plates, the oxide falls to the bottom and builds up shorting out individual cells... or just a short between 2 plates will cause a discharge. When a battery is moved it can dislodge those little flakes and they fall to the bottom and if the cell isn't shorted it will take and hold a charge. Pretty simple really. You can renew an old dead battery that wont take a charge by flushing and cleaning the inside and refilling with new electrolyte, it jst depends on how old and damaged the plates are inside. there are youtube links on this process.
Again, both batteries were fine before. And both have been fine since put back into use including the battery sitting inside Mom's camper on a carpeted throw rug. I don't know why a battery sitting on concrete drains it when according to all of the "scientist" here it shouldn't, but it does.
They are fine - essentially I'm doing that now with Mom's camper battery. It is inside her camper (so someone doesn't steal it) until I get there to winterize her camper and I bring it to my house to put on the trickle charger again.Does the car get used daily or often? Is the caper on any kind of trickle charger?
The car is not driven daily, it can sometimes sit for 3 or 4 weeks without being so much as started. The battery has been fine since put back in the car. The camper battery was on a trickle charger all winter until we moved stuff around and then it wasn't. Two weeks it was dead. Sitting in Mom's camper (without connected to anything) for almost a month and it too also still charged.
It would be interesting to take both batteries out of service for two weeks, place them on a block of wood or carpet and see what happens.
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Reminds me of years ago, worked with a kid that came back with a string trimmer in pieces and declared to the boss : “It ruined itself Chuck!”