The AA battery pack grafted onto the back of the Hobo datalogger recording groundwater temperature showed a 50% level during its most recent dump, so I swapped in a pair of new AA cells.
The pack hack dates back to 2009-09 and the Duracell Ultra cells have a “best used by” date of March 2013. Call it 5.5 years of service and, figuring an average current of 10 μA, that’s a total of 480 mA·h.
The datasheet shows many graphs at much higher currents, but a capacity of 3500 mA·h to 0.80 V at 5 mA seems pretty close. Given that they produce 2.87 V with no load, they’re still in decent shape.
However, the logger’s opinion of their voltage is what counts. To estimate that number, I checked the reports from the attic: the death planet for lithium cells.
Starting with an old Energizer failing after a few hours in December:
Two new Maxell CR2032 lithium cells also had trouble, with the first reporting a low voltage in January:
The second in February:
I think the Maxell cells failed from low temperature, but dead cell is dead.
That happens just above 2.85 V, so the attic datalogger now carries an AA alkaline battery pack.
The Smell of Molten Projects in the Morning 