Hobo Datalogger vs. Hacked AA Alkaline Battery

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:

Attic - Insulated Box - Early battery failure

Attic – Insulated Box – Early battery failure

Two new Maxell CR2032 lithium cells also had trouble, with the first reporting a low voltage in January:

Attic - Insulated Box - Maxell battery failure

Attic – Insulated Box – Maxell battery failure

The second in February:

Attic - Insulated Box - Maxell battery low - 2015-02-25

Attic – Insulated Box – Maxell battery low – 2015-02-25

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.