Re: Does the 5V Adafruit data logger work with the 3.3V Arduino Due?
Subsequently I discover the Olimexino 32U4 board was supplying 5v to the datalogger shield, despite the processor running at 3.3v (and 16MHz). The SD card appears to have survived.
Having dropped the battery voltage to 2.5v and further to 2.09 volts through a series of diodes, I cannot communicate over the I2C at 3.36 volts (testing with a Xino board running at 3.36v, 8MHz). The DS1307 RTC will retain time within a minute at a battery voltage of 2.5v since the post above.
I give up. I'll retrofit a DS1340 to the protoboard area, on a SOIC adapter.