[GeoffR]

I have been testing a new micro SD card (Sandisk Ultra 16GB) for a few days and there seems to be little difference in power consumption compared to no card at all. I estimate about 30-35 hours reading time with wifi off. (Can't be exact without a much longer trial).

But when I put my old card (Kingston 4GB) back in the slot it is clear within a few hours that the battery is being drained at a much higher rate. Even with no books on the card, just having it in the card slot is enough to cause the drain. So it seems clear that the fault is with the card itself.