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I have a Touch. In sleep mode, as reported by others, it seems to use very little battery power, but it also seems to use quite a lot once it finally does start to register. My thinking is that the battery gauge may not be very linear -not exactly unheard of with battery powered devices. I think the only workable way to be near accurate is to monitor uptime between fully charged and the low end cutoff (around 2.75Vdc for lithium) while accounting for varying loads based on device state. With a profile generated in that manner the device could get a fairly accurate idea of total available power upon recharging and then track remaining power based on usage and turn that into a fairly accurate % on demand. But, that has limitations as well, since batteries are often rated at C/10 or C/20 discharge rates, but fail to deliver that much capacity when used at higher loads. Basically, battery gauges are more an indication of a trend than an accurate depiction of remaining power. It's like trying to guess how much water is left in a glass by looking at how fast it's being poured out. Worse, the only time the unit can accurately estimate the total capacity is when you completely discharge it.
I would advise anyone not planning on reading for 12-18 or more hours to consider shutting the Touch off between usages if they're looking for longer run time between charges. I'd also suggest that the occasional deep discharge may not hurt the device's ability to determine total capacity and more accurately estimate the remaining charge. However, I wouldn't carry deep discharges to the extreme either. OTOH, if Kobo properly determined minimum voltages and set them a bit higher than any internal protection circuitry it should be safe to run a device all the way down until you get the recharge message and then recharge it soon.
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