...Bottom line here is that the system is pulling a *lot* of power for a battery based device. The cost of the power system is indeed "non trivial". I would suggest carefully evaluating the power in each sub assembly and seeing if money spent there would make more sense than money spent on giant / expensive batteries. I would also take a look at things like the back light -- how bad is it at 1/2 power ? Is that "ok" or not? On battery powered system I have ever done, we spent a *lot* of time beating on the electronics (and firmware) to get the power down. There simply was no other way to do it that made sense cost or size wise.
Bob
The MCU is a real beast mainly because it's a multicore parallel processor, so, there's no changing that. In order for it to do what I need it to do, it needs to be constantly running at maximum frequency, so the MCU is a constraint. The TFT is relatively small as it is, and the visibility at maximum viewing angle would just be sub-optimal at anything less than full power.
The battery capability is more of a leisure than a necessity. It would just be nice if the user weren't confined to wherever there is a outlet.
Hi
Ok, not directly related to your application (I don't know what that is):
In some cases, a "people in range" sensor is used. One example is the IR sensor on your cell phone. It shuts off the display (and likely other stuff) when the phone is at your ear. Other examples are seen in other areas. The idea is the same, if nobody is there to use the display turn it off.
If the battery is a "luxury", how much is it worth to the customer? Are you better off with a battery option than a "everybody gets a battery" solution. In some areas, the gizmo comes in one rack box and the battery takes up a whole other box. If they want the battery, they order it. All of the gizmo boxes have chargers etc in them. Those who don't want the hassle do not have to deal with it. Even lead acid batteries are a hassle to ship ...
If you go with a rechargeable battery, consider the whole issue of recharge time (and heat). I have no idea what the turn around at the end of the 84 hours needs to be. A 24 hour charge may be fine. If a 1 hour charge to 50% is not acceptable, you are back to either disposable batteries or swappable battery packs.
Lots of details.
Bob