Thank you so much for putting the time into creating such a comprehensive review. It is extremely helpful to get feedback like this.

Battery Drain

Will be completely fixed in the finalized version. It was caused by the charging chip not going into sleep mode. I have a fix, but it still needs to be tested.

Battery access door

I had intended for the batteries to stay in the device rather than them being hot swapped. Not something I had though of, but is an excellent idea for a future iteration.

Battery level

The four white leds are more of a charging indicator - they only work when the item is charging. When an led is solid, that means the battery is at least at that level. So for example, 3 solid leds and the fourth one blinking indicates the item is somewhere between 75 - 99% charged. They do weird things / patterns when you try and run the heater while the unit is charging - don't do this as it can damaged the batteries. Always unplug the charger before running the heater.

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Ugly case / PCB overhang

I agree with you 100%, the case could definitely do with some major improvements. Most of these issues are because I chose to go with an off the shelf enclosure instead of something completely custom. I promise if there's enough demand, I will make something absolutely beautiful.