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it's actually 3.7, just 4.2 when fully charged. same nominal voltage as the lithium coin cells.
The screen is 1.7", so that makes each side about 1.2" of the screen, so we'll be generous and give it a 1.5" square. call it 40mm. You need the o-ring, and sealing faces for the outer shell and all that. A circle then needs to fit inside of it, so with threads and another o-ring, the largest diameter you can get is around 30mm. Largest square inside of that is 21mm. For reference, a standard 400mah flat battery cell is 50mmx22mmx4mm. So to get it in there, you'd have to double the thickness if you can get a battery mfg to make a custom cell and now you have 8mm of battery, plus at least 3-4mm for the door, and then the rest of the watch. would be an inch thick. you gain nothing. They make a 25x20x5mm lipo battery that would fit, but again thickness is a problem, and the capacity is 170mah, so you lose a lot to gain a less reliable design and convenience for something that doesn't actually matter in the real world.
If the battery lasts 2-3 years and you have to pay $150 to replace it, so what, it's not the end of the world. With Scubapro, price may actually come down, who knows, but the price you pay for a watch sized computer with a screen like that is you have to use a sealed design. Huge part of the reason seabear was the first to do it, it's not easy to do, otherwise the other guys would have done it years ago and the reason they're still using the casio style screens. They take almost no battery power so they can use tiny batteries and have a long service life. If you had to change the battery every 15 hours, it would get real old real fast and you'd be at an infinitely higher risk of a flood than having to pay $150 every 2-3 years at the worst case to replace the battery
The screen is 1.7", so that makes each side about 1.2" of the screen, so we'll be generous and give it a 1.5" square. call it 40mm. You need the o-ring, and sealing faces for the outer shell and all that. A circle then needs to fit inside of it, so with threads and another o-ring, the largest diameter you can get is around 30mm. Largest square inside of that is 21mm. For reference, a standard 400mah flat battery cell is 50mmx22mmx4mm. So to get it in there, you'd have to double the thickness if you can get a battery mfg to make a custom cell and now you have 8mm of battery, plus at least 3-4mm for the door, and then the rest of the watch. would be an inch thick. you gain nothing. They make a 25x20x5mm lipo battery that would fit, but again thickness is a problem, and the capacity is 170mah, so you lose a lot to gain a less reliable design and convenience for something that doesn't actually matter in the real world.
If the battery lasts 2-3 years and you have to pay $150 to replace it, so what, it's not the end of the world. With Scubapro, price may actually come down, who knows, but the price you pay for a watch sized computer with a screen like that is you have to use a sealed design. Huge part of the reason seabear was the first to do it, it's not easy to do, otherwise the other guys would have done it years ago and the reason they're still using the casio style screens. They take almost no battery power so they can use tiny batteries and have a long service life. If you had to change the battery every 15 hours, it would get real old real fast and you'd be at an infinitely higher risk of a flood than having to pay $150 every 2-3 years at the worst case to replace the battery