Ask HN: Most interesting tech you built for just yourself?

Oh, I think I have a good one. I had an HP-25 calculator as old as myself, and couldn't use it. The original battery pack contained two sealed NiCd cells, which obviously failed many years ago. Most people replaced their NiCd cells with new ones, then with NiMh cells, or even alkaline AA batteries. This was always problematic: newer batteries were slightly larger and never fit well. Also, the power consumption of a calculator with an LED display was significant, so frequent battery replacements were needed. And the original HP charger was risky and could easily destroy the calculator.

So I designed and built a wirelessly (Qi) charged battery pack for it.

https://partsbox.com/blog/wireless-charging-for-a-hp-25-calc...

After a year of use, it's totally over-engineered and has so much energy and so little idle power consumption, that I have to remind myself to charge sometimes, the thing lasts for months.

I'm the only user. There are many people who wanted to buy one, but the step from a hobby design to small-scale production is a big one and it simply doesn't make business sense. Especially with Li-Po batteries being difficult to ship and potentially a hazard. I guess maybe if I found a manufacturer that would be willing to take the design and manufacture it on demand, taking over all of the shipping/support issues…

The HP-25 was a hand-held programmable scientific/engineering calculator made by Hewlett-Packard between early January 1975 and 1978.

Wirelessly charged battery pack. Whhhhhatttttt

I love this. I used to have a bunch of things that would benefit from a project like this.

There probably are some contract manufacturers that'll handle all that, but they'll need a decent sized minimum order.

I doubt anyone would front the money to produce 100k of them (to recover the cost of molds) unless you literally give them the design fro free.

Maybe there are smaller companies using 3D printers willing to take on a 1k sized order?

Love this.

Semi relatedly, the reason I chose my current smart watch is that it can go weeks without charging, I simply don’t worry about it. Yes it has a limited number of colors and I can’t browse the web on it or w.e. but that’s a feature.

Maybe I’ll seek out a phone with similar battery use profile…

My HP-29C could use something like that. I have it on the shelf beside me I use AA batteries and the seem to have both a good shelf life but I do now about life span because I don't use it much but when I do it still works.

What's your current smartwatch?

Li-Po isn't difficult to ship. You just slap a certain sticker on the outside of the box and pay a little extra. Otherwise, it's no more effort.

LiPo is an absolute nightmare to ship in nontrivial quantities. The trickiest part is all the paperwork. Shippers really do not like it, because it places a lot of extra restrictions on the way they are allowed to ship it. This is made 10x worse when you are trying to ship bare batteries.

This is also why companies like Sparkfun don't sell batteries internationally. Adafruit used to have this restriction too, but they seem to have recently changed that.