You are thinking of what is known in prisons as a “stinger”, but to the rest of us as an “immersion heater”. Amazon has a “travel” version for just over $2, FFS. Yes, they work quite well. It wouldn’t be terribly difficult to add a thermostat and a little logic, so it achieves/maintains a selected temperature, but I prefer the far cheaper “dip a fingertip in it” test.
Here’s a good example:
Sorry, I wasn’t clear then, I want something that does not heat it, it should only register the temperature and chime when it reaches my desired, set, drinking temperature.
I think that’s a fine idea for a product, as long as the engineers can make it work without connecting to the internet 
In the meantime, however, I’ll pour the “freshly hot” contents of my mug into a second, room temp, mug-- tends to pull the heat out fairly quickly, especially if using a thick ceramic mug.
He likes to live dangerously. He pays $80 for a hand-wash only, battery powered mug, and washes it in the dishwasher.
Meanwhile, back on Earth, $8 will get you this:
https://www.amazon.com/Mr-Coffee-Warmer-Office-MWBLKPDQ-RB/dp/B000CO89T8
starcucked
The only way I would pay $80 for a new mug would be if I was the protagonist from Darkman.
i bought this sucker years ago and found it too terrifying to actually use regularly.
the positive retention from the rings is nice but its not something you want near your face
I fail to understand exactly how he got conned by in store advertising/displays.
Dude. It’s not a trick. It’s in all the manuals.
Cut out the middleman: force the baby to mine bitcoin.
I can’t wait until the Internet Of Things utopia breaks and everyone has to put their coffee in the microwave to heat it up again.
If you’re into tech reviews and ASMR, Dave 2D is your guy.
he bought an overpriced negative entropy machine, what did he expect was going to happen
No temperature control on that one though 
ive seen photos of the spaces above mining rigs turned into racks for drying fruit and jerky
I bought one of these the moment I heard about it, and didn’t think twice about the price. And I love it. Hot plates don’t work nearly as well. I don’t take a sip every 45 minutes, but without this it will definitely cool off too much while I drink it (maybe I consume the whole thing in 20 minutes?)
I agree with the reviewer that it doesn’t hold enough, and washing is a bit of a hassle. I haven’t had any problems with battery life (I use it twice a day, recharge for a few hours between the first and second use, and I’m good).
For me, it’s (nearly) perfect.
"I don’t know how to explain this, but that Bluetooth is coming from inside your own house!"
I’d pay $80 for a beer mug that kept my beer at a specific temperature.
2nd law of thermodynamics be damned
