Hi guys,

I'm new here and I searched on the forum and I didn't find anwers to my questions so I decided to start a new thread.

I want to make myself a home made warmer to keep my regular size coffee hot for a long time. A couple hours would be great. I know only the basic stuff about electronic so I'll try to follow you as best as I can! And I know there is plenty of other good solution to keep a cup of coffee hot but I like a little challenge from time to time!

So here is the way I see my project: I would use a resistor as the heater that I'd drop it at the bottom of my coffee. Of course, it would need to be waterproof while keeping good heat dissipation. I'd like it to keep the coffee around 55 to 65*C (130 to 150*F).

I read that I could use a diode in the coffee that would trigger my power on and off between 55 and 65 or I could use a thermistor but i'm not exactly sure how i would have to use that last one.

I'd run wires from the heater element to a battery outside of the cup. I want to use small rechargable batteries like 3 or 4 AA or AAA or a Cellphone battery but I don't know what would be best.

It's been a couple of days since i'm trying to figure out what sort of hardware I gotta buy on websites like digikey and others but I really don't know what I need.

Maybe someone here could give me advice on the kind of setup I need to use and buy? or how I can choose the right stuff?

Thank you!

Alex

 

Insulation is your friend. The less heat that escapes, the less your batteries need to supply. That's important, because offhand I'd say you have little chance of getting enough heat from the batteries to keep a normal cup of coffee warm. If it's really well insulated, maybe. For sure you'll want a cover, like a fast-food cup.

For convenience, I really think you're going to want to heat from the bottom of the cup and not use an immersion heater of any kind. Every time you take a sip, you don't want to fight with reposition a heater.

A simple heater is a lightbulb, or several of them. The visual feedback is great, and they're designed to handle the heat. You probably won't even need a thermostat if you can, with experimentation, get the amount of heat set right. I envision a switch that shuts them off when you lift the cup off the heater, and turns them right back on when the cup is replaced.

Just thinking out loud.

 

...I'd say you have little chance of getting enough heat from the batteries to keep a normal cup of coffee warm....

+1, I think you will spend more on batteries than coffee, or more time charging batteries than making coffee.... Either that or you'll need a battery pack the size of your cup

I saw a USB powered one somewhere - a heating pad the cup sat on from memory....

 

The solution might be a different form of chemical energy. Instead of battery, maybe a pack like they use for applying to sore muscles. It's a super-satured solution of a salt, and you initiate crystallization with a little clicker built into the bag. It then throws off a lot of heat at a constant temperature until it's fully crystallized. Then you microwave it to regenerate the solution. They're not as hot as I like my coffee, but a darn sight better than room temperature.

 

How about a propane burner, the camping type. I have single burner Coleman model that works great and is very versatile.

 

----> http://bulkpurchase.in/Upload/29066249314206786573539.jpg

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----> http://ecx.images-amazon.com/images/I/51N0xrtiKWL._SX425_.jpg

----> http://www.dhresource.com/albu_790243264_00-1.200x200/cute-cartoon-rabbit-usb-powered-coffee-tea.jpg

 

Thanks for all this help guys!

But! As a real stubborn person I really want to try my concept! As I said, I know there is plenty of other ways to keep a cup of coffee hot. But most of the system I see is 12v or USB powered but nothing is cordless rechargable. And I'd like to know how much time a small battery would last. The best way to know would be to give a try to the concept...

So my question is still there... How do I choose the right heater element and the right battery to have the best heating/batterylasting possible? And where do I buy this?

Thanks again all!

 

If you insist:

http://www.minco.com/Heaters

Start here.

 

Nothing wrong with being stubborn and having a go, that is how we learn. But I thought I would have a go at some numbers and this is probably just a demonstration of how bad my maths is:

The specific heat capacity of water is 4.18KJ/Kg/K, taking a cup as 250ml, call it 1KJ/K for a cup of coffee. Assume the temperature drop is 0.05K/s (i.e. 3 degrees per minute - this is just a wild guess) then you need 50J/s input, which is 50W. i.e. think of running a car headlight bulb for a few hours...

Perhaps that place to start is with some measurements of actual volume and temperature loss over time to get a better handle on the power requirement.

 

why not modify a hand warmer? there are chemical, catalytic, and battery powered , as well as battery powrered socks to keep your feet warm, something like that should keep coffee warm.

 

... most of the system I see is 12v or USB powered but nothing is cordless rechargable.

Hook it to a rechargeable battery.

And I'd like to know how much time a small battery would last.

"small" means nothing.

So my question is still there... How do I choose the right heater element and the right battery to have the best heating/batterylasting possible? And where do I buy this?

Search portable tea warmer and vendors will show up. The "right" heating element depends if you want it at 99C or at 40C, the ambient temperature, and the amount of coffee; and if it is 5V or 12V.
The "best" heating/lasting could end being a motorcycle battery, or a laptop pack.

 

I like the idea of hacking a heated sock. Insulation and heating in the same package.

The estimates of heat flux by sirch2 may be overestimated, but the approach is right. For whatever cup you choose, measure temperature vs. time for a known quantity of water. It takes one calorie to heat one gram of water by one C°. So the slope of temperature vs time - while near your desired temperature - should allow you to calculate calories per minute, and ultimately watts. Multiply watts by the time (hrs) you want to maintain temperature, and you'll know how much battery capacity you need, in watt-hours. A typical rechargeable AA battery delivers 2500mAh at ~1.2V, or 3W-hrs. As we've been warning, 3W-hrs isn't much heat.

 

externet, small may mean nothing, but smaller batteries have smaller power storeage capacity.

 

How much power does it take to keep 8 oz of coffee @ 140 deg F?
Started with insulated cup holder, about 1 cm of polyethelene foam, paper lid @ 140 deg. 4 AA Ni-Mh not fully charged driving 3 33Ω, 1/2 W in parallel, temp falling. Switched to 12V supply & rheostat giving 7 V across 11Ω, 3-33Ω; after 1 hr, 140.5 deg. So about 4.5W about right. Will try battery after charging.

 

How much power does it take to keep 8 oz of coffee @ 140 deg F?
Started with insulated cup holder, about 1 cm of polyethelene foam, paper lid @ 140 deg. 4 AA Ni-Mh not fully charged driving 3 33Ω, 1/2 W in parallel, temp falling. Switched to 12V supply & rheostat giving 7 V across 11Ω, 3-33Ω; after 1 hr, 140.5 deg. So about 4.5W about right. Will try battery after charging.

Thankyou! I'm even kinda jealous you tried before me! haha!

So I tought I could maybe try with a battery like that:

http://www.all-battery.com/li-ion1865074v3350mahbatterypcbmodulewith24awgbareleads-34042.aspx

They look like a nice size and if I compare your datas to what that battery can push I think it could be a good combo!

So, if my calculation are good... 4.5w at 7v makes 0.643amps and as per digikey's website:
http://www.digikey.com/en/resources/conversion-calculators/conversion-calculator-battery-life

3350mAh at 642mA would give me 3.65hr battery life

Correct me if I'm wrong please!

Also,
I'm wondering what kind of heating element you tried with? And where I can find those elements?

Thanks again all!

 

For the test, resistors were directly emersed in water, not so good for coffee as water was turned a dirty brown. For a small fish bowl heater, I glued resistors to an Al plate. An external heater will be considerabley hotter than imersion, so proper insulation is necessary, glass cloth??
Maybe a " tin cup" with open ended handle to fit into holder.