I was wondering if someone could help me to understand what is going on with my battery as it supplies a heatpad. I have an Arduino Uno running a couple of environmental sensors, LED's and a heating circuit (via a relay) for a high altitude balloon payload. The heat pad is designed to trigger when the temperature inside the payload reaches <3 degrees, it is supplied via a variable voltage regulator set to 5v output.
I've been testing the heat pad using a 9v Lithum battery claiming 1200mAh capacity. At a regulated 5v, the pad heats to around 29 degrees within a few minutes and then the temprature starts to fall off gradually but over a period of an hour when it reaches room temp again. The battery gets hot in the first spike of temprature and gradually cools.
Weight is an issue (Id rather not carry another battery) but I want to give the other batteries on board (for the arduino/cameras) the best chance by taking a little of the chill out of the payload, the overall flight will be about 2 hours but the internal temp will not always be below 3 degrees.
I guess the million dollar question for me is, can I squeeze more heat out of the circuit for a longer period of time by modifying any of the electronics? I'm using a non-rechargeable Lithium battery as I understand these are the better option for cold environment. I will wrap the battery in bubble wrap for the flight to try and protect it a bit from the cold.
I have considered using hand warmers but apparently they give off water vapour and I want to avoid any risk of condensation on the lenses.
Any suggestions gratefully received, thank you in advance.
The data sheet for that heat pad gives zero information as to current demand.
You need to advise what current it draws when you apply 5 volts to it. From there you can calculate power consumption and then determination of what battery capacity you need to achieve the desired running time.
Have you considered using 3.4 volt lithium cells and doing away with the step-down regulator. You also need some form of temperature regulator as the data 'curve' given suggests the pad keeps getting hot until it reaches a delta-T of around 40+ degC which is way above the temperature you require.
A single simple wire-wound power resistor could also be used as a heater module as all resistive power drop is expended as heat.
First, I would maximize insulation and air infiltration. What weight/size restrictions do you have? What sensors? Led's, why? What temp are you trying to maintain for what components?
Show your battery - converting 9V to 5 may b not efficient, your pad is taking to much current from that why your battery is geting hot = battery is overloaded,replace relay by MOSFET= minimum current consumption.
Use Heating Pad rated for 3.7V and 18650 battery - no voltage converter.
5V USB Power Bank - voltage converter in it is the most efficient.
Thanks for all your replies, lots to look into regarding the above suggestions (all of which at first glance seem to be a better set up than mine!). I'll have a go at all of these, many thanks.
The pad draws approx. 700mA of current according to a Sparkfun tutorial I watched. That was from a power supply at 5v rather than a battery.
So the very best you could ever hope to achieve with 100% efficiency = 10.8 / 3.5 = 3hours
Assume say 50% efficiency then run time might be in excess of 1 hour
If you are only getting a few minutes and your battery is getting hot then I suggest you either have a heat pad that consumes more than 700mA or you have a serious wiring error.
Do you need it to run full time to keep above your minimum temp?
If the heating pad provides a 40 degree differential over ambient, when you are only a few degrees below your min temp, you could use PWM with a very low duty cycle to save power. Then, when it gets colder, your duty cycle will go up.
How long is your flight? Typically our launch-to-land is about an hour, as the balloons are designed to rise til they burst. A 9 volt alkaline battery is sufficient for that time span. We no longer use the Sparkfun heating pads. Nichrome wire on a kapton taped enclosure works well. If you have access to a freezer that goes really cold, you can get a handle on the current needed to keep your particular package at the proper temperature (it's about -40 at apogee.) An insulating styrofoam enclosure is also good for protecting the stuff when it crashes lands.
How long is your flight? Typically our launch-to-land is about an hour, as the balloons are designed to rise til they burst. A 9 volt alkaline battery is sufficient for that time span. We no longer use the Sparkfun heating pads. Nichrome wire on a kapton taped enclosure works well. If you have access to a freezer that goes really cold, you can get a handle on the current needed to keep your particular package at the proper temperature (it's about -40 at apogee.) An insulating styrofoam enclosure is also good for protecting the stuff when it crashes lands.
Hey Chris, thanks for sharing your flight experiance.
In short, I'm not sure how things will behave during the flight as the freezer only takes me down to -18. The payload is in a polystyrene box, the elctronics and batteries are seated in foam pads that are velcro'd to the inside of the box. This has essentially reduced the internal volume and therefore air to be kept warm. If all goes to plan, I'm hoping there will be useful flight data on the SD card (assuming we can find the thing of course!).
I suspect I am over thinking/engineering this but I am keen to keep all elements of the payload working. The camera's and Arduino seem happy working at 4 degrees (about 90 minutes in the freezer). I have a TMP36 monitoring internal temp, this controls the relay that closes the heat pad circuit at a given temp (i'm thinking of 5 degrees as the trigger temp?). I think I am going to try a 3.7v 18650 large mAh battery on a cheap 3.7v heat pad and see what happens in the freezer. This would be a weight saving on my current set up as no voltage regulator and heavy 9v batt!
Quick question, is your nichrome set up on all the time? This will be my nexxt experiment :O)
vinceherman:
Do you need it to run full time to keep above your minimum temp?
If the heating pad provides a 40 degree differential over ambient, when you are only a few degrees below your min temp, you could use PWM with a very low duty cycle to save power. Then, when it gets colder, your duty cycle will go up.
Hi,
Many thanks for the suggestion, I'm not really sure how things will behave during the flight so it's difficult to know what to do for the best. I guess this is essentially a test flight so, assuming I get the payload back, I can study what is on the SD card and determine how to tweak things for next time :O)
ted:
put your toys in styrofoam box + thermostat no power from battery will be taken only a few minutes per hour = fridge effect
Hi, thanks for your suggestion.
That might work (will be able to tell when I get the temperature data back from the flight). The only problem is that the payload is imperfectly sealed as a fridge would be (two camera holes although the fitting is snug).
Working from peoples contributions from this post, I think I am going to go with the following:
3.7v heat pad (40 to 50 degrees of heat)
3.7v 18650 3000mAh battery (no voltage regulator required)
The isssue I have is no data about the current the heat pad draws but, assuming it's 700mA, that would give over an hour at 30% of the theoretical 100% (I think). More than enough formy flight as I suspect 30% is very conservative. Will test beforehand though!
Electronic components, transistors, IC ..... like low temperatures, for noise reductions they are cooled ....
TDA3020 work better at - 50 deg then at +100 deg
On north pole people are using cameras ....
Only the battery do not like cold so keep them worm.
I think your looking at the thermal requirements from the wrong end. The rate of heat loss through the insulated container is a function of the differential temperature between the inside and the outside of the container along with the thermal transmission coefficient of the insulation material. You do not need a heater that will produce 40 to 50 degrees of heat above ambient. Note that that phrase is totally subjective. You specify heat requirement in watts and once you've done the maths on the thermal losses from the container you'll end up with a figure for the number of watts you'll need for a differential of 40 to 50 centigrade degrees. Once you have the wattage, all you need is a simple wirewound resistor that will dissipate that number of watts when supplied with 3.7 volts. Of course that covers the worst-case temperature scenario so you will also need some form of thermal (proportional) control to to reduce the heating and so maintain a steady temperature within the enclosure if the external temperature is higher than the lowest expected external temperature.
Phil_Cox:
The isssue I have is no data about the current the heat pad draws but,
Put your styrofoam box with thermometer inside to the freezer and observe how much temperature is drop in 1 hour, then you can have idea how much power is need from heating pad.
Why not use a smaller Arduino, like a Wemos D1 mini, or even a bare ESP-12.
Power usage is about the same as an Uno, but in a much smaller size.
It's own power use might already keep it warm enough (smaller, same power, so running 'hotter').
Those modules also have plenty of onboard flash (SPIFFS), so the SD card can also go (assuming no video is stored).
With the right (very low drop) regulator, a 3.7volt LiPo should be able to power it for hours.
Smaller also means a smaller enclosure, or thicker insulation. Less heating, smaller battery.
Leo..
jackrae:
I Of course that covers the worst-case temperature scenario so you will also need some form of thermal (proportional) control to to reduce the heating and so maintain a steady temperature within the enclosure if the external temperature is higher than the lowest expected external temperature.
Thanks for this, puts things in a different light and I will test this too. I have found examples (and data) from payloads using a power resistor and a load controller, I'm guessing this is the bit of electronics that keeps an even temp rather than switch the heat source on and off when a trigger temp is reached i.e. less of a peak and trough profile with the load controller? I have seen one example of 8 degrees being maintained inside the box for 3 hours in a freezer at -18 (2w resistor). I'd be happy with 2 degrees for 2 hours.
