Low temperature design problems?

[Artlav]

I have an autonomous weather station sitting outside.
It's pretty simple - an AVR micro, an RF module, a battery with solar charger and a few sensors.
Everything but the sensors and solar panel is in an almost-sealed plastic box.


It worked fine throughout the summer.
But winter is approaching quickly, and temperatures drop down to -12*C at night already.
Apparently, this causes problems.

First time it died in the morning, when the temperature went below zero for the first time and the battery was almost discharged.
I changed the battery, let it warm up indoors, and all worked fine for a week more, surviving through as low as -12*C.

Lacking sunlight, the battery ran out again, so i brought the thing indoors, let it warm up, did some tweaks to reduce power consumption, replaced the battery and got it back outside.
It was -7*C outside. An hour later, it stopped transmitting.
In the morning i brought it back indoors, and apparently the radio module won't transmit, while everything else worked.
I was preparing to replace it, when suddenly it came back alive.

So, i packed it up, and got it outside again.
It's -3*C now, it haven't failed yet after a couple of hours.

The question is - what could have caused these drop-outs?
It seems almost random, and only started with the cold.
All the components are rated down to at least -40*C (record low here is -42*C, so good enough).
What else could be a problem?

[mikerj]

Are you certain it's directly temperature related rather than moisture from condensation?

[mzzj]

How much tolerance you have on battery voltage?
I see that you are using 18650 size li-ion battery. Battery voltage will drop in the cold possibly causing the malfunction.
Try running your box in a -20 cel freezer with external power supply to see how much headroom you have.

As a side note your battery is probably your weakest link and you should NOT charge li-ion batteries below 0 cel.
Charge : 0 to 45?
Discharge: -20 to 60?

http://batteryuniversity.com/learn/article/charging_at_high_and_low_temperatures
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Many battery users are unaware that consumer-grade lithium-ion batteries cannot be charged below 0°C (32°F). Although the pack appears to be charging normally, plating of metallic lithium can occur on the anode during a subfreezing charge. The plating is permanent and cannot be removed with cycling. Batteries with lithium plating are known to be more vulnerable to failure if exposed to vibration or other stressful conditions. Advanced chargers, such as those made by Cadex, prevent charging Li-ion below freezing.
----

[Artlav]

Are you certain it's directly temperature related rather than moisture from condensation?

It fails outside, not when it's taken inside, so there should be nothing to condense.
To the contrary, dry winter air should suck up any moisture from warm things, no?

How much tolerance you have on battery voltage?
I see that you are using 18650 size li-ion battery. Battery voltage will drop in the cold possibly causing the malfunction.

The regulator IC can work from as low as 2V.
The maximum current draw is about 30mA in short burst when transmitting, which is about 0.01C.
I don't think there is much of a chance for any voltage drops.

Try running your box in a -20 cel freezer

Would have been nice if i had one...

As a side note your battery is probably your weakest link and you should NOT charge li-ion batteries below 0 cel.
Charge : 0 to 45?
Discharge: -20 to 60?

http://batteryuniversity.com/learn/article/charging_at_high_and_low_temperatures

Yeah, i'm aware of that.
From what i could find, it is possible to charge li-ion below freezing at very low current (chance of lithium plating is proportional to the charge current and inverse proportional to temperature).
With 0.02C peak charging, i hope it would survive.
With 0.01C peak and 0.001C average discharge, i don't expect the cold-driven ESR increase to matter.

As this is a hobby project, i'm more interested in seeing what would happen.

[wraper]

That module looks like some junk with a blob on the pcb. It might be that some bonding wire from that IC to the pcb became loose.

[mikerj]

It fails outside, not when it's taken inside, so there should be nothing to condense.
To the contrary, dry winter air should suck up any moisture from warm things, no?

Any water vapour within the box would condense when the temperature drops past the dew point.  You said the box isn't completely sealed, so it's entirely possible that moisture will get in.  Are the PCBs suitably protected from moisture, e.g. conformal coating?   This is just a suggestion to rule out though, not saying this is the problem.

[Seekonk]

You don't have the power for this, but I saw a pump control that had to work outside in real cold.  They bonded a resistor to the top of the micro and powered it all the time.  That's one way to use consumer rated chips in severe environments.

[Niklas]

The battery at its voltage is one of the things that you must check as already pointed out.

On the PCB to the left, the one with the green wires attached, you seem to have a crystal oscillator. What is the temp spec for that? Far from all crystals will operate during cold conditions.

Is the current consumption constant or do you have some periodic, rapid increase of current draw?
If you are already on the limit, this can trip brownout resets

[SeanB]

Your capacitors could also be freezing, as they might only be rated for -10c to 70C operation. You probably have issues as well with the battery getting cold.

One solution would be to insulate the electronics and battery very well with polystyrene shehting so that the heat they generate in use keeps them above freezing.That might require a bigger box to get 4cm on polystyrene foam on all sides with no large cracks or ways for cold air to circulate. Wiring just can be placed between the edges. You will need to place the solar panel dump resistor outside, or use a mosfet to short the panel at full charge.

[wraper]

-7^o C is not the temperature low enough that even consumer grade parts normally would stop working. Try to bend that RF module. Cold spray wold be helpful too. That Junk RF module cold be at fault easily as the blob ic die and PCB where it is bonded have very different coefficient of thermal expansion.

[TonyStewart]

There was a day when al plastic IC's were only rated to 0'C because moisture ingress of insufficent sealant of the epoxy lead interface.  When the moisture froze the gold wire bonds were sheared. Nowadays, Sumitomo epoxy has improved.

Most RF transmitters use temperature compensated crystal oscillators  or TCXO's for resonators which control the Tx frequency. But then considering the unit had a LiPo battery, there was no thought to this unit design ever working below freezing.

[Artlav]

Any water vapour within the box would condense when the temperature drops past the dew point.  You said the box isn't completely sealed, so it's entirely possible that moisture will get in.  Are the PCBs suitably protected from moisture, e.g. conformal coating?   This is just a suggestion to rule out though, not saying this is the problem.

Moisture.
Moisture indeed.
I was able to reproduce the problem reliably by bringing the thing cold indoors for a minute, to let moisture condense, then getting it back outside.
It freezes, things stop working.

Nothing actually breaks, the radio just gets detuned.

First time it broke after an hour, i closed it indoors, so all the humid air condensed inside.
Second time it still works, i closed it outdoors after an hour sitting there open.

As far as i understand it, the box, not being airtight, would accumulate moisture inside of it with variations in pressure, temperature and humidity - easier to get in than out.
So, after some time it would start to condense and freeze.
Sounds plausible?

If so, would a coating of polyurethane help?
Or perhaps putting some silicagel desiccator into the box?

That module looks like some junk with a blob on the pcb. It might be that some bonding wire from that IC to the pcb became loose.

RFM12B, not the best, but i haven't heard much bad about it either.
Datasheet says it's rated to -40.

The battery at its voltage is one of the things that you must check as already pointed out.

It's one of the parametres being read out, and it's stable.

Is the current consumption constant or do you have some periodic, rapid increase of current draw?

Near-zero for most time, about 17mA bursts (1/4s) every 5 to 10 seconds.

Your capacitors could also be freezing, as they might only be rated for -10c to 70C operation. You probably have issues as well with the battery getting cold.

Aluminium polymer capacitors, rated down to -55*C with 1.25 Z ratio to +20. Should be good enough, i guess?

One solution would be to insulate the electronics and battery very well with polystyrene shehting so that the heat they generate in use keeps them above freezing.

Less than a milliwatt average?
Don't think that would do much.

[mikerj]

As far as i understand it, the box, not being airtight, would accumulate moisture inside of it with variations in pressure, temperature and humidity - easier to get in than out.
So, after some time it would start to condense and freeze.
Sounds plausible?

Very plausible.  Heat cycles cause the air to expand and contract, so any moisture in the outside air will be drawn in.

If so, would a coating of polyurethane help?
Or perhaps putting some silicagel desiccator into the box?

Conformal coating can help, as long as you can fully seal the vulnerable parts.  It makes rework a ball-ache though.

A big bag of silica gel will likely work for a while, but it will eventually get saturated.  Displacing as much of the air within the box as possible will help, preferably with insulating material of low thermal mass e.g. expanding foam mix.

[ciccio]

Maybe this is not the case, but I remember a ceramic capacitor in a crystal clock circuit that  below 5°C will not allow the circuit to start at power on. Replaced it with a film type and everything was ok. Before discovering the cause of the problem we were obliged to keep the unit constantly on.
Best regards

[wraper]

Any water vapour within the box would condense when the temperature drops past the dew point.  You said the box isn't completely sealed, so it's entirely possible that moisture will get in.  Are the PCBs suitably protected from moisture, e.g. conformal coating?   This is just a suggestion to rule out though, not saying this is the problem.

Moisture.
Moisture indeed.
I was able to reproduce the problem reliably by bringing the thing cold indoors for a minute, to let moisture condense, then getting it back outside.
It freezes, things stop working.

Nothing actually breaks, the radio just gets detuned.

There is nothing to detune by moisture/temperature except crystal oscillator. Try to change crystal, pcb accepts 3 types of them, so anything 10 MHz will fit. Clean the pcb around/below it. If the 2 capacitors located together are PLL related, then PLL might stop working normally too if the leak appears there.

[TonyStewart]

There was a day when al plastic IC's were only rated to 0'C because moisture ingress of insufficent sealant of the epoxy lead interface.  When the moisture froze the gold wire bonds were sheared. Nowadays, Sumitomo epoxy has improved.

Most RF transmitters use temperature compensated crystal oscillators  or TCXO's for resonators which control the Tx frequency. But then considering the unit had a LiPo battery, there was no thought to this unit design ever working below freezing.

Moisture has a dielectric constant of 60. Ice expands and so is only slightly less. So frost is not the issue.   It is simply poor stability with temp.
All SAW filters and Crystals have a tolerance and temperature stability error   That is the issue.,

... but there may be other mickey-mouse design errors if a used LiPo for a weather station and Lipo cant handle cold temps. Looking at this DIY project, it clearly wasn't designed for outdoor use.