Yes, tonight I was curious what the charge got up to. Unfortunately I got too busy to check it directly after the sun went down. Roughly 6 hours after the sun was shining on the panel, it was still -38c I was getting 4.9v. So I have to assume they took a full charge. Today I would say it only got 3 good hours of direct sunlight.
And since I had it open, I cycled the power on the TLora and it connected to my phone via Bluetooth. I was able to then send a message out to the TBeam inside the house. So as for functionality, it seems all the electronics are still performing even at -40C
As for reducing the humidity, nature is doing a great job at that. I usually just place towels on the windows to soak up the water as it warms up. It usually only last a few weeks. Im excited its finally this cold, it means spring is close by.
@satkiwii I want to do similar testing (not as extrema weather, but still freezing temperatures) but I want the node to report the battery voltage in a message once an hour. I think it is a better over all test of the device and network, gives more data to chart against my weather station, and doesn’t require me moving the device and connecting the multi meter.
I believe someone in the forum built a voltage divider for the TLora, and was succesful at reporting back a voltage on the screen. I havent done that because it does leak current to ground, which would drain the Capacitors. But if you were to double the capacitance from what I have, it shouldnt drain enough to be a problem. The other option is to use a TBeam, it already has this ability to report back the Voltage. I have removed the battery holder on the TBeam and successfully provided 5.8V in place of the 18650. Once it gets over 4.2V the Tbeam reports %100 battery.
1 down side I have found with using the TBeam, the battery management shuts down the TBeam on low voltage at less then 3V, so your losing a lot of your capacitance only working in 3v to 5.5v vs 1v to 5.5v
This is why Im using a boost converter to maintain a constant voltage. But as I said you can for sure build a voltage divider and report the volts of the Capacitors back.
Also, once the TBeam powers down on low voltage, you have to physicaly press a button onboard to power it back up. The other option would be to build a watchdog circuit where once the voltage comes back up you pulse a 5v relay to break the connection to the capacitors on the + when it reconnects it powers up. But you have to physically break that connection. However, all of this requires more power to operate.
If some are wondering why I am not using the USB on the chip, I noticed you are limitted to 500mAh of charging. So you require longer period of sunlight. Where as charging directly from the MPPT to the capacitors allowes for the max of ~2Amps
I remember someone said the heltec don’t have a low voltage cut off bad for batteries but maybe ok for capacitors, I wonder if it would be a better option for you. Eventually I want to find the cheapest device, likely one without a screen or GPS, that has voltage monitoring build in.
Hi, due to cold temperatures in the northern hemisphere at the moment we realise how much the performance and lifespan of solar nodes are affected from deep temperatures. The danger zone begins when the temp drops below 0 C and your controller tries to charge the battery:
" Doing so even once will result in a sudden, severe, and permanent capacity loss on the order of several dozen percent or more, as well a similar and also permanent increase in internal resistance. This damage occurs after just one isolated ‘cold charging’ event, and is proportional to the speed at which the cell is charged.
To solve most of the problems in this topic I suggest you take a look at this: LiFePO4wered/Solar1 | Hackaday.io …charge controller with heating mode I found on Hackaday. I’ve ordered this controller in combination with a few TBeams, 2 LiFePo4s and a 6W (MPP at about 17V) Solarmodule. I will share my success with this controller, and my setup as a remote solar relay. I’ve considered wrapping the cells in insulation and heater foil and let the controller to its work high above the ground sticked on a tower with magnets. My only concern is that the controller keeps heating and heating and it never comes to charging the batteries - what do you think?
I think, thats why I went with the Capacitors. Although, if a battery did explode and catch on fire, it is winter time and the extreme cold is going to prevent a fire in the bush. Though no one ever wants to find a melted plastic enclosure anywhere.
Battery would be smaller and.light weigth, where the capacitors are larger and you need quite a few. But the trade off I think is worth it. A Maintenance free device without the worry of a fire just because its cold.
I’m not really worried about degrading cheep 18650 cells. And my test device is placed in an area with minimal risk of fire spreading.
From what I’ve seen the very common garden solar lights, and solar flood lights do not contain freeze protection circuits, and yet we do not have reports of these devices exploding in winter (or summer after freezing in winter and being charged).
This was done with what ever I had laying around. i have had 2 up for a year, one i
would take down to update and the other one I just left up, no updates or anything. mostly cause I didn’t want to take it down. if a OTA upgrade failed, i was just testing for longevity. well to see if it would last all 4 session in the Midwest they did fine, one went down during the winter for a couple weeks I just figured it was dead didn’t bother taking it down cause it was cold but it did come back up. The materials are as follows. 1X enclosure should probably be Fireproof unlike mine , 1X 5V 1A 18650 Lithium Battery Charging Board. 2x 2.5W 5V/500mAh Solar Panel. and Old batteries. one had 2 5500mAh batteries and the other had 1 6500mAh. This was difficult to post after seeing satkiwii Sweet Build but here it is
It was working well when it was cold out. I just left it outside. Then spring came and it got placed on a shelf in my garage. When the outside air temperature is above 0, I dont spend too much time inside. So I have not been on my computer much either. Since I saw your question, I grabbed it and placed it in the sun. Let us see if we can charge those capacitors from a complete discharge by only placing the panel in the sun. Time will tell. I also have not had a chance this year to get out to do any real world trials. Its been a busy summer.
As for costs… I was getting the capacitors in a package of 5. They do not seem to be easily accessible today. But it looks like the protection circuit is ~3$, the capacitor is ~10$ the MPPT charge controller was ~35$ and the USB boost I think was the cheapest ~$2. I already had the panel, which I got from a trash bin. I think I have spent ~135 USD. Seems expensive, but it was a good little project to keep the mind occupied when one does not want to be outside.
@satkiwii That sounds great. I’ll be awaiting your test results!
Also, I found an old garbage dump hill that would allow me to bridge the gap between cities, so I can link up with my brother’s family and their pre-teen kids. Now I just need to get a working setup and of course, permission.
It’s an old topic but how are your builds going? I have had plans for an outdoor repeater for quite some time but charging @ down to -35C is a challenge here as well. Supercaps are still expensive and not really optimal, i was again looking at battery options and LTO cells seem decent and can be charged at down to -40C. The voltages are not as common (2,8v max charge voltage) but three cells match the charge voltage of a 2S Li-Po (8.4V), and there’s cheap buck charger boards for that. What do you guys think? Have you already had a look at such a setup?
Welcome! And I urge you to come join the Discord. We have folks working on both LTO and automatic heated battery solutions. For the latter, some have had great success in having a second solar panel strictly to power a heating element and is controlled automatically with a bi-metal temp switch set to a few degrees above freezing.
i dont have the very-low temperature problem here,
but i just found these:
specs say, they deliver at -40° (maybe somewhere available but not very cheap, cheaper than a heater )
if combined with a not too big solar panel for very slow charging, this would be an easy option.
maybe an installation where SUNlight also warms up the battery a little bit when delivering Solar electricity at the same time
My latest LTO charger iteration arrived and it’s working well. It supports multiple battery chemistries including: LTO, LiFePo4, Li-ion and 2S LTO. 2S LTO will need a balancer which I have some ideas for.
This board handles charging and solar input but it doesn’t really do anything else. I’m currently designing a regulator “hat” that can be mated to the board to extend functionality. The regulator hat will include overdischarge protection for 1S LTO, INA219 for battery voltage readings, and a buck/boost chip. The output voltage will be selectable at 3.3v, 4v, or 5v.
The two boards together will make a complete solution for solar charging LTO batteries while providing a stable/usable output voltage.
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