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I live in a place where the temperatures can range from -20°C to +40°C in different seasons. I need to power a small micro-controller that uses very low energy with a solar panel and a battery. Which kind of battery is more suitable for this work? It must have the following characteristics:

  • It should resist to temperatures from -20°C to +40°C.
  • It's not important to store a lot of energy, 500mAh will be fine.
  • It should output at least 5V. If less I will have to put two or more batteries in series.
  • It should not require maintenance.
  • It should live for a decade or more.
  • It will be pretty always charged since the solar panel will be directly connected to it (or maybe, via software, I can make some cycles of charge/discharge).

which battery do you recommend?

Explanation of the project

I'm following an IoT course at my university, and I'm free to propose a custom project that will be part of the final exam. I thought about a series of small boxes that will allow us to check if a parking slot is free or not. The box will be build with my 3D-printer and it will include: a small solar panel with a maximum power of 9V * 50mA in the best case (size 60x80mm); a battery of at least 500mA that will be charged by the solar panel; a proximity/distance waterproof sensor that will check if a car is parked on the slot; a MCU with WiFi to send data to an other sensor or to a master with a raspberry pi. I will take inspiration from internet for the routing of packets through this "network" of boxes. Not all boxes will be directly connected to the master box.... We are already experimenting plastics to build a box strong enough to resist to the wheels of a car. The solar panel will be protected with a non-scratchable piece of plexiglass. Sensors will be cleaned from leaves and earth by rain, by road-cleaners and by the wind. Our roads are very clean. The major problem we are facing is to choose the best battery to be used in this "extreme" application. In winter, at night, the temperatures can drop up to -20°C while in summer, during sunny days, it's possible to reach +40°C. Finally, we will develop an App that will give you the nearest free parking slot.

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    \$\begingroup\$ What is a lot of energy? How much current is required? What voltage do you need? Better requirements are needed. -20 is pretty cold, will you need to run a heater? \$\endgroup\$ – Colin Sep 24 '18 at 13:58
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    \$\begingroup\$ Welcome to EE.SE! Keep in mind that questions about optimization (i.e., "What is the best ...?") require a definition about what problem dimensions are to be optimized for your application, such as size, speed, energy consumpation, user experience, etc. Since these can't be optimized all at once, you need to have a good idea of which ones are most important to you, and be able to articulate that clearly to us. \$\endgroup\$ – Dave Tweed Sep 24 '18 at 14:03
  • \$\begingroup\$ @Colin I’ve updated my question \$\endgroup\$ – Luca Di Liello Sep 24 '18 at 14:07
  • \$\begingroup\$ This article says "Currently, lithium-ion batteries stop operating around -20° Celsius." \$\endgroup\$ – CrossRoads Sep 24 '18 at 14:10
  • \$\begingroup\$ Is -20°C storage temperature or will it also be charged and discharged? \$\endgroup\$ – Alexander von Wernherr Sep 24 '18 at 14:11
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Without knowing your exact requirements, I would recommend you Ni-Mh AA batteries with nominal voltage of 1.2V (e.g. Eneloop brand) The standard ones have operating range down to -20deg C and they have very high energy density. But attention: At -20deg C you need to fine control the charging voltage and current. See this site for details: batteryuniversity.com

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  • \$\begingroup\$ I've updated my question with details \$\endgroup\$ – Luca Di Liello Sep 25 '18 at 6:27
  • \$\begingroup\$ Given that probably with very low temperatures there will no be solar power to charge the batteries, at the moment your option is the best. thanks \$\endgroup\$ – Luca Di Liello Sep 25 '18 at 11:14
  • \$\begingroup\$ Thanks. But if you do not intend to use rechargeable batteries, then primary cells like Li-iron-disulfide (energizer) would be even better for your project. \$\endgroup\$ – Stefan Wyss Sep 25 '18 at 15:13
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Sounds like you need a couple of alkaline D cells. Run your micro off solar when the sun shines and off the batteries at night. 10 years might be pushing the life of the batteries but not by much.
You can isolate the batteries with a Schottkey diode.

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  • \$\begingroup\$ Rechargeable alkaline batteries? \$\endgroup\$ – Luca Di Liello Sep 24 '18 at 18:36
  • \$\begingroup\$ @LucaDiLiello, not rechargeable, but "alternate power", solar when sun shines, battery when not. Saves battery life. \$\endgroup\$ – Ale..chenski Sep 24 '18 at 19:13
  • \$\begingroup\$ 10 years is 88,000 hours. Waking up every 30 sec with 10 mA consumption for 1 s gives you, say, 0.3 mA average. Assuming 30% solar, you will need a battery of about 18,000 -20,000 mAh, which is about a good D-cell capacity at this discharge rate. So this might work. After doing this estimate, +1 \$\endgroup\$ – Ale..chenski Sep 24 '18 at 19:26
  • \$\begingroup\$ It’s a good idea, but there is no space for 4 D cells. Moreover, i think that waking up, reading a sensor and send the result via wifi will take more than a second, maybe 2 or 3. \$\endgroup\$ – Luca Di Liello Sep 25 '18 at 5:55
  • \$\begingroup\$ @LucaDiLiello Depending on the MCU and your network, it may be a lot more than 2 or 3. Hint: don't use DHCP. \$\endgroup\$ – jcaron Sep 25 '18 at 12:17
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Your combined requirements

  • It should resist to temperatures from -20°C (to +40°C.)
  • It should live for a decade or more.

sort out any standard accumulator but lead. Lithium may reach this but I personally won't count on the quality of the cells I could get my hands on. Car batteries of high quality are in contrary available in the next car parts shop.

As an alternative, you may want to oversize your accumulator and put it into an electrical heated, insulated compartment. Also good for the electronics. Condensation is hell.

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  • \$\begingroup\$ I've updated my answer......this is the reason why it should live for many years without maintenance \$\endgroup\$ – Luca Di Liello Sep 25 '18 at 6:28
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    \$\begingroup\$ I understand what you are trying to do, but these two items from your requirements are among the hardest things to do in engineering. They give 50-year practice engineers an uneasy feeling and the need to do more testing, using a climate chamber and doing a lot of calculations just for that. And still it could fail in a year without warning because of some thing you haven't thought about. You project is complex. You have hundreds of such pits you haven't thought about yet. \$\endgroup\$ – Janka Sep 25 '18 at 11:52
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    \$\begingroup\$ The reason why most of these "maker" style things haven't been rolled out isn't nobody thought about it yet, but poor reliablity (if poor cost performance doesn't suffice to cancel it). Don't betray yourself. If you want to create such a thing, declare it art. It's allowed to fail then. People may share your dream and not complain about the sillyness. \$\endgroup\$ – Janka Sep 25 '18 at 11:54
  • \$\begingroup\$ I'm doing this project for an university course and probably (or for sure) it will never be implemented. Despite this, it will be very interesting to create such a simple plug-and-play box to monitor parking slots. You are right, there are many difficulties to face wrt the batteries and the temperature, but I'm sure that in some years there will be new technologies available. At the moment, I will only succeed in building a first working prototype. \$\endgroup\$ – Luca Di Liello Sep 25 '18 at 12:33
  • \$\begingroup\$ Moreover, I'm a student of computer science and this electronic problems will not be so important. But it should work.... \$\endgroup\$ – Luca Di Liello Sep 25 '18 at 12:38

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