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I'm thinking about using a solar panel to extend the life of an IoT device. However, I'm questioning if this will give me good results or if it will only be an expense that wouldn't give any benefits.

I'm thinking about using eight 3.7V 2600mAh 18650 Li-Ion batteries in parallel connected to a solar power manager circuit (this one, with MPPT) that provides up to 900mA to all batteries (112.5mA to each). Given that the Charging Factor (CF) would be very small (112.5/2600 = 0.04C), could it be harmful to the batteries? Maybe it reduces the number of cycles, thus their life span? Or perhaps it has even other negative implications that I'm not seeing?..

Also, I notice that many IoT devices that use both batteries and Solar Panels to power up their circuits usually use less batteries (e.g. one or two ~2000mAh cells only). Is it for an expensive reason? Sparing 2 or 3 batteries for example is really that money-saving? Or maybe they do it to keep the CF higher? if so, why is it necessary?

Thank you in advance from a rookie hardware developer.

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  • \$\begingroup\$ The only downside I can think of is that most datasheets call for 4.2 V and 0.1 C cut off current. If you are always below that you may need to prematurely terminate charging and “only” reach 98 % SOC. \$\endgroup\$
    – winny
    Dec 6, 2021 at 16:04

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You can't damage battery by simply charging too slowly, at least I'm not aware of any direct negative consequences.

As for battery capacity vs charging rate, your total capacity is 20800mAh@3.7V, which makes it 77Wh. So your charger will need 77Wh/5W = 15.4h (all are rough estimates of course, talking about ideal components) to charge the whole thing. More than a day from the sun unless you live in Norway or something and the sun doesn't go down in summer.

You never mentioned intended power consumption, and I believe it's the key to determining if this kind of setup makes sense at all.

Battery is supposed to be a consumable. They lose properties over time, and they live the longest if they're kept in the range of 50-70% (basically, around nominal voltage, since fully charged they are ~4.2V). Also, as they age, they become more prone to various failures (stop charging, significantly lose capacity, start leaking - discharging on their own, etc.).

As you mention your devices will also be partially powered by the solar panel itself and not from the battery, and since you're talking about IoT, which is generally low power (but please specify!), I doubt a setup with such high capacity is necessary, or can't be sure at least. If you have too much capacity, you will have a lot of batteries ageing while being barely used and spending much time outside most "comfortable" voltage range. At the same time, almost all our devices also spend most of the time on the full 3V-4.2V range, and the world hasn't crashed.

However, from practical point of view, I doubt you will notice much difference even if capacity drops. The real question you should ask should be "how long do I need for my devices to be able to run autonomously even without solar"? Calculate necessary capacity from consumption, add a little more to compensate for ageing (to your taste), or potential increased consumption due to future modifications/additions. As for charging with small current, it should be no problem in itself.

I fix laptops, see all sorts of dead batteries a lot, but I am not a battery specialist.

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