Powering small device on solar, but without battery? - Page 1
Forum section: electronics / beginners

author: Faranight
author_rank: Unknown
timestamp: April 24, 2022, 05:00:37 pm
content: Hello.I'd like to power a small DIY audio device on a remote location where there is no power, so I'll likely need to use solar.Yeah, I know I could just buy a battery and one of those 12V battery solar chargers, but that's gonna be overkill for this application. Also, I don't want to constantly cycle a battery or possibly over-(dis)charge it. The audio device is basically a small DIY "scarecrow" sound player that will be installed on a farm (middle of lawn). It idles most of the time, but also plays chosen loud noises over a speaker every once in a while to scare away certain animals and predators. It is powered at5.0VDCand consumes up to10 mAof continuous current, but while a short sound is being played on its speaker (2W speaker, 8 ohm, class-D amp) it will draw up to400 mAfor like2 seconds, then return back to 10 mA max @ 5V. The device doesn't have to operate at night, so absence of a battery is totally fine. It will simply cease to operate in the dusk and restart in the morning.What I'd like to know is how to power this thing properly.Obviously I can't connect a solar panel directly to it.And I want to have at least some power bank present to reduce power fluctuations.I had something like this in mind:Solar cells (i.e. 18V) => MPPT 9V => Super capacitor bank => Buck converter (5V) => BoardI thought about adding a supercap bank (At 9V??; not sure) so that when the device draws the 400 mA of current, it will draw them from the supercap bank, not the solar panel. And while it's not playing audio, the cap bank will have enough time to recharge even in cloudy weather.Thoughts?

author: SmallCog
author_rank: Frequent Contributor
timestamp: April 25, 2022, 12:07:24 pm
content: I’d grab an off the shelf solar lithium charge module, an 18650 or similar battery and then sort out some sort of low voltage disconnect and add in a boost module if necessarySome inspiration perhaps:https://youtu.be/baCLqPzuiF8

author: Doctorandus_P
author_rank: Super Contributor
timestamp: April 25, 2022, 07:02:34 pm
content: https://hackaday.com/2022/04/07/wireless-weather-station-gets-solar-powered-supercap-upgrade/Also, 10mA idling is atrocious. Idle current can easily be 3 orders of magnitude better

author: Faranight
author_rank: Unknown
timestamp: April 26, 2022, 06:33:30 pm
content: Thanks.I did some research on this. Apparently many supercaps only support very low voltages like2,7V, but there are some (reasonably prices ones) that go up to 8V. If I am to use cap voltages over 5V then I would need a buck-boost regulator to get the final 5V. However one little detail I noticed is that there are lots of buck/boost regulators that only work with input voltages down to 2V. This means I will get energy only from voltages above 2V up to whatever cap max voltage I am using. For a 2,7V supercap I would only be able to store the 0,7V difference. For 5,5V that's 3,5V usable voltage. Basically it means that 2 volts of storage is wasted.Luckily, I also found some boost-only regulators from TI that can work down to like 0,7V, which is pretty impressive. Some of them are even specially designed for supercap backup power likethis one, which is unfortunately out of stock on mouser/digikey/farnel. I'd still need to add a MPPT in front of it that does regulation down to 5.0V and have this act as a backup.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: April 26, 2022, 07:22:12 pm
content: It's important to realize that energy in a capacitor is quadratic with the voltage. When you start with 5V and discharge to 2V, then there is only 2*2/(5*5) = 16 % of the energy left in the capacitor.Supercaps are quite expensive and bulky, and even then can not store much energy.Maybe a LiFePo4 is a better choice.I'm not sure about LiFePo4, but often the number of charge cycles is an important limiting factor.Maybe it's a good idea to put in two batteries, and discharge one, until about 40%, and then switch to the other battery.Maybe it's a better strategy to keep one battery as a spare (at around half charge), and then only start using it if the primary battery has worn out.With either of these strategies, you should be able to get over 10 years of lifetime before maintenance is needed.