[HN Gopher] SolarPi experiment 2: Finally something that works ___________________________________________________________________ SolarPi experiment 2: Finally something that works Author : marbu Score : 28 points Date : 2023-08-21 20:57 UTC (2 hours ago) (HTM) web link (blog.rfox.eu) (TXT) w3m dump (blog.rfox.eu) | dragontamer wrote: | 1. Lead Acid is far cheaper, though its bigger and heavier than | LiFePo. 12V @12Ah is only $35 from a brand-name | (https://batteryinthecloud.com/products/ps-12120), and closer to | $25 from no-name brands on Amazon. | | 2. Modules to charge LeadAcid are so cheap, they don't even make | them. This specified PS-battery hsa a 13.5V to 13.8V "standby" | voltage, meaning that 13.65V from the Solar Panel is all you need | to have a UPS. Connect the 13.65V source from the Solar-panels to | the + and - leads of the 12V battery, and volia. You get 13.65V | when solar is available, and 12V from the battery when the solar | cuts off. The end. Isn't that easy? | | 3. Grid-tie should be similarly easy, though I don't have too | much experience with it myself. IMO, buy a professional AC->DC | converter, probably at 19V or some other suitable voltage and | then get a DC-DC buck converter to go from 19V to 14.35V, and | then a diode (0.7V dropoff) to hook up to your batteries in | parallel. You'll also need a diode in your Solar-panels cause you | don't want your Grid-tie system "charging your solar panels" | (that'd probably create a fire and/or damage them...). | | Hobbyists should NOT deal with main-power themselves, but there's | a gross-many number of AC-to-DC converters available from $10 to | $40. | | 4. Don't battery balance. Just buy bigger batteries. If 12Ah | isn't enough, buy a 20Ah battery. If 20Ah isn't enough, buy a | 33Ah battery, etc. etc. The limit is whatever you're comfortable | with (the bigger batteries give more current which can be more | dangerous) | louwrentius wrote: | Don't do lead acid. I say this from experience. Lead acid | charges too slow. You want a battery that can capture every ray | of sunshine - especially with intermittent sunlight - and | charge at full speed. Lifepo4 can do this. Lead acid can't. | dragontamer wrote: | A 12V 12Ah battery is 144Wh. I'm fairly certain that Lead- | acid can accept 0.3C, and you're right in that a 60W panel is | slightly more than the 43W that Lead-acid can accept. | | But sizing 12V 20Ah (ie: 240 Wh), and now 0.3C is 72W or 6A | of safe charging (less than 0.3C). | | ------------- | | See #4: just buy a bigger battery if you need a bigger | battery. Bigger battery provides more power and energy | proportionally for the chemistry. | | And a 12V 20Ah lead-acid is just $40 on Amazon. | | --------- | | I guess your overall point is that "Trickle-charge" isn't | enough for Solar? Which is something I admit I didn't think | about earlier (trickle-charge just doesn't send as much | current to the battery due to the lower voltage). But I guess | there's a "simplicity of circuit" advantage of trickle- | charging. A more complex state-of-charge circuit (likely a | microcontroller keeping tabs on the battery) is needed to | safely send 0.3C down to the Lead Acid. | | EDIT: On the other hand, having your "charge controller" just | be like, two power diodes, is a gross benefit to simplicity. | Its something you can do with Lead-Acid that's fully | impossible with LiFePo (and is why LiFePo4 needs expensive | charge controllers to work). For hobbyist purposes, there's | something to be said about using simpler technologies, even | if their specs are worse (and I'm not entirely sure if Lead- | Acid has worse specs than LiFePo4 in this use case). | louwrentius wrote: | The key issue is that lead acid at these low capacities may | be able to take one or two amps above 50% SoC. Especially | at 80%+ if the panel can do 4 amps but you can charge only | with 2, you are wasting precious sunlight. | | Lead-acid can charge at 0.3C but only when empty until 50% | or so. In the absorption phase, current drops | (dramatically). | | Also lead-acid means 50% usable capacity for longevity so | the 12v 12Ah lithium is 12v 24Ah lead-acid (minimum). Which | also means you can never charge with 0.3C. And even then is | longevity of lifepo4 so much better it's not funny. | | Please spend the extra money on lifepo4, save yourself some | headaches. | gh02t wrote: | I've been looking for the same sort of mini charge controller | with grid backup that is mentioned in the problems and challenges | also with no luck. Is this really not a product that exists? | | I've long wanted to do a sort of small scale migration to solar. | Offset the load from my home server with a few solar panels and | source any excess power needed from the grid while also charging | the batteries/providing power from solar when the sun is up. I | could probably engineer something myself but doing things right | on high power circuits is not worth the effort/risk. Any | suggestions? | colechristensen wrote: | Just grid tie a few panels with microinverters (one small | independent inverter per panel), and forget the battery. | | As long as your solar capacity production is not often more | than your usage at the same time, batteries won't be of any | particular use ... they have to be fairly large to have any | great utility in your home anyway, battery backup and solar | production are orthogonal goals until you are producing a lot | of power. | | If you do want battery backups on a small scale, just get a UPS | the same as you would without solar. | thebears5454 wrote: | I like that this guy has been struggling with this for so long | lol | | Just some random guy that I'm connected to via his frustrations | dylan604 wrote: | I too feel the pain of starting a project possibly because | someone feels they can do something for cheaper than a retail | version. When pricing all of the components, this often looks | to be the case. Oh how many times have I then had to buy things | a second time or decide something else is needed once elbow | deep into the project and many weekends later. If everything | was calculated to include the amount of time invested, it would | have been so much easier to just buy the thing. But something | about pride or some such just won't allow that to happen. We | could wax poetically about the act of accomplishment and the | self learning blah blah, but it's pride. | | Edit: Just wanted to add the relevant quote: "My only regret is | that you can't power the raspberry from frustration, because my | levels are lately spiking so high, that I am continually | wondering why isn't reality bending around my hate." | conorh wrote: | Is there a cheap(ish) solar regulator like the one he is using | that does have the ability to output data somewhere for logging | purposes? | louwrentius wrote: | The entry-level Victron mppt (I recommend an mppt charger) is | the 75/10 by Victron. You also need their ve.direct cable which | is not cheap, but if you are willing, you can make your own. | louwrentius wrote: | I started out like the author of the blog post with a 60Watt | panel and a cheap non-mppt solar controller. My balcony | orientation was shit and that didn't work out. Since then I | upgraded "quite a bit"... | | The challenge is more difficult than it seems. A raspberry pi 4b | does ~3.4 watt idle so that's 84 watt-hour per day. This may not | sound like a lot, but for battery-powered devices it quite a bit. | Microcontrollers can run months on that amount of energy, but | they aren't as convenient as the Pi. | | The 12v 12ah battery used, contains 144 watt hour. Only enough to | cover less than two full days with little to no sunlight. | | The 50 watt panel is already small, but the non-mppt controller | makes it even more inefficient. With good sunlight, the panel can | easily run the pi and charge the battery during daytime. | | The "real" challenge is to keep the Pi online during days of | overcast weather. A 150 watt panel may only do 4-8 watt under | those circumstances and that's not enough. | | But I always love these projects none the less. | dragontamer wrote: | I know Rasp. Pi is all the rage these days, but if I were to | make a solar-server, it'd be off of Beaglebone Black instead. | | * Beaglebone Black uses slightly less power and is slower than | Rasp. Pi. Lower power is a big benefit however. | | * Beaglebone has the "Programmable Realtime Unit" (a | microcontroller-like hard-realtime subsystem with GPIO pins). | | * These PRU subsystems can probably (???) be utilized for the | battery-state-of-charge and possibly even provide a software- | control for mppt solar chargers. Theoretically of course, | but... I have to imagine that a Cortex M4 has enough MHz to | handle these kinds of calculations. | | * If not, the Beaglebone Black has a built in ADC that could be | used as the basis of power calculations. Worst case, add in a | proper uC to handle power / build my own MPTT / Battery | charger. | znpy wrote: | I remember tuning down the clock speed of the raspberry pi cpu | when playing with flashrom, maybe that could also be used to | reduce its power consumption? | | Not sure what rPi version the author is using, i think i remember | i was using the original one ? It was 700mhz by default i think i | clocked down to 200mhz and 50mhz. Granted, it was mostly unusable | (bye bye openssh) at 50 mhz. | sleepytimetea wrote: | I have recently gone down the solar panel rabbit hole too but I | never tried the "fancy portable mini panels" cause they seem to | weak and overpriced. | | Instead, I acquired two large 140W panels that had been removed | from an old installation for the princely sum of $20 (USD). | | After struggling with trying to find connectors that I could pair | with the MC3 connectors (remember, these panels came off an old | installation), I bought a dozen MC4 and just cut off the old MC3 | connectors...MC4 is the way to go these days. | | Next came the surprise at finding out about "MPPT" - in all my | years looking at solar panels on roofs, I never knew they were so | finicky about voltage vs current curves...real MPPT charge | controllers start at $70 for a Victron 15Amp and I decided to try | my luck with Chinese sellers on AliExpress...turns out the | charger was a PWM fake sold as a MPPT charge controller. | | Anyway, I also needed a battery since the charger doesn't work | without a battery source. That's another saga but short story - I | built my own 3S battery pack out of 18650s. ___________________________________________________________________ (page generated 2023-08-21 23:00 UTC)