[HN Gopher] SolarPi experiment 2: Finally something that works
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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.
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