Physicist answering solar related questions

Rando

Explorer
What that plot is showing is that at one specific cell temperature/Battery voltage combination, PWM is as efficient as MPPT, at every other cell temperature/battery voltage, PWM is significantly less efficient than MPPT. So if your battery is nearing full (14V) and the cells happen to be at 70C, then PWM does as well as MPPT, however if your solar cell temperature is at 25 C or 85C or your battery is at 13V, PWM is 10 -20% worse than MPPT.

The efficiency of a decent MPPT controller is actually pretty high - 98% for the Victron units that are popular in these parts, and a buck converter is at peak efficiency when Vin is close to Vout so at low panel votlages the controller should be hitting this mark. I don't know what typical PWM efficiency is (ie the Rds(on) of the MOSFETS), but it is likely about the same. The power consumption for the controller itself (again based on Victron numbers) is 30mA at 12V, or about 0.36W.

I hate to call you out on this, but the 'reliability' point is also a red herring. If you are worried about this, there is absolutely no reason you can't wire your panels in the same fashion for an MPPT charge controller.
 

dreadlocks

Well-known member
what about partial/diffuse lighting conditions? I personally have no desire to park my trailer in the sun in the summer time.. I camp in the woods, wouldn't higher voltage house panels and MPPT harvest far more energy? The system I'm designing now has far more raw solar wattage than I'd ever need in direct sunlight, but I'm only expecting to see a fraction of that wattage IRL when the trailer is not in tow.. When I do have good lighting conditions and high heat absorption there will be no difference at all between PWM or MPPT or anything else because either way I'll be collecting far more than I can actually use.

for me cost is barely on the priority list, I want to be completely self sufficient in far from ideal conditions.. the last portable solar setup I had could of been entirely sufficient if I kept it in the sun all day long, but when I left camp the sun moved and I came back at end of the day I found I was never collecting more than I was consuming and it was just extending the runtime by a day or so.

My understanding is higher voltage panels will get and stay above vbatt for far longer in lower lighting conditions?
 
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Rando

Explorer
Partial illumination is one of the areas where MPPT really shines (ha!) . Rod Collins (aka 'Maine Sail') did a pretty good comparison between PWM and MPPT controllers under partial illumination conditions (ie a standard day in Maine): https://marinehowto.com/mppt-vs-pwm-solar-controllers/

To jump to the punchline, he found that MPPT outperformed PWM by about 20% on average:
05-MPPT-vs.-PWM-Controllers.jpg


As you note, performance under marginal conditions is normally more important than under full sun conditions. When there is plenty of sun, power is abundant and an extra 10 -20% doesn't really make a difference. It is on the third cloudy day in a row and your battery is running low when every little bit counts.

To address the larger point, arguing the advantages of PWM charge controllers is kind of like arguing the advantages of carburetors - I am sure there are some niche circumstances where carbs still make sense, and there are some diehards who swear by them, but the rest of the world has moved on.
 
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emtmark

Austere Medical Provider
Is there a weight savings durability ?

Supposedly amorphous do better in the shade but I’m leery of marketing wank. I pulled the trigger on the aus eBay blanket, it’s 600$ cheaper than the ampouphous offerings and shipping is 80$

I have a renology panel but I can’t fit it anywhere in my xterra, tent on the roof and alll. Space is at a premium and fold up 120watt is going to be tested out by me I guess


Sent from my iPhone using Tapatalk
 

llamalander

Well-known member
"My understanding is higher voltage panels will get and stay above vbatt for far longer in lower lighting conditions?" -Dreadlocks

This is what I'm seeing with an rooftop panel (36 volts or so) on top of my truck. I was excited to move beyond "12v" panels by using an MPPT. The cost of a 310 watt panel and Victron controller was less than 3, 100 watt Renology panels without any controller. It may even have a smaller footprint, the single pair of cables and fused disconnects were less expensive and easier to install.

I have a few amps charging my battery long before the panel gets direct sunlight because the Vbatt is already surpassed. I guess I could be running 3 Renology panels in series and see similar results. My single panel has far fewer bypass diodes than a ganged set, so what are the conditions where that is disadvantage? Is it errant shading by branches as opposed to diffuse light from overcast/rainy days? How might one balance the tradeoffs of series vs. parallel connections, bypass diodes and the buck conversion properties of some MPPT controllers?
 

llamalander

Well-known member
Re. "temperature induced lost power", do higher voltage panels suffer more than lower in the same conditions? I see that the voltage is generally a linear drop as the temp increases, but with a lower voltage panel (or parallel panels) you must drop below the threshold of the battery voltage much sooner. Having a surplus of voltage and an MPPT that can maintain the required voltage for the charging profile (fairly efficiently) seems like it would allow a longer period of charging. Basically, the buck conversion seems like it is worth the price difference between MPPT and PWM, and any Bluetooth connectivity is just extra.



On a vaguely related note, has anyone found inexpensive logging thermometers one could use to collect data from a vehicle mounted electrical system? I would love to be able to look at my battery/panel/engine compartment/water tank/fridge/transmission temperature in real time or even just over time-
Between the mountains and the deserts where I travel, the outside temperature varies a considerable amount. Having an alert when a component moves beyond a workable temp. could help.
 

J!m

Active member
Omega Engineering is a good resource for data loggers and probes. More fancy = more money but they are reasonable.
 

Rando

Explorer
Re. "temperature induced lost power", do higher voltage panels suffer more than lower in the same conditions? I see that the voltage is generally a linear drop as the temp increases, but with a lower voltage panel (or parallel panels) you must drop below the threshold of the battery voltage much sooner. Having a surplus of voltage and an MPPT that can maintain the required voltage for the charging profile (fairly efficiently) seems like it would allow a longer period of charging. Basically, the buck conversion seems like it is worth the price difference between MPPT and PWM, and any Bluetooth connectivity is just extra.



On a vaguely related note, has anyone found inexpensive logging thermometers one could use to collect data from a vehicle mounted electrical system? I would love to be able to look at my battery/panel/engine compartment/water tank/fridge/transmission temperature in real time or even just over time-
Between the mountains and the deserts where I travel, the outside temperature varies a considerable amount. Having an alert when a component moves beyond a workable temp. could help.

http://www.sensorpush.com/ makes some great little blue tooth temperature monitors and data loggers. Easy to move around as needed, and you can set low/high alarms.
 

dreadlocks

Well-known member
If you've got to play tetris on your roof I can see multiple smaller panels with more diodes being better than a single large panel, trailer/camper roofs have enough crap up there (vents/aircons/etc) to cast shade and knock a panel out its a no brainer when your trying to cram as much as possible up there.. but then I'd still personally run in a series, or parallel groups in a series so I push more volts and less amps, thus requiring lesser cabling with less transmission losses.

my case its not a concern because I have racks/nerf bars higher than the aircon/power vents, so I want the biggest panel I can recess evenly between the crossmembers.. the house panels do give greater output in less space and if finances are a concern you can probably find some used house panels on craigslist for stupid cheap and no shipping.. I was finding used Poly ~300W panels for as little as $120 locally.

unless you're a desert rat the solar formulas/experience for fixed installs with predictable and repeatable expectations is pretty much worthless for most of us, for mobile expo setups you want alot more solar than you'll think you need (probably 4x as much).. Here in the Rockies I am blessed with over 300 days of sun a year, however I play alot in the PNW.. and its nearly reversed with 300 days without sun over there.. the conditions are so variable you really should try to plan for the worst and hope for the best.

the big lesson I learned from my first wack at trying to be an off the grid gypsy was I should had designed for poor lighting conditions and not the ideal lighting conditions, because the former was the norm and the latter is what all the internets solar expertise assumed.. which makes sense once you think about it, of the billions of panels being deployed nobody is going out and putting solar arrays up in the middle of a dense forest but a very few dumbasses like my self.. I dont have any solar on my house because its shaded by a massive cottonwood and all 3 companies quoted me numbers that made no financial sense.. $40k in panels would had barely made a dent in my electrical bill.
 
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