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isochronic Guru Joined: 21/01/2012 Location: AustraliaPosts: 689
Posted: 06:36pm 12 May 2017
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Does the voltage of a panel at max output alter a lot when the light intensity alters ? Eg if a panel is at max output and producing say 20 amp at 15 volts in bright sun.. and the sun dims to approx half intensity and the load is adjusted to draw about half ie 10 amp .. does the panel voltage stay about the same ? I am wondering if the max-power-voltage can be set-and-forget to some extent.
flc1 Senior Member Joined: 20/11/2011 Location: New ZealandPosts: 242
Posted: 12:49am 13 May 2017
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Gday Chronic, From what I have seen with my setup and inverters there seems to be only a small change in voltage with the change in light intensity, eg a variation from about 61 v to about 57v when useing a 45-90v gti inverter. The big change is with the current, the voltage seems to stay pretty stable,I guess thats how the cells work. Even with a panel only hooked up to a volt meter and open circut, the voltage never seemed to change much more than say about 4 or 5 volts with the change of light, diffrent when it is morning and evening of coarse. I know of other bad quality panels that have a larger drop in voltage and really poor perfomance in anything except direct sunlight. So I sappose you could set and forget to some degree, depending on how your panels perform? I think with panels hooked up in series that voltage drop will be multiplied per panel. Fred
Madness Guru Joined: 08/10/2011 Location: AustraliaPosts: 2498
Posted: 01:25am 13 May 2017
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This graph for some panels I have says volts stay the same at different light levels.
There are only 10 types of people in the world: those who understand binary, and those who don't.
isochronic Guru Joined: 21/01/2012 Location: AustraliaPosts: 689
Posted: 01:49am 14 May 2017
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thanks for that
I am thinking, if a panel power max is aligned to a reasonable battery charging voltage (and the battery is protected from over-voltage) the panel will automatically be operating close to the optimum point
Edit - I guess that then leads to whether the temperature of the panel needs to be allowed for, as well.. If it gets hot, the voltage drops..maybe too much..hhmm.Edited by chronic 2017-05-15
flc1 Senior Member Joined: 20/11/2011 Location: New ZealandPosts: 242
Posted: 03:12pm 14 May 2017
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mppt should keep the voltage up,allowing curret to decrease, when panels get hot. Is'nt that how mppt works?, Are you programming an mppt ? The open circut voltage of one of my string of panels is 84v, peak power volts is 60,so I sappose if you have floating battery voltage slightly below the peak power volts ,then all good, aslong as you have dumpload or whatever to control the battery voltage like you say. sounds good in theory anyway lol.Edited by flc1 2017-05-16
Madness Guru Joined: 08/10/2011 Location: AustraliaPosts: 2498
Posted: 04:39pm 14 May 2017
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MPPT as the name says is tracking maximum power point, the graph below is from one of my Midnite Classic MPPT charge controllers. It operates at a lower voltage earlier in the day and late but during the day with clouds coming over there is little change in the volts.
Top red line is PV volts, other red line is PV amps, violet line is Amps out of the controller.
Edited by Madness 2017-05-16There are only 10 types of people in the world: those who understand binary, and those who don't.
isochronic Guru Joined: 21/01/2012 Location: AustraliaPosts: 689
Posted: 05:40pm 14 May 2017
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What is the load (controller out) voltage, I am guessing 24 v ? [nice chart too..]Edited by chronic 2017-05-16
Madness Guru Joined: 08/10/2011 Location: AustraliaPosts: 2498
Posted: 08:55pm 14 May 2017
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It is 48V, there is a lot more information available but I made it simple for clarity.There are only 10 types of people in the world: those who understand binary, and those who don't.
isochronic Guru Joined: 21/01/2012 Location: AustraliaPosts: 689
Posted: 09:40pm 14 May 2017
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Doh ! I was looking (squinting) at wrong scale ! (30 etc on left but that is current not volts, now I see volts scale is on right ). thanks, I am not building a MPPT just yet , more like incrementing my (slow) learning curve
Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406
Posted: 06:05pm 15 May 2017
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You are quite right. The voltage hardly changes form weak twilight to full mid day sun, and if you can strike a happy combination between the number of battery cells, and the solar panel max power voltage, you cannot go too far wrong.
The relationship is not constant, the MPPT voltage does move around a bit, but its not worth worrying about.
You will just about always get more by adding one extra panel than trying to chase the last 0.1% with an expensive software driven MPPT tracking gadget. The MPPT "peak" is not really a sharp peak but a rounded "hump". A few volts either way hardly makes any difference to power.
Notice. New forum software under development. It's going to miss a few functions and look a bit ugly for a while, but I'm working on it full time now as the old forum was too unstable. Couple days, all good. If you notice any issues, please contact me.
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If it gets hot, the voltage drops..maybe too much..hhmm.
, more like incrementing my (slow) learning curve