Solar Panel Exceeding Output - then some wee shade - Page 1
Forum section: electronics / general

author: metrologist
author_rank: Super Contributor
timestamp: February 25, 2018, 11:46:46 pm
content: I have a 100W newpowa solar panel and recently switched to an MPPT SCC ().I connected my 36V Li-ion eBike battery pack to charge and noticed the panel output was exceeding 140W and around 20V, 6A. That is measured with an ebay module connected to the panel. I am surprised because when I got the panel new, I pointed it direct to full sun and measured short circuit current at spec, just over 5A.Well, the MPPT SC is telling me that I am charging over 5 to 6 amps, and I only want about 4 amps to keep within 1C. Should I not worry about it?Another point, the panel output drops to a mere 20W if just a 1/8 square corner of the panel is partially shaded. That seems odd...

author: IanB
author_rank: Super Contributor
timestamp: February 25, 2018, 11:50:24 pm
content: I think solar panels are wired with lots of cells in series to get a higher voltage on the output. If you shade some of the cells they become like resistors blocking the output from the illuminated cells. Hence the drop in output.

author: digsys
author_rank: Unknown
timestamp: February 26, 2018, 12:02:31 am
content: Maximum current or maximum voltage is rarely maximum power !! That's the huge advantage of an MPPT controller, and there is also a bigdifference in their efficiencies, depending on which one (or more) of the "scan" profiles they use. A 20-30% increase is well expected.IF the cells specify 1C max, I'd definitely try to keep it at that. You will get some lifetime degradation otherwise, but it is impossible to tell unlessyou deliberately run the tests yourself. Usually there's a current limit setting on the MPPT?As for shading .. the "better" brand panels have bypass diodes fitted, which "drop-out" poorly performing sections, but there is a slight penaltyto pay when you do this. When we make up our panels, we break the area down into sections, with a separate MPPT for each.BUT, without bypass diodes, make SURE NONE of the panel has shade spots. It's the MOST COMMON error in installations.

author: Seekonk
author_rank: Super Contributor
timestamp: February 27, 2018, 07:45:36 am
content: Certainly your measurements are just wrong. You just don't have a chance of getting more than 105W under ideal conditions. There is extreme penalty for shading even a small part of the panel and MPPT won't help that unless your array is volts is quite high and are able to kick out a whole section with a bypass diode.

author: metrologist
author_rank: Super Contributor
timestamp: February 27, 2018, 03:47:32 pm
content: I vaguely recall the panel ratings at max power was 18.xV and 5.xA. It was 100W. When using the panel with a PWM SCC, I don't recall seeing power over 80W.There might be something else happening as my setup is a Volt/current/watt/Whr display module from ebay that is connected between the panel and SCC. I suppose load pulsing on the meter's output side could fool its measurement.

author: Seekonk
author_rank: Super Contributor
timestamp: February 27, 2018, 09:55:47 pm
content: As these are designed to feed into a battery, generally not a lot of filtering on the output.

author: metrologist
author_rank: Super Contributor
timestamp: February 28, 2018, 05:14:54 am
content: How does that translate into what I observed?

author: Seekonk
author_rank: Super Contributor
timestamp: February 28, 2018, 09:23:49 pm
content: I very much doubt that it does, but who knows what you have or how it was measured. All I know is it didn't happen."Seeing and believing are often both wrong." FOW

author: metrologist
author_rank: Super Contributor
timestamp: February 28, 2018, 11:28:08 pm
content: I'm sure a reasonable explanation is hiding under the covers.

author: paulca
author_rank: Super Contributor
timestamp: March 01, 2018, 07:16:01 pm
content: If I understand correctly any measurement from a PWM controller can be all but ignored as it's switching on and off it will never be able to measure the panel properly as it's constantly disturbing t.The ebay module. I assume it's a panel meter thing that measures current and voltage? Usually they need to be powered from an external source if you want to measure both volts and current together. I can't remember why but it's something to do with the current sense part causing a ground loop with the voltage sense part and they interact in funny ways skewing the readings.Even the current sense only ones it is recommended to connect them to a separate source from what you are measuring. If you are connecting them to measure and get powered by the same source you need to disconnect one of the grounds as a minimum.I'm sure more knowledgable folks will correct or confirm my vague memory of this.

author: metrologist
author_rank: Super Contributor
timestamp: March 01, 2018, 11:51:21 pm
content: It is a panel meter, check this out:http://www.mpja.com/download/32541medata.pdfIt is installed between the panel and controller. It goes off when it gets dark. Next time I will connect a couple DMM's to measure current and voltage, and may connect my scope to see the signal.