24v Van - Series or Parallel

[shogran]

Hi guys!

Possibly another silly question, but I'd like to make sure I fully understand before building my system.

I'm in the process of converting a van into a camper and have a 24v 280ah battery system. It will be charged via a Victron EasySolar GX II 24/3000/70 that I got second hand.

Going into this, I knew that I would most likely have to get panels that were wired in series in order to deliver enough voltage to charge the batteries. However I wanted to avoid this if at all possible due to the fact that there will be limited roof space to begin with and my understanding is that if a panel in series is shaded every other panel in the chain is knocked out too. I would not have enough space to wire pairs in series and then connect them in parallel.

Then I discovered the Perlight 415w mono panels which I could comfortably fit 2 panels of, and have space for additional fans etc.

The specs are:

Maximum Power Voltage - Vmpp(V)	38.9
Maximum Power Current - Impp(A)	10.67
Open Circuit Voltage - Voc(V)	46.7
Short Circuit Current - Isc (A)	11.12
Panel Efficiency (%)	21.2

I contacted the company selling them to check my configuration and they are saying that even with these I would need to run them in series to have a hope of getting enough voltage to charge the battery. Their entire business is solar installation so I have no reason to disbelieve them, but I wondered if anybody could help me understand why these would not work in parallel at a reasonable level with the 46.7 open circuit voltage. I may be missing something very simple!

Thanks in advance!

 

[littleharbor2]

Vmp. is your working number. Open circuit is mostly to figure out the maximum voltage at startup, before any current flows. At 38.9 Maximum volts under load you MAY be fine, but that's to be expected at 25 degrees, Celcius. In real world conditions, and especially mounted on the roof of a van , your panels will be working at considerably higher temps. The higher the temps, the lower the voltage output of the panel. MPPT controllers need to see a certain minimum voltage to do their magic. Too low a voltage and they work like a PWM type controller .
I would look for some higher voltage panels like Sunpower 327 watt panels. Check power data and dimensions.

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https://si-datastore.s3.us-west-2.amazonaws.com/documents/vKh3KEhmBcSPqiTIxViDpmeolIY8y7zJRFw0Inzn.pdf

 

[shogran]

Thank-you for your response! Definitely helped with my understanding. Those panels look fantastic but outside of my budget (at least over here in the UK), but I've found some similar panels to my planned ones with a slightly higher wattage and voltage output that were cheaper than the ones I had found.

I've also been informed that bypass diodes are in all modern panels which eliminate my concern of a panel taking out the entire series of them! Found Will Prowse's video on the subject and much happier about the system plan moving forward!

Thanks again

 

[littleharbor2]

Those Sunpower panels are long out of production AFAIK. Look for used panel sellers in your country There's many in the US and they sell at huge discounts. You've got to have something like that over there. There were two things that led me to suggest those panels for you. One was the high cell count which gives higher than typical voltage output. The other was the short stature which I'm sure you are dealing with in trying to mount two of them, sideways, on the roof of a van. It's a hard combination to find but they are out there. I think Panasonic has similar panels.

 

[littleharbor2]

These are very similar.

Especificaciones - VBHN325SJ47 HIT - Panasonic México

Especificaciones - VBHN325SJ47 - HIT - La estructura de los módulos solares HIT™ no sólo es innovadora sino también diferente de los diseños convencionales. Con ella se consigue una mayor eficiencia de la celda, que se traduce en: más energía por metro cuadrado. Sus materiales de primera calidad...

www.panasonic.com

 

[JK14]

Generally, the SCC needs about 5-6 volts higher input than the battery bank to work. So a 25.6V nominal bank would need at least 31 Vmp to turn on charging. 38.9 Vmp should be plenty.

 

[mikefitz]

The Victron Easy Solar is designed for high string volts, I suggest using the panels in series. There are multiple segments in the 415 panels each with bypass diodes, so the shading effect won't be too bad.

 

[shogran]

Thanks all for the input everybody, ended up getting a pair of 425W Trina Vertex S Mono Black Frames that I'll be running in series!