Adding a second string of panels?

[PeterH]

I am planning an upgrade to my RV solar power system. I presently have two NewPower 165 watt panels mounted on the roof of my camper. They are wired in series as a single sting with a rated PMax voltage of 35 volts and PMax amperage of 9.43 amps. These panels and MPPT charge controller feed a battery bank of four lithium iron phosphate batteries with roughtly 400 ah of storage capacity.

I'd like to add a second string of 110 watt SunPower flexible panels that can be deployed as needed. This string will have a PMax voltage of 36.6 volts and a PMax amperage of 5.9 amps.

Can I simply parallel these two strings into my Victron SmartSolar MPPT 100/50 Charge Controller?

 

[HRTKD]

Probably not. Mixing strings that have different specs usually does not end well. It works, but not optimally.

I have 640 watts on the roof of my RV going into the same Victron 100/50 solar charge controller you're using. I then also have 640 watts of panels that I deploy on the ground. Those go into a separate Victron 100/50 solar charge controller. The two controllers work well to charge my 560Ah bank of LiFePO4.

 

[PeterH]

Thank you for your input!

When you say "does not end well"... are you saying I'll let out the magic smoke? Or that one string or the other will not produce as much as expected? I selected the second strings max panel voltage to match the original string as closely as possible, trying to reduce that loss in efficiency. Given a choice, I'd rather avoid purchasing a second charge controller; thinking it would be easy enough to arrange appropriate wiring, switching and fusing to enable switching in and out that second string. I was also thinking that if adding one string of these different panels would work, then I could add a third or forth string, using the same panels, and a combiner box, for the ground deployed panels.

Ideally, I could just add extra panels to the roof of the camper, but at 25+ lbs each, I'd rather avoid loosing that much cargo weight carrying capacity when fully loaded. These flexible panels weigh less than 5 pounds each and don't take up much space when carefully stowed for transport.

 

[Hedges]

I think you can parallel two strings of similar voltage. They differ by 1.6V or 5%, which will pull voltage down slightly from peak of Watts/Volts curve of the higher one.

You'll be adding 200W array (string) to an existing array of 350W, for 550W total.
50A charge controller at 12V is 600W.
Seems OK to me.

 

[HRTKD]

When you say "does not end well"... are you saying I'll let out the magic smoke? Or that one string or the other will not produce as much as expected?

My mistake. I misread your specs and thought they were much different. Hedges has it right. Close enough that you may not notice any less output from the stronger string.

 

[PeterH]

Thanks Jim for sharing your opinion. I am going to order a couple of the SunPower flexible panels and setup to test for ground deployment. Hoping that by setting them out in the sun only when I want extra capacity, that the flexible panels will last longer. I understand they tend to loose power output after just a couple of years.

 

[HRTKD]

My ground deployment panels are full sized panels. The only times I regret using them for ground deployment is when I carry them out of the trailer, when I carry them into the trailer and when I store them in the trailer. It's really a two man job, they're that heavy/bulky.

Knowing what I know now, I would go with flexible panels. Some sort of frame that allows you to tilt the panels and keep them from blowing over/away is helpful.