I'm adding another panel to my trailer - mixing Vmp ratings?

[plh]

I am planning to add an additional solar panel to my trailer roof (shiny aluminum sheet). I am looking at a Bifacial 100 Watt Solar Panel with a Vmp of 18.15V. My current panels are traditional 100W mono panels with a Vmp of 19.23V. Please note that the new panel will be added in series for a system voltage voltage of 56V - my battery is 36V nominal and my MTTP charge controller can input up to handle 90V. Is there an issue mixing different Vmp and cell technology panels?

 

[Dave in AZ]

You can mix panels, in series if they are close in Voc, in parallel if they are close in Ioc.

You don't use Vmp, you use Voc for all "is this OK?" Calcs.

If you add them in series, then you will get the LOWEST of the two panel's amps. If you add them in parallel, then amps add but you will get the lowest of their voltages.

With 2 panels, doing series tends to let them start generating earlier in day and later, due to increased voltage. If there is shade at all on either, or they are placed such that one will get shaded more than other at times of day, then parallel can be better, so at least one panel still outputs. In series, much shade on either panel will stop the output. For those you mentioned, I mean like shade the size of a football.

 

[plh]

Old panels are Voc 22.12VDC and the tentative new panel Voc is 21.45VDC - I'm thinking it is OK with a 0.67VDC mismatch. For a "system VOC" would it be 3x 21.45 = 64.35VDC in series?

 

[Dave in AZ]

With 3 in series, yes. Then figure 15 to 20% cold temp increase, it is probably on back of panel.