17v Vmax NewPowa vs 18v Vmax Renogy with Victron MPPT

[sunrise]

Hi,
I am working a RV solar application with very limited roof space, the standard 100W panels won't fit. My goal is to have at least 200w panel on the roof so my panel options are as following. (due to AC unit shading, panels will have to be connected in parallel).
(Vmax/Voc)
1. Renogy 100w + 50w x 2(21.6v/17.9v), total of 200w. consistent Vmax and Voc.
2. NewPowa 75w x 3 (17v/20.3v) x 3, total of 225w. consistent but lower Vmax.

They will be used with a Victron SmartSolar 20A MPPT controller.
#2 is my preferred config, as it provides 25w more and would be simpler to install and better aesthetics as well. But I don't know the 1v drop in Vmax would have any negative impact on output. My Victron MPPT charger requires Voc 5v over battery voltage to start charging and 1v over battery to stay in charging mode. My lithium battery (3.4x4) has 13.6v near full charge, so I am looking at minimal 14.6v Vmax and 18.6Voc, which is within 17v and 20.3v.

But my questions is, in practice, would this reduce the duration of a day that I get solar charging from these panels? compared to Renogy panels which has about 1v Vmax over NewPowa?

Thanks!

 

[gnubie]

It's a bit complicated but there will be little difference in the hours you will be charging. What will potentially change is how well your controller can make use of the power from the panels.

The outright charging part for a Victron is dictated by the Voc of the panel. So long as that stays above the battery technically power is flowing from the panel to the battery / load and will continue until the array Voc falls <1 over battery voltage. There will be almost no power towards the edges of the day.

What happens to the Vmp is important. If the array Vmp drops to battery voltage or worse under battery voltage, voltage decrease due to panel heating for example, the controller will not be able to hold the array at Vmp and provide current to the battery so it will let the array voltage float up to at least 1V over battery. That means you are now working on the wrong side of Vmp where current quickly collapses as voltage increases. You can see this in the IV graphs that Google will find for you. Some controllers give up the ghost when Vmp is too close or below battery voltage, Victron doesn't but your charging power drops off.

If you are going to sit this low in terms of voltage you want to, IMO, use mono panels rather than poly panels. Mono panels have a lower thermal derate so voltage for voltage a mono gives you a better chance of staying on the correct side of or even at Vmp.

Some MPPT controllers have a pseudo-PWM mode too. In situations where the panel Vmp is this close to battery voltage you may find they never enter MPPT mode, staying in this PWM mode.

There is an efficiency curve to consider too. I can't speak for your particular charger but all chargers have a sweet spot when it comes to panel voltage vs battery voltage vs charge current. If you contact Victron via your place of purchase or their community support forum you may be able to get hold of the efficiency graphs for your charger so that you can make an informed decision as to what to do regarding array voltage.

 

[sunrise]

Thank you @gnubie, all the panels(newpowa or renogy) I am considering are mono panels. I think similar config (2-4 panels in parallel for a 12v system) is pretty common with travel trailer < 20 feet so it must worked for other folks. And for an RV the optimal state is probably never achieved due to the uncontrollable camping site conditions. What I wanted to know is if the 18v Vmax renogy panels would have an advantage, under same condition.

 

[gnubie]

As an isolated thing the newpowa panels, if they meet their specs, would do better but it's not in isolation. The efficiency curve for the charger comes into play and IMO this is going to be the significant factor. If you can't get the efficiency graphs my feeling to go with the Renogy panels to get a higher Vmp.

In terms of hours of charging, it won't make any practical difference as the amount of time Voc will stay 1V over battery won't be much different but more significantly at the start / end of the day where light levels are low enough to cause this to be an issue there is more or less no useful power available anyway. At this time of year my 1.9kW array is high enough in voltage at 5:15am to cause my Victron charger to switch to bulk mode but there is so little power that it doesn't even register 1W until some time later.