by lfhoward » Sun Aug 20, 2023 10:55 am
Been thinking about the challenge of increasing the solar panels on the trailer roof by adding a parallel set of two 100 watt Renogy units, as mentioned on the previous page. My math gives me 398 watts for the array when accounting for the inefficiency of combining these new panels in series with my 210 watt panel.
I went back and checked the PV input from my previous camping trip. The 210 watt roof panel pretty much maxed out at 120 watts given that it was flat and not angled, and that 120 was at noon. Given 120/210 is 57% of the max, that means I could probably expect 114 watts out of a pair of new 100 watt panels also laid flat on the roof. So the practical wattage the roof would generate at noon in July on a clear day would be something like 227 watts given 57% of 398 watts, but usually less.
This brings up whether the current charge controller would be up to the task, and if not, which one I should get to replace it. From a space standpoint, I could go up to the Victron 100/20 without an electrical & logistical redesign. The 100/30 has a much larger footprint.
Wired the way I want to do it, the roof array would have the following stats:
Voc (open circuit voltage) = 50.2 volts
Isc (short circuit amps) = 9.85 amps
Vmp = 42.8 volts
Imp = 9.3 amps
Max power = 398 watts
These numbers are comfortably below the 75/15 charge controller’s limits of 75 volts and 15 amps. So my current charge controller won’t blow up, even on a super cold day. However, the max output of this controller, regardless of how much it is overpaneled, is capped at 220 watts (15 amps x 14.5 volts). If the roof would typically max out at 227 watts of real production with the panels flat on the roof, then all I am missing out on is 7 watts. Not bad. However, there are times when the existing 210 watt panel can be angled up and pointed south. In that case, efficiency goes up, adding maybe another 40-60 watts of real power. If I stuck with the 75/15 charge controller, I would not be able to access that additional power from lifting the panel at midday because the controller would already be working at its max. (I am sure it would help on the shoulders of the day though to angle the panel when production is not at max.)
Alternatively, I could replace the 75/15 with the Victron 100/20 MPPT charge controller, which has a max power output of 290 watts at 12 volts. With that I would be able to capture and use 73% of the roof array’s 398 watts to charge the battery, which is probably as good efficiency as I would ever see from the roof array where the 210 panel is angled towards the sun and the 2, 100 watt panels are mounted flat. Thus, I don’t think I would need the even larger 100/30 charge controller to run my roof array without a loss of efficiency, and I can avoid the rewiring I would have to do to make the larger 100/30 fit. Although the max output of the 100/30 is 440 watts at 12 volts and could theoretically use 100% of the roof array’s output, I would never see anything that good from the roof array even under ideal conditions.
So I think I am going to look for a good deal on 2 Renogy panels.
Would you keep the 75/15 charge controller or upgrade it to 100/20, or would you go crazy and get a 100/30? After adding the 2 Renogy panels there would be no more space for additional solar on the roof, so having extra future capacity is not a factor.
Last edited by
lfhoward on Sun Aug 20, 2023 11:51 am, edited 1 time in total.