Add and additional panel

[wifibeagle]

I have an existing 180 watt solar panel on our boat dock charging 2 12V 55 Ahr gel batteries in series. It has kind of worked for 11 years before the 2 previous 55Ahr batteries died. It turns out the guy who sold me the solar panel and the Morningstar PWM controller either didn't know what he was doing or took advantage of my ignorance. The Vmp/Voc of the panel are specified as 25.8/32.7V. I have learned this is not a good match to bulk charge my batteries at ~29 volts. I plan to add another panel in series and change to a MPPT controller, probably a Victron Energy SmartSolar MPPT 100V 15 amp 12/24-Volt Solar Charge Controller. The batteries power 2 cameras ~1A for 12 hours/day, charging boat batteries ~2-3A 2 hours 3 times a week and ~22A occasional boat lift blower ~2-3 minutes..

My question is what criteria should I use to select the additional panel? I understand the current will be limited by the the lower of the two panels. I don't understand the criticality of matching the other parameters especially the Vmp. Most of the readily available panels on Amazon are nominal 12V with a Vmp less than 20V. This one "24V" panel is the closest match I have found. Panels with a Vmp around 25V don't seem very common.

Existing panel Potential 200 watt new panel
Vmp 25.8 24.38
Imp 6.98 8.2
Voc 32.7 28.98
Isc 7.65 8.7

Today (1/2/24) in full sun I measured Vmp/Imp/Voc/ as ~24/5.64/31.68 with my EY1600 solar multimeter. The local weather station reported a solar radiation of about 460 at that time. I suspect in late June the Vmp might be closer to the 25.8V in the specification.

So, is this potential new panel a reasonable match? Do I need to match the Vmp or only the Imp. I recognize paring it with the old panel will limit it to a max output of ~170 watts. However, The resulting bulk charge current would be in the range of 8-12 amps in full sun. A big improvement over the ~3A max I get now.

 

[sunshine_eggo]

Your numbers check out.

IMHO, ditch the 11 year old panel and replace with a single new panel (72 cell, "24V" panel) or identical smaller panels in series.

 

[wifibeagle]

My original plan was to connect 3 100 watt "12V" panels in series with the Victron. I even thought of 4 in a parallel/series combination to maximize the Victron capacity. The 100 watt panels are cheap and also light weight. The weight is a significant factor for me as I'm also 11 years older!

 

[sunshine_eggo]

My original plan was to connect 3 100 watt "12V" panels in series with the Victron.

3S would work.

I even thought of 4 in a parallel/series combination to maximize the Victron capacity.

2S2P would work too.

The 100 watt panels are cheap and also light weight. The weight is a significant factor for me as I'm also 11 years older!

LOL... I refuse to admit this.

 

[BryceFreeman]

Plan should be doable...capacity fits, company with good reputation...as long as the left space is enough for the 3 or 4 panels

 

[wifibeagle]

Two related questions based on ditching the old panel/controller and replacing it with 2-4 100 watt panels and adding a Victron MPPT.
Most gel battery manufactures recommend 20% of the 20Ahr rate as the bulk charge current which in my case is about 11 amps. The batteries I have recommend less than 16.5A initial charge. Assuming that as a goal at least 3 panels are required based on the Victron calculator. What about multiple cloudy days?

The two choices I find using 100 watt panels based on the calculator are
3 panels in series with a 100v 15A MPPT max 10.2/14.1A hot/cold 75% undersized
4 panels series/parallel with a 75/15 MPPT max 13.5/15A hot/cold 100% undersized

Not sure I totally understand the undersize part but both seem more than adequate for sunny days. I would rarely use the full capacity of either choice on a sunny day. Is there a preference for cloudy days?
Cost is not the primary driver. I'd rather spend a bit more and not have to get on my dock roof again in the future. I also want to protect the expensive batteries. I'm 70 and still able to tackle this but who knows about next year. For reference, the dock metal roof is inclined about 20 degrees, facing due south and our latitude is about 36.6 degrees in southern Virginia. There is no shade after about 9am.

 

[sunshine_eggo]

Charge rate of lead acid is a maximum. The optimal for all varieties tends to be around 10%. Some manufacturers are good with down to 5%. Below that and the slow charge rate can encourage sulfation, but that's less of an issue on GEL and AGM batteries. Victron controllers can be set to output less than their maximum if you want. I did this for a time when my array could provide more current than my lead-acid bank could take.

Cloudy days are cloudy days. PV sucks no matter how you look at it. Give that you're comparing 300W array vs. 400W array, the 400W array would perform better on cloudy days because you have more panels.