I have a MPPT 75 | 15 with a 180 Watt 36 Volt solar panel. Problem is most of the time I’m not getting even close to 180 Watts due to angle of the sun, clouds, etc. and want to add just a little supplemental, say 75 or 100 Watts more solar. The first primary panel sits on the back of the boat facing backwards and I want to add another panel which sits on the side of the boat, to pick up on early morning or late day sun, at a 90 degree angle from the first panel. So I’m not worried about pushing say 280 Watts as when one is getting strong power, the other will be basically idle.
So I’m trying to understand the limits of the 75 | 15 and mixing solar panels. 75 is for max volts and the 180 Watt panel at peak times often puts out about 42 volts, and maxxes out at about 10 amps.
So can I add another 36 Volt panel from the same manufacturer which maxxes out at about 5 amps and be safe?
Due to space and budget concerns I’d like to avoid buying another MPPT if possible as the boat is a money pit in countless other ways
I’d like to stick with 36 Volt as it seems very efficient. Is that the main part that must be matched when adding a panel?
While I wait for answers on why my brand new RS6000 doesn’t work I could offer some thoughts for you to consider:
(i) If you have 2 panels which are not facing the sun at the same time better wire them in parallel. This will increase the amps but leave the volts the same (assuming the 2 panels are the same model..recommendable!) Panels mounted in series don’t perform well when one is in the sun and another in the shade and you risk getting only marginal benefits from an additional panel if you wire them in series. You obviously should ensure that your MPPT device and battery can handle the slightly higher current.
(ii) If your single panel is not performing well even in full midday sun then I would suggest checking your cable and connector quality. It may be that they are hindering getting a higher yield off your panels and this would be a relatively cheap fix. Some solar PV cables are not even copper and have relatively poor conductive properties. Also make sure all your connections are tight and have a good contacts.
(iii) I’m no maths genius but above you say you get 42W and 10amps from your single panel. that works out at 420W (from the formula W=VxI). Something doesn’t add up there.
(IV) If your inverter is designed for that you could add a small wind turbine to supplement the panel and allow for overnight charging to keep your batteries full enough.
Post scriptum: I checked the 75/15 and 15 is the max amps. If you max out at 10A peak with your existing panel you are highly unlikely to surpass 15A if you have a second panel at 90° to the first.
75V max on PV side. Use voltage open circuit (Voc). Mind voltage at lowest feasible temperature. Use Victron MPPT calculator with you panel or panel parameters.
15A max charge current on battery side.
13V x 15A = 195W
26V x 25A = 390W
MPPT can be over-paneled 130%.
Though on 13V battery you can have 250Wp PV.
According to datasheet the MPPT 75/15 has a limit of 15A on PV side and should match amperage short circuit (Isc) . This is independent from the naming scheme.
Your figures are not feasible.
42V x 10A is 420W not 180W.
On the other hand assuming a 12V system, 10A on battery side might be 14V x 10A = 140W.
140W / 42V = 3.3A on PV side.
Assuming your 36V panels deliver 3.5A at 42V, you can parallel two of you panels. Parallel will keep voltage and sum up current.
2 x 3.5A = 7A which is well below the max input current of 15A.
You should not put PV panels with different direction in series (which is called a string), as the panel with lowest current will determine the overall current.
Probably you will have less earnings with two panels in series than with a single panel.
If you parallel 3 or more panels, use diodes to prevent back current.
Thanks for the responses and the detailed info. So I looked into the numbers and I’m getting 42V on the solar panel side of the MPPT, sorry for that confusion.
Also my panel is working quite well. It is rated at 180W, but often gets over 190W and sometimes just over 200W, but only for very short periods of time and only if the panel is angled correctly at the right time of day. Most of the day it’s getting about 100-120W. So now we’re getting about 13 hours a day of sunlight and it’s making about 1000W total per day according to the MPPT.
Also I forgot to mention I’m looking to buy the same brand and model. So it sounds like going with another 36V and putting them in parallel I should be OK, with another 80 or 100W? Then I would have to decide if it goes on the sunrise or the sunset side of the boat, but basically it will be at a 90 degree angle from the ‘main’ panel.
About the numbers actually now I’m confused. On the History tab it says Solar Panel P Max and V Max, and these number indicate the panel regularly hits 190W and 40-45V. On the Trends tab however I see the Battery Amps usually maxes out about 8.5-9 Amps and Voltage 13-14. 8.5x14 however is only 119W. Is this normal, or are my cutoff switches losing a lot of power? Should I be seeing 190W regularly at the battery? If so something is not as great in this system as I first thought. Hmm…
