Judging Solar Array Performance

[howson]

This is a continuation of the discussion started in the Solar Charge Controller switching from lithium to sealed thread, specifically post 43 and 44. The reason the discussion is moved here is after reviewing my own post (43) it veers off of that thread's topic. If you go back and read that other thread but want to comment on this side topic, do so in this thread!

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What I asked DaveH to do is to post the performance of his array. According to the information I gleaned from the other thread, Dave has a Furrion 165W panel (see post 20) paired with a Victron 100/30 SmartSolar controller.

The Furrion panel's specs are below (focus on Vmp, or the expected operating voltage of the panel when paired with an MPPT controller and VoC, the "Open Circuit Voltage").

The hypothesis behind my question to Dave is that since it's known the Victron won't start working until the solar voltage (PV) output of the array is 5V above the battery voltage level, the single Furrion 165W panel's relatively low operating Vmp results in a narrow operating range while charging the battery.

Example: Assume a battery voltage of 13.2vDC in the morning. Further assume shore power is NOT connected (or the converter is turned off). The solar controller won't start working until PV = 18.2vDC. Since it operates at 18.72v, that's a very narrow operating window.



The VictronConnect app records performance, so DaveH sent me the following screenshots as shown below.

TRENDS



HISTORY



What's interesting (looking at the Trends images) is that PV appears to nominally operate around 23vDC. This is nearly the Open Circuit Voltage (VoC) of the panel.

So what's the takeaway? An educated guess: when these screenshots were taken, the camper was plugged into shore power and the converter had already charged the battery, thus there was only a short and low power output from the solar controller before the battery rejected any further charging (short bulk mode, which quickly switched to float).

How did I come to that conclusion? When a battery is fully charged and a solar controller is operating in full sunshine, the resistance of the battery to accepting a charge is so high the solar array sees the equivalent of an open circuit--thus the high Solar Voltage recorded in the screenshots (nearly at VoC instead of Vmp). The guess is supported by the 10Wh yield over an entire day as shown in the History screenshot.

My conclusion is the array's performance cannot be judged based on the data provided. To do a true evaluation of the performance, DaveH will need to remove shore power from the camper (best time is at sundown) when the forecast for the next day promises plenty of sunshine. When shore power is removed, leave a small light on inside the camper to induce a small drain on the battery bank overnight. That should result in a partially-drained battery bank when the sun rises.

Turn off the small light inside the camper in the morning and do not use any items in the camper. Now let the array do all the work of charging the battery.

On the subsequent morning (after a full day with nothing but solar charging the battery), plug the camper back into shore power.

In the VictronConnect app, look at the History and Trend data of the day's array performance when the only source of battery charging power was PV.

DaveH -- I will try this experiment on my camper and will post the results to see if a judgment can be made about the performance based on the Trends and History information. Then, if you're willing, we can see if the methodology can be reproduced on your setup.

More to come in a day or two.

Howard

[DaveH]

howson this trailer had been in storage since November 1st. I have a switch that disconnects the battery from all loads and it was only connected to the mppt/solar panel.

I made the snapshots 12/27/23 before I left on my trip the next day.

I have a 100 AH Battle Born lithium battery (unheated).

When I get set up in Rockport next week I can flip the 120 volt converter breaker off, run my batery down then set my battery switch mppt only (after dark) and turn my converter back on to run the trailer.

Is there any particular voltage you want to me to run my battery down to?

[howson]

DaveH
I have a four 100aH Battle Born batteries, thus 400Ah of capacity. The bank was fully charged this afternoon at 16:29 (4:29 PM for civilians!) so I disconnected shore power and turned off the solar panels so there was no further charging from any source. I turned on two exhaust fans and one Coleman/Mach air conditioner fan (on low) so the camper is pulling about 30 amps from the battery bank. (See screenshot below.)



It's now 1829 (6:29 PM). The State of Charge (SoC) is now 85%--about where I expected it to be.

I turned off the air conditioner fan and now with just the two small exhaust fans running the draw is just over 7.61A.

There's about 12 hours to sunrise, so if my math is correct the camper should consume another 13 * 7.61 = 98aH of battery power. That represents roughly 25% of the capacity, so SoC should be about 60%.

Prior to sunrise I'll turn off the fans and leave shore power off with only the parasitic 12vDC loads on in the trailer. I'll turn off the Victron Multiplus, too, as there will be no need for it's charging function and won't be using it's inverter capabilities. Then I'll power up the solar controller so we can watch the "wake up" as panels start to produce PV.

Bottom line (to answer your question): I think between 50 and 75% SoC should suffice for our test. Battery voltage is so linear in a LifePO4 it's better to track the SoC IMO than the battery voltage since we're using Victron battery monitors.

More to come!

Below is current status as of now...

[TucsonJim]

I'm looking forward to your results howson. I'll be on the roof tomorrow starting to install my panels. I'll repeat the same type of experiment soon. I hope to be boondocking in a few weeks to give the rig a real trial. If all goes well, I should be able to fully charge my battery bank and let my nephew plug into our coach so his converter can charge his in the afternoon.

Jim

[Country Campers]

I would love to have input on this experiment but the sun will not shine here until April, if I am lucky, and the camper is in the garage. I will do some testing on this in the spring though.

I too will be watching all the results and data that will be provided.

Brian

[howson]

DaveH, TucsonJim & Country Campers

Turned off the fans and Multiplus this morning at 06:01. I'd guessed in post 3 that I'd see 60% SoC this morning. Actual is 64%.

Nothing is on now in the camper except the CCGX, SmartSolar Controller, and the normal always-on items (sensors, clock on the radio, etc). The baseline draw in my camper is ~.8 amps (higher than I expected).

I'll attempt to post an update around midday showing how the solar charging is going and what, if any, observations can be made based on the data.

Howard

[howson]

Midday update. First topic--using the Trends screen in the VictronConnect app with a SmartSolar controller.

What I found is tapping the - or + magnifier icons (item 1 in pic below) zooms in or out, or could my fingers to pinch or spread (common action on an Android phone). Then, by moving the vertical line (item 2) to a specific spot the exact values I want to examine are displayed (note the 22.56V and .03A).



Note in the pic above that Solar Current and Solar Voltage were selected as the graphed values. Those values allowed me to see exactly when the controller started working (supplying power to the RV).

Why solar current vs battery current?

There are parasitic draws in the camper as recorded in a previous post. The draw is quite low but those draws will slightly delay when the battery starts charging. Specifically (for my RV) the first .8 amps from the array will go to those parasitic draws. Once the array provides more than .8A the battery will start charging. Showing when current actually flows from the controller eliminates that issue--doesn't matter where the power goes, just that there is power being provided. (Not sure I explained that very well!)

In the screenshot posted above, the solar voltage was a surprisingly high 22.56V before current started flowing. Note in the screenshot in post 6 that the battery voltage (at 64% SoC) was 13.06v. Therefore I expected power to start flowing when the array voltage reached 18.06V. I can't definitively explain the discrepancy (why the voltage was almost 10V above the battery voltage before there was evidence of solar current).

As of 1:47 PM the yield from my 1200W array according to the VictronConnect app is 1.25kWh (History view, labeled as item 1 in pic below). What is that in amps? I think using the SoC is the easiest method to figure this out. Since we started with 64% this morning and it's now at 87% (item 2 below), that's a 23% difference (increase). The battery bank holds 400aH, so 23% of 400aH = 92aH added to the battery bank by the solar array. (Hope that makes sense!)

What I'm looking to see from DaveH's setup is the voltage level that his array operates (the Vmp). Item 3 in the pic below shows approximately where my array operates. Why at this voltage? I'll explain below.



The 1200W array on my 315RLTS consists of twelve Renogy 100W panels. There are six series pairs (six sets of two panels wired in series). Those series-pairs are wired in parallel. So six 200W panels wired in parallel for 1200W total.

Here's the specs of a single panel (remember to double the Vmp since there are two wired in series in my array):



To repeat myself for emphasis, with two panels wired in series, the voltage of each panel is summed so the Vmp should be about 35.4vDC. The fact that there's six pairs wired in parallel does not change the Vmp.

Look back up at item 3 in the previous screenshot and you'll see the red line (I added this line in PowerPoint) is about 35v--exactly where the panels are expected to operate.

A key point to remember is an MPPT controller will try to keep an array's voltage at it's Vmp (when current is flowing). As power output of the array goes up, current increases (again--Vmp stays about the same). When power output decreases, current decreases.

Last thought for this post. Some may wonder at what seems like poor performance from a 1200W array. Keep in mind the sun is about as low in the sky as it gets this time of year and due to where my trailer is parked it is affected by lots of shade from trees (again, due to the angle of the sun). Look closely at the pic below and you'll see that even in the middle of the day some of the panels are not fully exposed to the sun and the panels are definitely not at an ideal angle.



There will be one final post later today when the SoC reaches 100% or the sun goes down. Any subsequent posts will be in response to questions.

Howard

[howson]

End of Experiment Update

Unfortunately the SoC did not reach 100%. I was hoping to show the spike in PV voltage at the controller as it approached VoC ("open voltage") due to batteries being full. Because of how I have my rig configured, it rarely goes into a state where no power is being delivered from the solar panels. (That's a whole other topic well outside the scope of this thread.)

What I can show is the final yield (via the History tab):



...and the point where the Solar Controller finally shut down due to lack of sunlight (just above battery voltage level + 5V as expected).



Conclusion
DaveH (or anyone else) -- if you're still willing to try this on your setup, here's the process suggested with what I currently (think) I know. Prerequisites are a Victron SmartSolar controller and a phone loaded with the VictronConnect app.

1) Check the weather forecast. If the following day will be mostly sunny, press on. Otherwise delay until the forecast looks favorable.
2) Ensure the battery bank SoC is 100%. Once that SoC is reached AND it's dark outside, disconnect from shore power. This ensures only solar will charge the battery on the subsequent day.
3) After turning off shore power, turn on one or more 12vDC devices in the camper so that the amperage consumption rate is ~2 amps (chosen since you have a single 100aH battery). Assuming there's 12 hours of darkness, that should consume 24aH resulting in a 76% SoC in the morning.
4) In the morning before sunrise, turn off all the 12vDC devices turned on the previous evening in the camper, leaving just the parasitic draws taking power from the battery. Let the battery charge throughout the day with just the solar panel.
5) When darkness sets in, go ahead and plug the camper back into shore power and enable the converter (if the breaker was turned off, turn it on).
6) Take screenshots of the solar charging results from the VictronConnect app's History and Trends screens (similar to what I've done in this thread) so it can be reviewed.

Howard

[howson]

I figured out a way to show VoC and VmP when my array is operating and when it stops generating power due to a full battery bank (see below) The point is to show that when the current output of the array drops to zero the voltage (as expected) rises. Not as much based on the specs, but there's still definitive evidence.