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RV Solar Series: Testing the System

RV Solar Series: Testing the System is not such an easy task and involves camping at various times a year without outside electricity. Testing in mid-winter will come at some later time. Testing in March is a good balance between summer and winter conditions.

How did I get these numbers?

The combination of our battery monitor and the solar controller feed my cell phone, via Bluetooth, all the data I need to make conclusions pertaining to the ability of our solar panels and our battery status.  Last year, (2018) operating on battery and generator I started making observations on how effective living on 150 amps per day was possible, and how long putting 150 amps into my battery would take using my generator.  My generator is well up to the task, my charger between the generator is lacking and I didn’t have solar.

Starting with the battery monitor this is a typical morning picture in the winter, before any solar or generator charging.

Screenshot Victron Solar Controller
Screenshot Victron Solar Controller

State of Charge

Here you can tell the status of the battery at 12.14 volts and the estimate of consumption at 33%.  Fifty percent consumption is the tolerable limit for a lead-acid battery.  More than 50% of consumption means that it will take an extreme time period to recover and that repeated visits to below 50% state of charge will decrease battery life.  The voltage of 12.14 seems to be about right for depleting 33% of the battery.  Given the 50% limit, I have 17% of my battery charge I can safely use. 

I am not relying on the voltage to tell me the state of charge for my battery, rather I am coupling the voltage reading with the amperage used, in this screen capture indicates that I have used 100 amps since the last time the battery was full. This also gives me a snapshot of usage for the moment I am consuming 10.3 amps.  This is how I calculated the actual draw of my different appliances. 

Screenshot Victron Solar Controller
Screenshot Victron Solar Controller

Solar Controller

This is the screen capture of the solar controller readings at 7:18 am on the same day. The sun has not yet risen but I am collecting and distributing 0.2 amps from the pre-dawn sky and sending it to the battery.  Obviously 0.2 amps into the battery at the same time as 10.3 amps is coming out of the battery helps, but not much.

These two numbers say that my overall consumption at that moment is 10.5 amps  What I want to see on the battery monitor is a positive number without the minus sign. There is a disparity between the voltages between the two screenshots because the monitor has been watching battery voltage and amperage all night long and the solar controller is trying to replace that with a little more voltage than the current state of the battery.  You can think of the controller number as volts going into the battery, rather than volts in the battery.

Screenshot Victron Solar Controller
Screenshot Victron Solar Controller

This is the history page for my solar controller on the same day, at the same time. Notice that nothing has been produced and the maximum yield on the panels as 4 watts. The previous day the maximum yield was 538 watts  The battery input voltage ranged from 14.45 volts on the previous day down to 12.04 volts.  This morning the minimum input voltage was 12.14.  The bar graphs above the numbers represent time spent in bulk (white) absorb (grey) and float (light blue).  This is how I can tell how long each charging phase lasted and what time each phase of charging started. 

RV Solar Series: Testing the System – Situation/environment

Our test started in early March, elevation 4300 ft above sea level, maximum air temperature 50 degrees Fahrenheit,  32 degrees north latitude, the sun at zenith was 42 degrees above the horizon and ideal panel tilt would have been 48 degrees pointing true south – unless we had a tracking system.  Obviously, with nearly flat panels, we were more than 40 degrees off optimum, pointed slightly southeast. This decreases the apparent size of the panel and causes loss by reflection.

Maximum Output

One of the interesting numbers is that our maximum output from 700 watts of panels (not on the testing day) was 545 watts. This number is a yield of 78% even given all the handicaps of the previously outlined. I find this number to be amazing even though it is not the 700 on the sticker. Remember to hit that high yield, I had to have the combination of two things #1 a clear sky with sun angle close to perpendicular to the panel and #2 a very hungry battery.  Another time our panels hit an output of 600 watts, which is an 85% yield. Amazing.

10 years old

Another handicap is that our panels are 10 years old, harvested from a previous installation and exposed to unknown heat cycles even before we ever purchased them. All solar panels decrease output with age. The final handicap of the test is that I did nothing to clean the panels.

One of the advantages my system has is that our wire loss is very low at .03 percent (calculated value). I ran more than double the wire to the roof just in case I wanted to expand the system in the future, but I put all wires to use to run the system as it is currently installed.

Stop Wasting

Even if I doubled the panels I would be at less than one percent voltage drop over the wires. Systems I have seen, waste 10% of the available electricity because the wires were too thin. However, you can’t make up for panel tilt by installing a fat wire. Likewise, if you tilt and track with the panels you better have a fat wire big enough for the higher amperage. Small wire, high amperage equals heat, push it too far and you are talking melting and fire. 

One of the problems with testing is that if you use the generator for even a few minutes, or in our case — drive somewhere, charging the battery, it invalidates your data. What we needed was a few days using nothing but solar to recharge the battery to validate our system design. 

RV Solar Series: Testing the System – Test Results – on good days

On test day one, starting at sunrise we needed 98 amps. Starting at sundown the previous evening we used 52 amps in the evening and another 46 amps while sleeping. I left the solar switch off until 8 am although sunshine hit the panels at 7:45.  Didn’t use the generator in the evening prior, to find out when the solar could recover to 100% battery charge. I started the previous evening with a 100% battery charge at sundown. 

We used the inverter and furnace during the test and lived in the RV normally. We didn’t monitor or artificially conserve energy during the test or the evening prior to the test. 


We had high clouds in the morning and it was not a perfect blue day. Bulk charge started the day a little handicapped by the high clouds. The bulk charge was finished at 11 am.  The battery still is not fully charged at this point, but the fastest feeding time is done. At that time, the current flow is reduced. Had the current not been reduced, the battery would resist the flow and create undesired heat.


The next step in the charge cycle is the absorption stage, during the absorption stage the battery is being fed; although not as rapidly as before, if the feeding continues the battery will be fed until “full”.  The absorb stage starts at the end of the bulk stage. We achieved the absorption stage at 11 am.  On this day we fed the battery until “full” for two more hours. Remember that full is a term that is not really maximum because the readings are based on a surface charge, not a true “deep” charge.  We were getting there but not there yet.


The following stage is the float stage which started at 1 pm. We were in float for four hours. The battery read full the entire time. Throughout the day, we didn’t quit using electricity because we wanted to live as if we were not trying desperately to charge the battery.

Overall during the day we produced and used 1.98 kilowatts of energy which is slightly more than 150 amps.  About 100 amps were used to fully recharge the battery and an additional 50 amps were used to run the house. We ended the day with a 100% battery charge. Then living normally (for us) we discharged the battery until sunrise.  (Note: we could have used more than 50 extra amps during the day and the battery would still have been full at sundown. Much of the energy the panels could have produced was wasted, not used by us or used to charge the battery.)

Day Two

Day two of the test provided similar results. We started the day needing 96 amps.  The sky was a perfect blue, the first sunlight on the panels at 7:45 am bulk stage was off to the races. We hit the absorption stage before noon. Remained in the absorption stage for four hours and followed by three hours in the float stage. Battery fully charged at sunset with power to spare.  On day two we produced and used 2.11 kilowatts of energy from the solar of which about 100 amps went to recharge the battery and more than 60 amps were used to run the house. Again we ended the day with a fully charged battery.

The following morning I was again down 94 amps, but we did not recharge using only the solar. Instead, we started with the solar and then drove to our next destination. During the drive, we used only the engine alternator and then upon arrival plugged into the power pole. Our test was done. Before starting the two-day solar only test we were without shore power for four previous days using a combination of solar and alternator power to recharge the battery on a daily basis.

RV Solar Series: Testing the System – Adverse Condition Test Results

Clouds are the number one adverse condition for solar energy production. (assuming the panels are clear of snow or shade)

First Day

Partly to mostly cloudy, needed 68 amps in the morning, solar produced 104 amps, battery not fully recharged because we also consumed about 60 additional amps during the test.

Second Day

Mostly cloudy, needed 102 amps in the morning, solar produced 112 amps, battery not fully recharged because we also consumed about 60 additional amps during the test.

Third Day

Cloudy with rain, needed 107 amps in the morning, solar produced 87 amps, battery not fully recharged because we also consumed about 60 additional amps during the test.

I gathered almost enough energy to recharge the battery without having direct sunlight, amazing and inadequate. Each day we had to run the generator to avoid battery damage. Remember during the test series, I expected to need about 100 amps each night until sunrise the next morning. Lead-acid batteries need to be fully charged right after discharge. Do not draw down a lead-acid battery and then continue drawing it down further below the 50% mark.  Each visit to below 50% decreases lifespan.

RV Solar Series: Testing the System proves that what we did works in March. I hope to have the same results in January on my lead-acid batteries. If it still functions as well as I expect, then I expect the system to provide everything we need when I switch to an easier to charge battery.

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