Portable Solar Trackers

RV Solar Battery Scenario

The one thing I have learned to accept in life is that we cannot anticipate what will happen from one moment to the next. We can only plan for what we can imagine, but what about the things we can’t imagine?

There are a lot of RV owners that want to use solar for boon-docking. Unfortunately, it is a little bit more involved than just mounting several solar panels on your roof.

The following scenario is an attempt to simulate what you might encounter while boondocking in a remote location with no power or water provided. The plan is to run for 4 days on solar and recharge the storage batteries using only solar.

 

Water Analogy


To better understand solar I have come up with a water analogy that may put things in perspective.

We start off with a full barrel of water (this represents the storage battery) that provides us with a given volume of water (this equates to the total amount of AH amp-hours available to us). At the base of the barrel is a valve ( this equates to a switch) with enough pipe to reach a water-propelled pinwheel (our load). As the pinwheel spins it is slowly emptying our the barrel. Two people with water hoses are filling the barrel (this equates to the two solar panels).

If the effluent (water out) is the same as the influent (water in) then we will maintain the same level of water in our barrel. In our scenario, we are lucky to just prevent the barrel from emptying too quickly.

Now, somebody comes along and parks a truck on both hoses (this equates to clouds or rain) the water will stop filling the barrel and the water level of the barrel will drop.

 

Our Solar Equipment


The system I chose for our mock scenario is designed to be used with flexible solar panels because they are lightweight, portable and easily stored in the RV.  The flexible solar panels are so thin they can be stowed under a mattress; if you have slept on an RV mattress you wouldn’t notice it if there as a field of rocks under there.

For this particular scenario, we will use (2) high-efficiency solar panels and an MPPT charge controller to give us the best possible output while still being portable.  My goal is to avoid installing anything on the roof of the RV.  Some RV’ers have no issues with mounting an array of solar panels at a fixed angle on their roof, some of them may never encounter water problems, I will not risk it.  I feel that I can produce enough power to recharge my batteries in a reasonable amount of time by making the panels track.

Equipment Used For Scenario

  • (2) Sunpower 110 Watt high-efficiency solar panels producing 12 amps per hour in full sunlight.
  • (1) MPPT Victron Solar Charge Controller 20 Amp
  • (1) Renogy 170 AH LiFePO4 Batteries

These two solar panels will produce 6 amps each for a total of 12 amps for every hour of sunlight. We will need to make some assumptions because we are not taking into consideration the time of year, the weather or look angle. We will just assume that we have only 5 hours of sunlight available.

Scenario Goal:  Boondock for 4 Days On Storage Batteries


Day #1

On the first day, we set up our tracking system facing south. We run the coffee pot, flush the toilet 5 times, fill the sink with water, watch the news and run a few LED lights.

By 8 PM we have consumed 30 amp hours. Fortunately, the Sun was out all day and we produced 25 amp hours. Theoretically, we produced nearly as much as we consumed. Through the evening we run the water pump to flush the toilet, charge the electronics and run some LED lights, we consumed another 15 amp hours.

The (1) LiFePO4 170 AH battery can be discharged to 75% which means that we have 127.5 AH to work with. During the day we lost 5 amp hours (30-25 amp hours) and through the night we consumed another 15 amp hours. We used 20 amp hours of our 127.5 AH leaving us with 107.5 AH to start day 2.

Day #2

The Sun rises at 6:30 AM and we start producing a few amps. The sky is mostly cloudy and by noon we have only produced 10 AH but we still need to run the same devices we did the day before, about 15 AH. So we have a net loss of 5 AH. By 8 PM of day 2 we consume another 15 AH but a mostly cloudy sky prevents us from producing any more power. So by the end of the day, we used up (5AH + 15AH) for a total of 20AH. We started the day with our storage battery at 107.5 AH (107.5AH – 20 AH) so we are left with 87.5 AH. Through the night we use another 15 AH leaving us with 72.5 AH (87.5 AH – 15AH).

Day #3

We start day 3 with 72.5 AH and we have a solid 5 hours of sunlight which allows us to produce 25 AH. Our consumption is still about 30 AH so again we lose about 5 AH during the day and 15 AH through the night. We are now left with 52.5 AH (72.5 AH – 20 AH).

Day #4

Day 4 is a disaster. It rains all day. Not only did we not produce any power during the day ( 0 AH) we consumed more than what we normally do because we stayed indoors all day. We lose 35 AH during the day and 15 AH through the night leaving us with 2.5 AH (52.5 AH – 50 AH).

As you can see we will have depleted our battery on the morning of day 5. This is based on a lot of assumptions but it gives you an idea of what we might expect. In reality, we will probably consume more power on some days and less power on others. Another thing to consider is that during the day we will be outdoors, this means we will not be consuming power.

 

Summary


In this particular scenario, we were able to boon-dock for a full 4 days even though we experienced days with clouds and rain. We had 127.5 AH to work with and we used all but 2.5 AH. We consumed 125 AH and that is with only producing a total of 60 AH during that period.

It would appear that (1) 170 AH LiFePO4 battery would be sufficient, but at what cost? One of these batteries would cost about $1500.

If we substitute these batteries with (3) 100 AH lead-acid deep cycle batteries we would have 300 AH (100 AH x 3) but we would only have 150 AH to work with. This would reduce our cost by around $765 (about half) but it would increase our weight by 150 pounds and we would end up replacing these batteries in about 3-5 years.

Maximum Solar Battery 100 AH
$255.99

You could save even more money by using a less expensive battery. The Renogy AGM 100 AH deep cycle battery below is a good reliable battery, however, the duty cycle of this battery is probably only 1-3 years.  Using lead-acid batteries is cheaper initially, but over the course of 8-10 years, the LiFePO will cost about the same.  The primary advantage this battery has over lead-acid is the rate of charge and discharge.

In our scenario, we consumed about 30-50 AH per day. If we had enough solar panels to produce 30 AH for each hour of sunlight we would be producing as much as we consume, in effect never depleting our battery.  In our scenario, we had days without sunshine and we still managed to last 4 days.  We cannot anticipate how much sunlight we will have but we can produce as much as possible in the shortest time frame and we did that without drilling holes in the roof of our RV.

 

windy nation AGM
Windy Nation 100-watt AH $199.99

Because we want to still be portable, and be able to easily stow the solar panels, we could add a folding solar panel such as the Dokio 200-watt solar panel.  This could be situated on the ground with anchors, or laid on top of the roof if there is no wind.  It would add another 11 amps, provided you connect them in parallel with your tracking panels.

dokio soalr panels

This is accomplished using MC-4 branch connectors.  By adding another portable panel we would produce about 2o-24 amps each hour. We are still portable, still stowable, easily deployed and we did not have to drill any holes.
mc-4 branch connectors