Portable Solar Trackers

Bigger, Badder, Better Solar

When considering solar for an RV it is important to remember that we are limited on space and the entire system must be deployed quickly. We also want to be able to pack it up quickly in case of high winds or heavy rain.

The system I designed is lightweight and it is very portable, it consists of a single solar panel and a tripod. Fortunately, the storage battery is contained in the RV so we don’t have to worry about carrying a battery around.

How do we create a system that is capable of producing more power, more efficiently and still be portable?

 

Better Solar Panels

We could start by adding a second panel, a larger panel, or both.  This particular build can only handle a 100-watt solar panel, but it can be modified to handle a single panel as large as 150-Watts. Anything larger than 150-watts, or a second panel should be mounted on something other than a free-standing tripod.

Most solar panels are only about 15-18% efficient. A 100-watt solar panel, like the one below, will produce about 5.1 Amps @18.1V.

This is not bad and for around $100 you would have a nice system. A better choice would be the Sunpower Maxeon that produces 6.3 Amps @ 22.8V. This panel is about 24% efficient. Please note that we are only considering flexible solar panels because they store very easily.

Sunpower Maxeon 110 Watt
$275

The dimensions of one panel are about 45.9″ x 21.9″ and it weighs just 4.4 pounds. With a little modification, it is possible to adapt the portable solar tracker to hold (2) panels side by side. This would change the footprint to 45.9″ x 43.8″ and the weight would still be under 10 pounds. We would be producing around 12.6 Amps and because the system tracks we would be charging longer. By tracking we improve system efficiency by 30%, by install more efficient solar panels we improve efficiency by 8%.

 

Better Mount

The tripod is more than adequate for a 100-watt solar panel, but if you really want to charge your battery quicker you should consider mounting (2) Sunpower solar panels on a hitch-mounted mast.

Hitch-Mount
Hitch Mount

Another option would be to mount the solar mount on the side of the RV ladder. The challenge will be tilting it into position. The Flag Pole Buddy comes in different sizes.

Rv ladder mount
Flag Pole Buddy

 

Badder Charge Controller

Modern charge controllers use Pulse Width Modulation (PWM) to slowly lower the amount of power applied to the batteries as the batteries get close to a fully charged state. This type of controller allows the batteries to be more fully charged with less stress on the battery, extending battery life.

It can also keep batteries in a fully charged state (called “float”) indefinitely. PWM is an improvement in older charge controllers but it can be improved upon.

The latest solar charge controller is called a Maximum Power Point Tracking or MPPT. MPPT controllers are able to convert excess voltage into amperage improving the overall efficiency by about 30%.

Solar panels can deliver far more voltage than what is required to charge the batteries. MPPT technology converts the excess voltage into amps, the charge voltage can be kept at an optimal level while the time required to fully charge the batteries is reduced. This allows the solar power system to operate optimally at all times.

The final function of modern solar charge controllers is preventing reverse-current flow. At night, when solar panels are not generating electricity, it can actually flow backward from the batteries through the solar panels, draining the batteries.

So to improve the efficiency of this system we will use a Victron SmartSolar MPPT 100/20 Solar Charge Controller .

Victron 100120
$157.25

 

Bigger Storage Battery

The traditional battery of choice for most folks that build their own systems is with lead-acid deep cycle storage batteries. They do this primarily because they are relatively affordable. The downside is that lead-acid batteries, typically, should not be discharged below 50% of capacity.

Lead-Acid Batteries

Pros

  • Readily available. You can find a lead-acid battery anywhere.
  • Very affordable – a Group 31 size deep cycle lead-acid battery with 100ah of capacity will cost $150 – $300.
  • Lead acids are reliable and durable.

Cons

  • They are heavy and have a low energy-to-weight ratio.
  • Shorter lifespan and cycle life especially when they’re deeply discharged. ( about 1-3 years)
  • Discharging deep-cycle lead-acid batteries below 20% (and sometimes 50%) permanently reduces the battery’s capacity.
  • Hydrogen gas discharge and acid leakage, only this is relatively rare on newer maintenance free batteries.
  • High current loads rapidly diminish rated capacity.
  • Deep-cycle lead acid batteries are designed for slow, steady discharge over a 20+ hour period.

LiFePO4 – Lithium Iron Phosphate

Renogy Lithium Iron Phosphate Battery
$1499

Pros

  • Long life span (5-10 years) vs lead acid (1-3 years), depending on the depth of discharge and assuming that the cycle limit doesn’t kill the battery first
  • Longer cycle life, as LiFePO4 batteries last 1,000 to 3,000 charge and discharge cycles, compared to similarly sized lead-acid batteries, which can range from 200 – 1000 cycles (again, assuming the depth of discharge is within recommended limits for both battery types).
  • LiFePO4 batteries are less susceptible to problems caused by the depth of discharge…a LiFePO4 battery can be dropped to 20% of charge without long-term damage. Most lead-acid batteries lose capacity or cycle life if they’re discharged more than 50%.
  • A third Lighter than lead-acid batteries.
  • Arguably, LiFePO4 batteries are more environmentally friendly than lead acid.
  • Very safe – the odds of a “thermal runaway” (aka battery fire) are very low. The same can not be said of other lithium ion chemistries.

Cons

  • As mentioned, LiFePO4 batteries are costly. (about 3 times more)
  • LiFePO4 batteries are hard to find. Most must be purchased online.
  • Susceptible to damage via overcharging (it’s very important to use a charging system that’s designed for LiFePO4 batteries if you want to maximize their life).

 

Battery Bank Rules

Perhaps the most important element of a solar system is the storage batteries. It does not matter how much power you produce if you have no way to store it when you need it. In the case of lead-acid batteries, we will need twice as many batteries because of the 50% rule.

This depends on how serious you are about solar. If you are just looking to recharge your RV battery for overnight stops I would not even worry about installing solar. Your vehicle will charge your battery while you are driving.

If you plan on boon-docking, (camping without supplied power or water), you will need to produce as much power as you can and as quickly as possible. This is one distinct advantage that LiFePO4 batteries have over lead-acid, you can charge them very quickly. You can also discharge them deeper, about 80%.

Your choice is to decide what you need for storage (base on your usage described below) and then decide if you want to spend more or install more.

 

More Power

The best way to calculate your power requirements is to run without shore power or generator for a number of days using just the battery. Remember you only want to run them down to 50%.

Do not even attempt this if you have a single 100 AH deep cycle storage battery, you will need at least 400 AH of storage to last 2 days.

State of Charge SealedFlooded
Lead Acid
battery volt
Gel battery volt AGM battery volt
100% 12.70+ 12.85+ 12.80+
75% 12.40 12.65 12.60
50% 12.20 12.35 12.30
25% 12.00 12.00 12.00
0% 11.80 11.80 11.80

You want to measure the voltage and record the battery bank with no load, and only after the battery has had a chance to recover. Take readings every hour or so and check the chart above to determine the state of charge. A “Watts Up” meter is great for measure current draw, voltage and power being produced.

Watts Up
$29.95

Two 100 AH deep-cycle batteries connected in parallel will provide 200 AH of storage. The problem is you can only use around 50% of that energy to before you start to reduce the life cycle of the battery.

That means you only have around 100 amp-hours to use. If it took (2) days to discharge your battery to 50%, or 12.2V,(refer to chart) divide that figure by the number of days it took to drain the batteries when you boondock (100/2) and you will get 50 amp-hours. This is the amount of energy you consume on a regular day.

Now that you are aware of the storage capacity of your battery, you have to figure out the number of solar panels required to replace the 50 amp-hours. Time of year, look angle and weather will greatly impact the number of hours you have to recoupe this energy. On average you can figure about 5 hours of sunlight.

A decent 100 Watt solar panel is capable of producing around 5 amps per peak-sun-hour on average. This means that you can produce 5 amps per hr. x 5 hours, or 25 amps per day. In this scenario, you will need (2) 100-watt solar panels producing 10 amps per hour for 5 hours to produce 50 amp-hours.

 

Reality Check

So, unless we reduce power consumption we will use 50 amps per day and we will need (2) 100-watt solar panels producing the maximum power for 5 hours a day to replace what we used the day before.

Unfortunately, most folks that install solar panels for their RV mount them flat on the roof. Some install a tiltable mount, but most folks have no means of tracking the Sun.

Flat mounted panels are about 30% less efficient than a single-axis tracker of the same wattage and another 5-10% less efficient than a dual-axis solar tracker.

To make matters worse, flat mounted panels don’t receive full Sun the first and last few hours of the day. This means the amount of time to produce power is reduced slightly.

 

Summary

To modify your solar tracker to include the components described above will require you to modify your frame and mount. The solar frame would need to be wide, the Charge Controller would need to be swapped out and you would have to install more batteries in your RV to increase storage capacity.

You would still build the basic portable solar tracker system but you would need to purchase extrusion wide enough to accommodate the two panels. I suggest making the frame foldable or easy to disassemble. I will submit a modification to the build plan explaining the process. Remember, to build this Bigger, Badder & Better system you will need (2) 110-watt Sunpower solar panels, a wider mount, a Victron Charge Controller and (2) MC-4 “Y” connectors and ideally a hitch-mounted mast.

 

If you elect to use LiFePO4 batteries you would only need this one battery for about 2 days, or more of boon-docking.

If you should decide to stay with lead-acid batteries you could simply add more batteries in parallel to the existing RV storage battery. This is assuming you have at least (1) 100 AH deep cycle battery and the space to install more. You could remove your existing battery and replace it with the (2) batteries pictured below. This would give you a total of 175 AH and that would give you about 2 days, or more of boon-docking.