Solar Power: Part 2-Batteries

Without batteries, you will only be able to use solar power when the sun is shining. Honestly that is not so convenient so I would say batteries are a necessity with a solar panel system. Of all the battery choices (wet cell, maintenance free, lithium, Tesla, salt water, etc), I only have experience with wet cell and maintenance free batteries. Therefore, I will limit my discussion to these two types of batteries. These are also the most common types used for solar power.

Most of us own or have owned a car; a battery is used to start the car and power parts of the car during use. Typically this is a wet cell or maintenance free battery. These batteries work great because they are given a continuous charge during use. However, once you turn off the engine, it does not take long for these batteries to drain. Another type of battery, common in golf carts, sailboats and electric wheel chairs is deep cycle batteries. These batteries are designed to be charged and discharged repeatedly; however they are not designed to be discharged completely. It is best if deep cycle batteries are not discharged below 40-50%. Typical life of a car battery is 3-4 years. Typical life of a well cared for deep cycle battery is 7-10 years. Typical life for a poorly cared for deep cycle battery is 1-3 years; they are much to expensive not to care for properly. Batteries will be about 1/3 (or greater) of your solar budget.

When I lived aboard a sailboat, I had wet cell golf cart batteries. I used two 6 volt Trogan T105 wired together in series to make a 12V battery bank. These were strong reliable batteries but it was a pain because wet cell batteries give off small amounts of hydrogen gas in addition to the possibility of sulfuric acid spills. Wet cell batteries need to be vented and also maintained (water added). I prefer to have my batteries inside my home and I did not want to vent them to the outside as that opened up another hole in my structure. Maintenance free, for me, are a better choice.

 (Image from amazon.com)

Maintenance free batteries are available as automotive, deep cycle gel cells and deep cycle AGM (absorbed glass mat). Automotive batteries are a no-go for the reasons previously stated; gel cells (in my opinion) are temperamental but I like the AGM batteries. This is a sealed maintenance free battery, it can be mounted normally, sideways or even upside down. AGM were designed to use in military jets.

Deep cycle batteries are rated in amp hours; this is like the storage capacity of a water tank. Amp hours is the amount of energy the battery will store if fully-charged. For example, my AGM batteries are 125 amp hours each. As previously stated, you should only discharge a deep cycle battery approximately 50%; therefore an 125 amp hour battery is only really providing 62.5 amp hours of energy for use before needing to be completely recharged.

In part one of this series on solar power, I mentioned this series of blog articles would be for a basic solar setup costing in the neighborhood of $3500-5000. This size setup can easily accommodate 4-6 AGM batteries. I have experience with both the Vmax 125 amp hr and the 155 amp hr batteries. Both are excellent. When combined (wired in parallel so it remains 12V but the amp hours are additive) into a system, four 125 amp hr batteries will provide 500 amp hr (125 x 4 = 500) of storage or 250 amp hr (50% of 500) of usage before recharging.

Shown below is my battery storage, two batteries on each shelf and enclosed in a cabinet that becomes a lamp stand next to my chair. This is the advantage of AGM batteries, they do not need maintenance so can be stacked.

 I have room under the sofa for further battery storage if I decide to expand my system at a later date. This is important, always plan for expansion of your system so you are not locked into a set system.

In the next installment of this series on solar power, I will discuss the inverter. The inverter converts the battery power to AC electrical current for use in typical appliances and other needs in your home.

Below are affiliate links for the batteries discussed in this article. As always, questions are welcome.

Solar Power: Part 1-Introduction

Entire books are written the subject of solar setups so I will try to digest it down to the basics.  The book I would recommend for learning about setting up a solar power system is the Solar Electric Handbook. (Purchase of any of these items through our links will give us a few pennies to cover internet costs and allow us to continue providing additional content, thank you).

 Designing a solar power system can be fraught with tension and stress. There are so many terms and it seems each device uses a different nomenclature. There are watts, amps, amp hours, volts, etc. It is enough to make you think you are at the United Nations and no one has a translator.

In a series of post, I will show you how I designed my solar power system and I will detail the reasons for each item I selected. I am assuming you want a basic system that cost $5,000 or less and will provide for basic needs such as a refrigerator, LED lights, tv, fans, laptop and mobile phone charger, and other basic needs. Appliances requiring high electrical needs (heaters, large air conditioners, clothes dryers, etc) can be operated with solar but require a larger system and most likely you will need a qualified professional for the install. What I will detail is a basic system I use in my tiny house.

Let us first list the required components: solar panels, solar controller, inverter, batteries, fuses, maybe a battery charger, transfer switch and monitors, and wires to connect all together. I installed the solar controller, inverter, transfer switch and my main electrical panel in a box on the tongue of my tiny house trailer. These items are outside my home but protected from the elements. The attached photograph shows the basic components. I will detail my reasons for each item in a series of blog posts but many other items work just as well if not better.

Batteries can not be stored outside in my region because they will loose amperage in cold weather. I have four deep cycle AGM batteries installed into a cabinet that is also my lamp stand adjacent to my chair.

Your first duty is to determine: Can you afford to go solar. If all you need is one or two LED lights and charging your mobile phone, you can setup a system for $100 or so. However, operating a tiny house with more comfortable needs will cost a bit more and will depend on your needs. A basic system will cost approximately $3-4,000 dollars (assuming new components). I define a basic system as one that can power an apartment size refrigerator, tv and dvd player, laptop computer, LED lights, fans, and other basic needs.  Therefore, moving forward, we are designing a basic system as I defined above.

Wow, $4,000 dollars, that is a bit steep for a basic system-some will say. However, the cost to get electrical service in the bonnies can top well over $30,000 or more. Solar is also an investment, being off-grid frees you from the invoices of the power grid. If you are a prepper, solar will be the only power available when the grid is shutdown by unrest or other problems.

Most books and professionals will tell you first need to add up all your electrical needs to design your system. I prefer to work backwards designing my system (figure out the system I could afford and then see what it would power) because I am on a limited income. I have no choice but to do solar because the electrical grid is not available to me.

So, this is how I started. I planned for a $3500 system with all new quality components. This would be able to generate 400 watts of power from four 100 watts solar panels. This is where it get confusing; it is not complicated but can be tedious because not all items speak the same language. Your friend is the following formulas:

Watts = Amps X Voltage

or the same formula rewritten as

Amps=Watts/Voltage

I may have lost a few of you, not to worry, this will not be a math class. I will give you instructions to figure your needs. Let us assume you live up north where there is less sunshine in the winter. If you live in Texas, Florida or other areas with abundant sunshine, you will have much more power available 12 months of the year. Living up north, we get about 7 hours of usable sunshine per day in the winter (assuming blue skies or light overcast).  Remember that figure from above, 400 watts from the solar panels. Take the hours of usable sunshine per day (7) and multiply it by the 400 watts to give you the total energy generated per day in the winter from your solar panels; in this example that is 2800 watts. You will get some loss of power because of resistance of the wires and such so let us assume you are getting 2500 watts of power added to your batteries each day. Of course, in the summer you will have more sunshine but you need to design you system for the times of the year with limited sunshine. Unless you have a summer cabin that is not used in winter or some other twist; I am assuming you are designing this system for year around use.

WooHoo, now we are making progress. But wait, batteries are measured in amps or amp hours.  No worries, just convert it. 2500 watts on a 12V system is 208 amps (2500 watts/12 volts = 208 amps). So, on this one day of sunshine you have added 208 amps to your batteries (assuming the batteries needed to be charged). Wait, you can not use all that energy because your batteries should not be discharged below 50% (assuming you have lead acid batteries, not salt water batteries). Therefore, divide the 208 amps by half to get roughly 104 amps for your daily use.

Ooops, here we are again at the United Nations; most electrical items are not rated in amps, many are rated in watts. For example, a 60 watt bulb or a 37 watt refrigerator. No worries, just convert it again: 104 amps X 12 volts = 1248 watts. You have 1248 watts available for your 24 hour day; the next day more energy will be generated. Some days, you might have excess and other day, you might draw more than what is generated from the batteries.

In the next blog post in this series, I will explain batteries and how to calculate your needs for battery storage and use.