Batteries are a major cost of any solar system and the most fragile item in the system. Solar panels, and charge controllers have life expectancies of 25 years plus, your batteries could last for only months if not treated correctly.
With the cost of solar panels dropping dramatically in the last few years, batteries have become the major cost of a solar electrical system. Knowing how to care for your batteries can mean the difference between your batteries lasting many years or just a few months.
Lead acid batteries are the most common battery used in solar systems. They are still the best option for the money, but they take a bit of knowledge to get the maximum life out of them.
Types of Lead Acid Batteries
There are many types of lead acid batteries but only a few are suitable for storing solar energy.
For example, a car battery, even though it is a lead acid battery, it is not usable in a solar system because it is not designed for deep discharge cycling. A car battery is designed for starting an engine only, meaning it is designed for a quick high power discharge to start the engine and not for long term slow discharge to power your needs through the night. A car battery or starter battery will only endure a few deep discharges before it loses its ability to store energy.
For solar system electrical energy storage a “deep cycle” lead acid battery is what is needed. A deep cycle battery has been designed for deep discharge and charge cycling. The lead plates in a deep cycle battery are thicker plates that have been designed for many charge and discharge cycles.
The two most common types of deep cycle lead acid batteries are sealed and flooded. Sealed lead acid batteries are just about maintenance free. Flooded lead acid batteries require that you open them up and check the level of the acid on a regular basis.
Sealed Lead Acid Batteries
Sealed lead acid batteries eliminate the need to have to mess with adding water to the battery and they won't spill if tipped over, but you give up a bit of control when it comes to adding more years to the life of the battery.
Lead acid batteries are very slow to charge so charge controllers employ charging algorithms that help charge the batteries faster by employing higher voltages and currents. These higher voltages and currents also help keep each cell on the battery equally charged which adds to the life of the battery. But these higher voltages and currents must be kept to a minimum on a sealed battery because above a certain voltage the electrolyte in the battery will begin to gasify. This limits the controller's ability to enhance the charging of the battery.
Flooded Lead Acid Batteries
Flooded lead acid batteries have caps that can be opened on the top of the battery. Like the battery in your car. This allows gases from the battery to escape caused by the charging and discharging of the battery, and for you to add water which is lost when the battery gases. This also allows for higher voltages and currents to be used when charging, discharging, and equalizing the battery which results in a longer battery life.
The Problems with Lead Acid Batteries
The Problems with Lead Acid Batteries in a solar energy system are that they charge slowly and if not charged completely will begin to lose their capacity to hold a charge. If over charged the positive plate will begin to corrode and lead to battery failure. On flooded lead acid batteries the electrolyte(water level) must be properly maintained or irreversible battery failure will occur rapidly.
What Determines the Life of a Lead Acid Battery
What determines the life of your battery is how well you keep the battery at full charge, how deep you discharge the battery, and how well you maintain the water level in the battery. These are the most important factors you can control to get the maximum life out of your battery.
Having enough solar power to serve your needs and keep a full charging status on your batteries for as many hours as possible during the day along with having a quality charge controller will take care of keeping a full charge on your batteries.
The next part is a little tricky, controlling how much you discharge your batteries when the sun goes down.
The average depth of discharge of your batteries is directly related to the life of the battery. The deeper you discharge your batteries at night the less life they will have.
For example, if you only discharge your batteries 10% below full charge your batteries will be able to deliver around 7000 cycles of use, that equates to around 19 years.
But on the other hand, if you allow your batteries to discharge to a level of 50% then you will only get around 1000 cycles or 2.5 years.
Having enough battery storage to handle your needs without having to discharge your batteries to a great extent will dramatically increase your battery life.
How to Determine How Many Batteries You Need
To determine how many batteries or electrical storage you need you first need to figure out how much power you will be using when on battery power.
This will take a bit of trial and error, again this is for a solar system on a shoe string. Batteries are very expensive and if you don't want to spend more on batteries than your house cost then you will need to figure your needs which can be difficult.
There are a number of ways batteries indicate their storage capacity but we are interested in, whats called, amp hours. The amp hour rating tells us how many amps can be drawn from a battery in a certain amount of time.
For example, a 100 amp hour battery will give you 50 amps for two hours, or one amp for 100 hours. But these numbers are only approximations like everything else when dealing with batteries. A battery will usually give you more juice if drawn slowly then if drawn fast. You might get 120 hours drawing one amp per hour, but only 1.5 hours of power at 50 amps per hour.
But remember that you only want to discharge the battery to a certain point, say 30%. For example, you need 100 amps hours to power you through the night. So to use only 30% of your batteries you will need storage of around 300 amp hours. If you need 100 amp hours of power and you only have a battery rated at 100 amp hours you will be discharging the battery 100% every night which will cut your battery life to just a few months, where 10% discharge rate will give you many years of use.
I use three 66 amp hour batteries to power my needs at night. This gives me around 200 amp hours of storage. I use only about 15 to 30 % of that every night. This is next to nothing compared to a full solar install with thousands of dollars of batteries. But I use 2 watt LED lights, small flat screen TV, and no fridge at night. I store water in the freezer part of the fridge which keeps the fridge cold at night. I charge the batteries in my laptop and tablet in the day for night use.
Use the power you get during the day to charge things up for night use. I store water in my freezer and freeze it during the day so at night the fridge goes off and the ice in the freezer keeps the rest of the fridge cold. That in its self save hundreds of dollars in batteries. I also use a 7” flat screen TV that comes with its own battery at night which I charge during the day and the pig screen TV during the day . I light my driveway with a 2 watt LED from Auto Zone and it lights up the driveway great for only 2 watts per hour of power.
There are many things you can do to meet your power needs half way which will save you thousands of dollars over time.
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