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Solar Power: Batteries & Terminology

By Edited Feb 8, 2016 0 0

Getting The Most For Your Money

Batteries are a requirement if you want to build an off-the-grid solar power home. Since power can only be collected from the sun for a limited number of hours per day and power needs are typically 24 hours per day, we need a way to store energy so it's there when we need it.

When discussing batteries, the term 'deep-cycle' refers to the fact that they can be deeply discharged (80%) over and over again and hold up well to this kind of usage.

However, battery construction varies a lot, since they are typically designed for specific types of service. If a battery is used in a motor-home, it may be called a deep cycle battery but actually be more like a car battery, but designed for occaisional deep-cycle use. By designing the battery for a particular type of service a manufacturer can be more competitive in that market. The problem for us is that terms like 'deep-cycle' become degraded and it makes it difficult to make the most cost effective choice.

True deep-cycle flooded lead acid batteries are an ideal way to store power. Some are designed specifically for alternative energy applications will give you 20 years of service. Other suppliers, more focused on price competitiveness are much lower in cost are designed for 5-7 years use.

One of the ways you can judge the durability of a deep cycle battery is to get the charge-discharge cycle rating for the model you're interested in. The manufacturer will have a spec sheet with a rating. You can usually look it up on their web site if the vendor doesn't know. With a calculator and a little thought you can pretty well calculate how long they will last. Deep discharges shorten life. Shallow discharges will result in longer life.

As you look at the specifications you'll notice that the output power of the battery decreases as you increase the rate at which you draw power. Charging issues are similar. High charge rates are more efficient with a large capacity set. Another benefit is that the larger the set the longer they will last. More or larger batteries create a more solid voltage that improves regulation of your inverter in high demand situations. When it comes to total amp-hour ratings of a battery system, go large!

Batteries all have an internal and sometimes an external self-discharge rate. Batteries that have acid and dirt collecting on the case can drain power from the battery by leaking current across the case. The internal self-discharge rate is not a big issue.

So to maintain the life and health of your batteries they need to be kept clean, charged and not left alone. Self-discharge is usually an issue only when systems are left for weeks at a time without a charge. (A small solar panel can keep a float charge on the system while you are away if you have a weekend home or cabin and you're not always there.)

There are several different construction types of batteries that used for alternate energy systems. They are:

  • Flooded lead acid (most common)
  • Sealed AGM
  • Sealed Gel cell

Flooded Lead Acid

These batteries are much like a car battery. For solar use they are designed for deep-cycle use so they have thicker plates and larger electrolyte capacity. They can be refilled with water and create a small amount of hydrogen gas when charging.  The term flooded comes from the fact that the battery's plates are surrounded with liquid electrolyte.

Sealed AGM

In these batteries the electrolyte is infused into a fiberglass mat. AGM stands for absorbed glass mat. These are very safe and easy to handle but suffer from comparatively shallow discharge and reduced number of charge cycles. They work best in a system where the batteries are left in float mode or when deep discharges seldom occur. Not very suited for full-time off-the-grid use.

Sealed Gel Cell

These batteries the electrolyte is mixed with silica dust to form an immobilized gel. Like Sealed AGMs, these are very safe and easy to handle but suffer from comparatively shallow discharge and reduced number of charge cycles. They work best in a system where the batteries are left in float mode or when deep discharges seldom occur. Not very suited for full-time off-the-grid use.

Caps for flooded cell batteries (Hydro-Caps) are available that recombine oxygen and hydrogen as it seeks to exit the battery reducing water loss and maintenance. They also increase safety. This makes the flooded cell, THE best choice in my opinion.

The amortized cost of batteries (cost divided be time in service) is much better with flooded cells. My favorite brand is Rolls Surrette, with the S-600 being the best value available. 

Battery Charge Monitor

The most used and important accessory you need for your system is a charge monitor. A gas guage if you will, that allows you to see how low your batteries are.

The TriMetric Model 2020 monitor is a good example. You can monitor how low your batteries are in bad weather and start the generator if you're getting too low. It allows you see how fast your batteries are charging and helps you keep your finger on the pulse of your power system. There are other good choices available, but make sure you can see the state of charge of your batteries. It'll save you far more than it costs.

My monitor is no longer available but it has been a real life saver. Weather issues can leave you blind as to the actual state of your batteries. You can also see how much current is being used by your system, which can help you conserve power. You can also see how much power is being used by appliances by watching and turning appliances on and off.

Go solar!

Power from the Sun: A Practical Guide to Solar Electricity
Amazon Price: $26.95 $13.30 Buy Now
(price as of Feb 8, 2016)
DIY Solar Projects
Amazon Price: $21.99 $10.83 Buy Now
(price as of Feb 8, 2016)
Practical Photovoltaics: Electricity from Solar Cells, 3rd Edition
Amazon Price: $18.95 $12.02 Buy Now
(price as of Feb 8, 2016)


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