Storing Power in Batteries


One of the major challenges associated with renewable energy systems is the requirement to store power until it is needed. Energy sources like solar and wind suffer from the shortcoming of not being available all the time. Photovoltaic panels only produce energy when the sun is shining, and wind turbines need wind above a certain velocity to produce power. Under some circumstances small-scale hydroelectric installations can provide reliable power without storage, but for the majority of renewable power installations, some form of storage is a fact of life.

The choice, care and maintenance of batteries can be one of the most challenging aspects of a renewable energy installation. Although batteries have been with us for many years, myths and misconceptions about them abound. First of all, we need to look at what kinds of batteries are used in a typical renewable energy installation.

The rapid rise in the use of portable electronic devices like laptop computers and cell phones has led to the development of a number of new battery types. However, for renewable energy use, virtually all installations depend on one of the oldest and most widely used of battery technologies, the lead-acid storage battery. With well over 100 years of development, this is by far the most widely used battery type in the world. From operating your trolling motor to starting your truck, the lead-acid battery is the number one choice in a wide range of uses. It is relatively inexpensive, readily available, and durable, when used correctly. Although there are many sizes and types available for different applications, the care and maintenance requirements are generally the same.

When it comes to lead-acid batteries, whether the starting battery for your truck, or the deep-cycle type you use for your trolling motor, excessive discharge is the primary cause of shortened battery life. Forget what you have been told about fully discharging your computer or phone battery - they are a completely different technology. For lead-acid batteries, running them dead is the kiss of death.

A good automotive starting battery can last many years with little or no maintenance. That’s because it’s operating in its nearly ideal application. You discharge the battery only slightly to start the vehicle, then the alternator immediately recharges it and maintains it fully charged. Except for momentary discharges when you idle at a stop light with all the accessories running, the battery remains fully charged, with the alternator carrying the entire electrical load.

Although deep cycle batteries of the kind you use for your trolling motor are somewhat more tolerant of being repeatedly discharged, their service life is directly related to how deeply they are allowed to discharge.

A battery that is only 20% discharged each time may last 4000 charging cycles, but discharging it over 80% may cut that to a few hundred cycles. There are charts available that show battery life vs. depth of discharge, but a common rule of thumb is that the best economy results from designing a battery bank for 50% discharge.

In practice, there may be other considerations in deciding on the size of the battery bank. Because the sun and wind are unpredictable, how much capacity do you need to get by if the weather doesn’t cooperate?

Is renewable energy your only power source, or do you have a gas or diesel generator to pick up the load when necessary?

Although a generator adds to the cost and complexity of the system, it provides a redundant source of power and also allows you to size the battery bank more conservatively.

What exactly is a battery bank? A standard, off the shelf deep cycle or industrial battery is likely to be large enough only for a very small installation, like an outpost camp. In order to achieve more capacity, it will probably be necessary to connect several batteries together into a bank of batteries. You could connect two 12-volt batteries in series and have a 24-volt battery bank, or connect them in parallel for a 12-volt bank. In either case the capacity or amount of energy in the bank would be the same, but the components of your system would have to be designed for the appropriate voltage.

Why not always connect in parallel for 12 volts to keep things simple? There are a couple of reasons. Because the size of the wiring you need is related to current, not voltage, doubling the voltage allows you to carry twice as much energy through the same size wire. This is the same reason that the big power using devices in your home, like stoves and dryers, operate on 220 rather than 110 volts. For this reason, many larger renewable energy installations are designed to use battery banks of 24, 36 or even higher voltages.

The second reason for avoiding connecting batteries in parallel is to keep the load on the batteries balanced. Slight differences between batteries make it possible for one battery in a parallel configuration to assume more of the load than the other. The problem does not occur with series connection. That doesn’t mean that you can’t successfully connect batteries in parallel, but it is less ideal and requires more careful monitoring.

All lead-acid batteries are made up of individual 2-volt cells, connected in series. Your 12-volt car battery is made up of 6 cells connected together inside a plastic case. There is a filler, where you add water, for each cell. Larger renewable energy systems typically don’t start with standard 12-volt batteries at all. Batteries specifically designed for renewable energy applications are available in a variety of sizes of individual 2-volt cells. Using individual cells for your battery bank not only give you much more design flexibility, it also allows for the replacement of a single cell, should one fail prematurely.

The other important component of your renewable energy installation is one or more inverters to convert the low voltage DC power in your batteries to 110 or 220 volt AC for your appliances and lights to use. Although some very small systems in trailers and boats use 12-volt DC, it is generally cheaper and far more convenient to be able to use standard AC operated lights and appliances. Modern inverters are extremely energy efficient and produce very clean AC power - much better voltage and frequency regulated than the typical output from generators. The very best inverters produce power that is indistinguishable from what you get off the commercial power grid. Many inverters also include battery chargers and charge controllers to allow you to switch over to battery charging when the generator is started.

A small-scale system for use in an outpost camp could be as simple as a large golf cart or forklift battery, some photovoltaic panels, a charge controller and an inverter. A small generator could be added for redundancy. Of course, the installation in a main base lodge would be larger and more complex, but the basic building blocks are the same.

It is also very important to remember that maximizing energy efficiency is always the first step in designing a system. Renewable energy systems are relatively expensive to install, and you don’t want to pay to generate power you don’t really need.

A properly designed renewable energy installation should not break the bank, and will provide cheap reliable power for many years. Photovoltaic panels are typically guaranteed for 25 years, and a properly sized and maintained battery bank can last ten years or more. Inverters, charge controllers and other electrical devices also have a very long service life.

One thing is clear. The cost of fuel like diesel and propane will continue to increase, as will the cost of transporting it to a remote location. It is also clear that your guests will find electric lamps and running water far more convenient than a bucket of water from the lake and hot, noisy propane lights. 

This article was taken from pages 9 & 10 of NOTO's "The Outfitter" publication, Spring 2006 Issue


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