Measuring Battery State of Charge

The meter shown above would be part of every off-grid solar photovoltaic (PV) system if it existed, but unfortunately it does not. Making an accurate State of Charge (SOC) determination is not a simple task. The purpose of this article is to demonstrate an easy way to make fairly accurate SOC determinations using a digital voltmeter.

Battery state of charge (SOC) at any given moment is perhaps the most important PV system statistic you can have. Not only will it help you decide when to conserve power in order to avoid a shortage, but more importantly it will help you avoid damage to your battery bank and prolong its life.

To determine the SOC, three things must be considered; battery temperature, battery voltage, and the amount of current flow (charging or discharging). The results of this procedure will only be accurate if your batteries are at about 70 degrees Fahrenheit, plus or minus about 5 degrees.

At any given time, your batteries are either charging, discharging, or at rest. To make SOC testing as easy as possible, this procedure includes the ability to test during any of these conditions. During the day, when sunlight hits the panels, use the “Charging” procedure. At night, use the “Discharging” procedure. If no load is connected, and no charge current is flowing, you can use the “At Rest” procedure.

Test Conditions:

For greatest accuracy when using the “Charging” procedure, a significant charge current must be flowing. This happens when your solar panel array is in full sunlight, during peak sun hours. A small PV array produces about 10 to 30 Amperes of charge current, applied to a battery bank of perhaps 500ah capacity or less. A larger PV array might generate 35 to 65 Amperes of charge current, applied to a larger battery bank. Both are examples of significant charge current flow.

For greatest accuracy when using the “Discharging” procedure, the load must be significant when considering the capacity of the battery bank. You can easily create the appropriate load by using incandescent light bulbs. For example:

A 50-Watt light bulb is a significant load for a 100 to 200 amp/hour battery bank.
A 75-Watt light bulb is a significant load for a 200 to 400 amp/hour battery bank.
A 100-Watt light bulb is a significant load for a 400 to 600 amp/hour battery bank.
For a battery bank larger than 600 amp/hours, use two 75-Watt light bulbs.

SOC determination when discharge current is flowing:

After applying a significant load, measure the voltage at the positive and negative battery terminals. Find the voltage nearest to the actual voltmeter reading in the “Discharging” column in the chart at the end of this article. Follow that row to the left-most column where the SOC percentage is indicated. For example, if the DVM voltage reading is 12.26 volts, then the battery has about 70% of its charge remaining.

% of Charge - - - - Charging - - - - At Rest - - - - Discharging

70 - - - - - - - - - - - - 13.30 - - - - - - - 12.36 - - - - - - 12.25 (Volts)

SOC determination when charge current is flowing:

Measure battery voltage while significant charge current is flowing. Look at the “Charging” column at the end of this article, and find the voltage nearest to the actual reading in that column. Follow that row to the left-most column where the SOC percentage is indicated. For example, if the voltage reading is 13.47 volts, then the battery is charged to about 80% of its capacity.

% of Charge - - - - Charging - - - At Rest - - - Discharging

80 - - - - - - - - - - 13.45 (Volts) - - -12.46 - - - - - - 12.30

If the actual voltage exceeds 14.75 volts, the battery is fully charged, and your charge controller may be attempting to apply an equalizing charge. To explain an equalizing charge is beyond the scope of this article, but is included here to let you know that it does not indicate a problem.

SOC determination when batteries are “At Rest”:

If the batteries are neither being charged, nor is there any load connected, compare the actual voltage reading with the “At Rest” column in the chart below. Batteries are at rest at night, when the sun isn’t shining, and no significant load is connected. Batteries can also be at rest when the array current (charge current) exactly equals the current required by the load (discharge current). For best results, the batteries should be at rest for several hours before taking a voltage reading.

You might experience a voltage reading far in excess of the fully charged voltage reading listed on the chart. This is due to an effect known as “Surface Charge”. This surface charge will dissipate after the batteries have been at rest for several hours, and you’ll be able to make a fairly accurate SOC determination. Because of nuances of “At Rest” battery voltage readings, you’re likely to achieve more accurate results when using the “Charge” or “Discharge” procedures.

Due to variables, such as the type of batteries and their condition, don’t expect 100% accuracy. However, the approximate SOC value will be helpful in determining when conservation measures are needed. Keep the chart and a good DVM near your battery bank, and check the SOC often.

State of Charge (SOC) Chart:

% of Charge - - - - Charging - - - At Rest - - - Discharging

100 - - - - - - - - - - - - 14.75 - - - - - - - - 12.70 - - - - - - 12.50
90 - - - - - - - - - - - - - 13.75 - - - - - - - - 12.58 - - - - - - 12.40
80 - - - - - - - - - - - - - 13.45 - - - - - - - - 12.46 - - - - - - 12.30
70 - - - - - - - - - - - - - 13.30 - - - - - - - - 12.36 - - - - - - 12.25
60 - - - - - - - - - - - - - 13.20 - - - - - - - - 12.28 - - - - - - 12.15
50 - - - - - - - - - - - - - 13.10 - - - - - - - - 12.20 - - - - - - 12.00
40 - - - - - - - - - - - - - 12.95 - - - - - - - - 12.12 - - - - - - 11.90
30 - - - - - - - - - - - - - 12.75 - - - - - - - - 12.02 - - - - - - 11.70
20 - - - - - - - - - - - - - 12.55 - - - - - - - - 11.88 - - - - - - 11.50
10 - - - - - - - - - - - - - 12.25 - - - - - - - - 11.72 - - - - - - 11.25


Tip: When batteries are deeply discharged it’s important to recharge them within 24 hours in order to prevent permanent damage.

Disclaimers:

This procedure and the accompanying chart applies to lead-acid batteries, and may not be accurate for other battery types. While the results of these tests can be helpful, they are only approximations. Due to many variables, do not expect a high degree of accuracy. If you need greater accuracy, consider a Tri-Metric Battery monitor. A good-quality hydrometer can be used if your batteries are not the sealed type.

Working with lead-acid batteries can be dangerous, use extreme caution.

John

Get Ready for a Renewable Energy Paradigm-Shift

Not too long ago it was perfectly acceptable, perhaps even “cool”, to smoke cigarettes in public. Now, smoking is frowned upon by many. A paradigm-shift occurred. Today, those who live in big homes and drive big cars are considered successful, but that perception will soon change as well. Conspicuous energy over-consumption will someday be frowned upon, just as cigarette smoking is today. When the paradigm-shift occurs, Jeff Foxworthy will have a whole new source of material. It might go something like this:

You might be an energy hog if:

· Your landscape lighting has more lights than Bush Stadium during a Cardinals night game.
· You drive to work in a vehicle that has more square feet of room inside than the average Cuban home.
· You run your air conditioner in the summer, just so you can have a fire in the fireplace. (President Nixon actually used to do that!)
· You keep your air conditioner running while you’re at work, just so your cat will be comfortable.

It’s easy to understand why these things will be frowned upon in the future. We’re living in the information age, and we know that excessive energy use not only increases pollution and consumes natural resources that are already in short supply, but also contributes to carbon dioxide emissions and therefore to global warming. We know that the electricity we use comes from coal-fired power plants, and we’re aware of the negative consequences of mining and burning coal. Perhaps we’re not yet mindful of these problems each time we flip on a light switch, but we’re getting there. A paradigm-shift is coming.

Renewable energy systems will be a part of our future, there’s no avoiding it. The sooner you install yours, the sooner you’ll be able to over-indulge in electrical appliances (if you so choose), without being considered an energy hog. Instead of following the crowd, be a pioneer. Solar panels on your roof are sure to be noticed, inspiring others to follow your lead. When neighbors ask questions, be helpful. Direct them to websites and businesses that specialize in renewable energy products, services, and information. Your reward will be a cleaner environment, and the preservation of natural resources which will benefit future generations. Wars will be less likely, since the demand for fossil fuels will be reduced. All things considered, you have much to gain and little to lose by embracing renewable energy and encouraging others to do the same. Lead by example and make conservation and renewable energy an ongoing part of your life.

Solar John

The $25 million Richard Branson Challenge - My Entry

You might have heard about Richard Branson’s offer of a $25,000,000 prize for a commercially viable design which will result in the reduction of greenhouse gases from the atmosphere. The idea may conjure up visions of massive machines that suck in polluted air in one end, and spew out clean air at the other. More realistically, some suggest ocean fertilization, and outer-space-based systems. I doubt that either of those plans will work, so I've put together a plan of my own. This will be my entry in the Richard Branson challenge:

Submission by: Solar John

Date: March 16, 2007

Abstract:

Removing carbon dioxide makes little sense as long as we continue to pump it into the atmosphere at an ever-increasing rate. Any physical device or system designed to remove carbon dioxide from the atmosphere would be ineffective, at best, without an accompanying program to reduce emissions. By the same token, an effective plan to reduce carbon dioxide emissions eliminates the need for a device or system to remove carbon dioxide already in the atmosphere. The remaining carbon dioxide emissions can be absorbed by plants. Therefore my proposal has little to do with removing it, but is instead a plan to stop polluting the atmosphere in the first place.

The Design:

A gradually escalating tax on the use of fossil fuels would accomplish the goal. Primarily, this would affect electricity providers. To remain competitive, those utility companies would switch from fossil fuels to non-polluting alternatives in the generation of electricity. This, of course, will reduce carbon dioxide emissions. Individuals who continue to use electricity from polluting providers will pay at an ever increasing rate, and therefore would be compelled to use less, and eventually switch to a non-polluting provider. The polluting provider will eventually stop producing electricity, due to high costs to do so, and a lack of demand.

To a lesser extent, gasoline used in automobiles also contributes to carbon dioxide emissions. In the United States, auto manufacturers are already mandated to produce more efficient, less polluting automobiles. These constraints should continue until it becomes more economically practical to use alternatives to gas and diesel engines. This trend will spill over to other devices that currently run on gasoline, such as lawnmowers, garden tractors, and small recreational vehicles. Hybrid cars and golf carts already use electric motors, proving that electric propulsion is practical, and that the technology is already in place.

Funds collected by taxing the polluting energy providers will be used to help clean energy provider’s ramp up quickly, and to help agriculture meet the growing demand for crops with which to make biofuels. A portion of the same funds will also be used to help individuals and organizations pay for non-polluting systems, and to fund small-scale renewable energy projects. These would include solar photovoltaic systems, wind farms, small to large hydro electric power facilities, and bio-fueled heating equipment to name a few. A significant portion of the available funds would be used for reforestation efforts, resulting in an increase of CO2 absorption. Practical carbon sequestration projects might also be a component of this plan.

Benefits and drawbacks of the plan:

In addition to accomplishing the primary goal, this plan has many other benefits. It will benefit the economy by creating new jobs, especially in agriculture and manufacturing, and will result in exportable technology which will have a positive economic effect. Another desirable effect will be an ecological improvement which will benefit commercial fisheries, tourism, and recreation. The elimination of coal mining will preserve mountain ranges that currently are the victims of Mountaintop Removal Mining, stop the pollution of groundwater sources, and will mitigate other harmful effects of mining, cleaning, and transporting coal.

Those who benefit from oil and coal industries will be forced to look elsewhere for their livelihood. However, they will have time to change careers, perhaps taking advantage of the many opportunities that will result from the implementation of this plan.

A portion of crops currently used for food will be diverted to energy production instead, creating a fear of food shortages. However, the American Farmer has always met the demand for food crops, domestically and abroad, and will continue to do so in the future. Crop yields will be higher because of a cleaner atmosphere and water, a byproduct of this plan’s implementation.

Conclusion:

This plan will accomplish the primary goal with few negative consequences. It is a common-sense approach to solving the massive global crisis we face. We need to become good stewards of the earth and sky, instead of just claiming to be. Our survival depends on it.

Solar John

Solar Electric System Measuring and Monitoring Instruments

I like to know what’s going on inside my photovoltaic (PV) system at any given moment, but since voltage and current cannot be seen, instruments are needed to acquire that information. In addition to instantaneous readings, I watch and log subtle changes over time. By observing trends in the data, I can optimize system performance, and spot small problems before they become big ones. Without this information, my system may fail to perform up to its potential at a time when I need it the most. For that reason an investment in monitoring equipment is money well spent. With these things in mind, here are a few measuring and monitoring tools that provide the needed information:

Digital Voltmeter (DVM):

The DVM can be used to measure output from the solar panel array, battery voltage, and inverter output voltage. It is the primary troubleshooting tool when checking a system that is not performing up to its potential. A DVM can detect subtle voltage differences, most of which would be overlooked by an old-fashion analog voltmeter. These subtle differences can point to the source of the problem, such as a badly crimped lug on a battery cable. Tip: Search for problems such as this when the system is under a significant load.

Many DVM’s include the ability to measure current and resistance. These meters are more accurately called Digital Multimeters (DMM’s). The added features are worthwhile, even though the cost of a meter with these features may be higher.

By entering periodic voltage readings into a journal, I’ve created a log of system operating parameters with which I can compare to future readings. Any significant performance decline will warrant further investigation. The solution may be as simple as cleaning the glass on the solar panels. To free me from the task of taking these voltage readings manually, I use a data logger.

Data Logger:

Shown above is one of the least expensive data loggers you’re likely to find. The Lascar Voltage Data Logger is programmed to take periodic voltage readings by plugging it into your computer and running a setup program. You then remove the device from your computer, and connect its plus and minus alligator clips to the voltage source to be monitored. The Data Logger takes voltage readings at a rate that you’ve specified in the setup program. These periodic voltage readings are retained in the data logger until you upload them to your computer. The data can be printed or displayed, but the program also creates a chart, making it easy to spot voltage trends over time.

When monitoring solar array output, you’ll see a dramatic voltage increase at dawn, voltage dips caused by clouds, and a decline to zero when the sun sets in the evening. The accuracy of the Lascar Data Logger is not as good as that of the voltmeter, but it is adequate. And, at less than one hundred dollars, this data logger is a bargain.

When monitoring PV panel voltage you'll see a significant increase when the charge controller detects fully-charged batteries. This voltage increase happens because the charge controller stops providing charging current to the batteries. This results in a lighter solar panel array load, which in turn results in a solar panel array voltage increase. To better explain this is beyond the scope of this article.

Monitoring battery voltage is another good use for the data logger. Subtle changes over time might indicate that the battery is nearing the end of its useful life, or that you simply need to add water. Because batteries can easily be damaged by abuse, constant monitoring is highly recommended. The data logger shown here also allows you to set alarms, perhaps to alert you when battery voltage falls below a preset value. This feature helps you avoid damaging your batteries by over-discharging them.

Some of the tests that you’ll want to perform on your system will involve monitoring voltage over time. I periodically perform system capacity tests by connecting a known load and waiting to see how long it takes for the batteries to drop to 50% of their capacity. The data logger frees me from the need to take periodic manual readings, and even alerts me so I’ll know when to abort the test.

While the data logger described here is good, a two-channel data logger would be even better. It would allow you to measure solar panel array output and battery voltage simultaneously.

Although battery monitoring is a good use of a data logger, the device shown below provides a better way to monitor your battery bank.

Tri-Metric Battery Monitor:

Like the data logger, the Tri-Metric Battery Monitor can highlight subtle changes over time. It can also alert you to problems via an alarm. However, unlike the data logger, the Tri-Metric device can also give you state-of-charge (SOC) information at any given time. This can be good to know for a variety of reasons. For instance; I like to maintain my batteries at a high SOC because I never know when I’m going to need power from them to serve in the event of a grid power failure. Still, I also like to use power from my system on a daily basis in order to reduce my usage of grid-supplied electricity. The Tri-Metric device can help me achieve a good balance between the two.

Determining the SOC without a device like the Tri-Metric is difficult, even with a high-quality voltmeter. For that measurment alone, the Tri-Metric is a good investment.

The Kill-A-Watt meter is another instrument that you’ll want to add to your arsenal. While it can be used to monitor the output of the inverter, it is typically used to provide information about the loads you’ll connect to your PV system. The inexpensive Kill-A-Watt meter is definitely a worthwhile investment.

The energy requirements for most of the devices you’ll connect to your system will be known. For example, a 60-watt light bulb for 4 hours results in a power consumption of 240 watt/hours. But calculating the energy requirements of your refrigerator, for example, is not so straight forward. Because the compressor kicks on and off at irregular intervals, you need to measure power over time. The Kill-A-Watt meter does just that. Simply plug the device to be measured into the Kill-A-Watt meter, and the Kill-A-Watt meter into the wall outlet. After 24 hours have passed, make note of the Kilowatt/Hour reading. The Kill-A-Watt meter has a built-in timer, making the chore easier. You can use this information to help you determine the size of a PV system needed in order to meet your energy needs. Additionally, you can use the Kill-A-Watt meter to determine how much power older devices waste, and to help you decide which model/brand to replace them with.

The Kill-A-Watt meter also measures instantaneous power (in watts). This is useful because many devices do not include the power rating on their labeling. You'll need this information to determine how large your inverter needs to be. Simply add up the total watts of each device you intend to use simultaneously. Your inverter needs to be able to be able to provide that much power on a continuous basis.

Besides measuring the energy needs of individual devices, you can also connect the Kill-A-Watt meter to the output of your inverter. This is helpful in several ways; you’ll be able to observe voltage dips that may occur if you exceed the capacity of the inverter, and you can measure the power provided by your system over time. The display also shows you the line frequency, which should be 60 Hertz in the United States. Tip: Don’t plug the Kill-A-Watt meter into a Modified Sine Wave (MSW) inverter. I’ve read that they can be damaged by doing this.

These instruments, and a good understanding of electronics, should be all that you need to maintain and optimize your system and to diagnose problems. Follow the links below for more information on the instruments mentioned.

http://us.fluke.com/usen/home/default.htm
http://www.lascarelectronics.com/
http://www.bogartengineering.com/
http://www.p3international.com/products/special/P4400/P4400-CE.html

Solar John

How to Survive a Bear Attack

There's an old joke about two hikers who meet a bear. The first sits down and starts putting on his running shoes. The other says, “What are you doing, you can’t outrun a bear!” and the first says, “I don’t have to outrun the bear, I just have to outrun you.”

It’s no joke when temperatures are below freezing, and you suddenly lose the ability to keep your home warm. The obvious moral of the story is “be prepared”, but there’s more to it than that. Neither of the hikers expected to be rescued. Their survival depended entirely upon their own efforts. The same lesson can be applied to other disasters. When hurricane Katrina devastated New Orleans, desperate people waited weeks for help. For many, help came too late. Like the hiker without tennis shoes, and many residents of New Orleans, most of us will not be adequately prepared for a large-scale emergency.

A large-scale emergency might be the result of a natural disaster, or perhaps an act of terrorism, and emergencies often occur with no warning. When it happens, you’ll have to make do with the system you have, not the one you wish you had. In addition to storing food and water, a small photovoltaic (PV) system can make the difference between life and death. The electricity generated by the system will make it possible to preserve and prepare food and medicine, boil water for drinking, and provide warmth. The energy is renewed each day that the sun shines, and batteries store power for use at other times.

While it is tempting to choose easy and inexpensive solutions to protect family and property, these are often inadequate or ineffective. Some choose gasoline generators to serve in the event of an electrical grid failure, but fail to keep a fresh supply of gasoline on hand. Unfortunately, gasoline may not be available locally when it’s needed the most, and it’s not practical to store a supply large enough to get you through a long-term emergency. Some choose natural gas-powered generators, but they would be useless if an earthquake or act of terrorism were to damage pipelines. While it is impossible to guard against all possible disasters, the most reliable source of electricity in the long term is that which you generate yourself with roof-mounted solar panels.

Although the cost of a solar PV system is high, you only need one big enough to meet your emergency needs, not your average daily non-emergency needs. Costs may be somewhat offset by taking advantage of federal, state, and local tax breaks. You can add to the system over time, increasing functionality and therefore your comfort level in the event of a power outage. Doing the work yourself is one way to get the most system for the money.

John