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Building a Battery Bank for Your Off-Grid Energy Needs

Solar and wind energy can be as fickle as the weather they depend upon. Therefore, anyone hoping to escape from the grid by turning to these renewable forms of energy must have good storage systems set up to supply all of their power needs when the sun has set and the winds have become still.

Battery banks are the best choice for renewable energy storage. By using batteries to deliver energy captured from the sun and the wind in a consistent and reliable form, renewable power enthusiasts will not be left dependent on the whims of nature as they attempt to leave the electrical grid behind them.

But putting in a system of batteries to harvest and store the output of the sun and the wind involves more than meets the eye. Before installing a battery bank, much thought, care, and calculation will be necessary to ensure that the system chosen will fully meet the energy needs of the household in a cost-efficient way.

Battery Basics

Deep discharge or deep cycle lead-acid batteries are the best choice for home energy systems. These terms ‘deep discharge’ and ‘deep cycle’ simply mean that the batteries in question will have the capacity to store and produce a lot of energy if and when it is needed. When direct energy from renewables is unavailable and battery energy must be put into use, it will be sent into the onsite electrical grid of a residence or homestead in the form of DC current, and a device called an inverter will then convert that energy into the AC current that is required to power household appliances and other electrically-powered machinery.

Lead-acid batteries come in two styles, sealed or flooded, and while the latter is cheaper it also requires greater levels of maintenance. Batteries for renewable energy storage usually cost between $80 and $200, and the lifespan of a lead-acid battery is usually somewhere between one and 15 years, depending on the quality of the battery chosen and on how well it is taken care of.

Batteries of this type come in 2-, 6-, and 12-volt varieties. Because home energy storage systems generally deliver 12-, 24-, or 48-volt outputs, more than one battery will be needed to meet the energy needs of the normal residence. In addition to voltage, lead-acid batteries also carry amperage ratings, and it is these two numbers together that determine the overall strength of an individual battery. Both voltage and amperage levels are cumulative, depending on how a battery bank is set up – batteries connected in sequence will combine voltages, while parallel strings of batteries that feed into an electrical output system separately will combine amperages.

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Running the Numbers

Deciding how much battery power will be needed for a battery bank is actually a question of mathematics more than anything else. First, a home owner will need to know how much electricity in watt-hours they are using on a daily basis. If the move to renewables has not yet been made, this information can be obtained by taking the total energy use figure listed on a utility bill and dividing it by 30. Since electric bills record energy use in kilowatts, to convert to watt-hours it will be necessary to multiply the number obtained by 1,000. Assuming that the home owner does not live in Hawaii or some other location where the climate never changes, it would be wise to calculate daily energy use for the coldest month of the year, since a battery bank should be set up with the capacity necessary to handle the highest possible energy load. If a house has left the grid and is already using a renewable system, the best way to calculate daily energy expenditure is to multiply the wattage of each appliance in the home by the number of hours is in use during a typical day, and then add the figures up to get a total energy use number.

The next step is to determine how many “backup days” there may be when the energy sources that power a renewable energy system – the wind and/or the sun, in most instances – will be cut off completely. Cloudy or still days, in other words, must factor into the equation, because on those days battery banks will have to carry the full energy load with no assistance from the system that charges them. In order to obtain a watt-hour usage number adjusted for unfavorable weather conditions, the number of watt-hours used on an average day should be multiplied by the number of expected backup days.

Step three is to determine the depth of discharge. It is a no-no to let batteries routinely discharge all of their energy to meet household power needs because this will cause expensive batteries to wear out very quickly. To prolong battery life – and save money – the depth of discharge should probably be limited to no more than 25% of an individual battery’s potential, although this percentage could be pushed as high as 50% if a battery bank will not be put into actual use very often. Once this percentage has been chosen – and it will basically be up to the home owner to decide how much strain they want to put on their batteries – this number should be converted to decimals (.25 for 25%, .50 for 50%, etc.) and the number obtained in the previous calculation should be divided by the percentage of discharge as expressed in this form.

Next, this number will need to be adjusted for the ambient temperature effect, which just means the lowest average temperature a battery bank will be exposed to over the course of a normal year. Lead-acid batteries are rated for temperatures in the 70s on the Fahrenheit scale, and they work less effectively when exposed to lower temperatures. Therefore, another calculation must be performed, as the number from the previous step will be multiplied by what is referred to as the ambient temperature factor. The chart of these factors is as follows:

Lowest average temperature Ambient temperature factor

80 degrees or greater Fahrenheit                                   1.00

70 degrees                                                                          1.04

60 degrees                                                                          1.11

50 degrees                                                                          1.19

40 degrees                                                                          1.30

30 degrees                                                                          1.40

20 degrees                                                                          1.59

Ideally, battery banks should be located in places where climate can be controlled, but it is also important to have good ventilation because lead-acid batteries give off gases that can be flammable if allowed to build up. So basically, everyone should try and protect their battery bank from extreme temperatures as much as possible, but good ventilation should never be compromised. Garages and sheds generally make good locations for battery banks, and if those garages or sheds have some form of temperature control so much the better. Because of the potential for gas buildup, battery banks should never be placed inside the home.

Once the number adjusted for discharge percentage has been multiplied by the ambient factor, there is only one thing left to do, and that is to calculate the amp-hour capacity of the battery system that is being planned or considered. This step involves division – the previously obtained number should be divided by the total voltage of the proposed battery bank, which will be either 12-, 24, or 48-volts. When the amp-hour figure has been calculated, this will act as a guide for the final battery bank setup.

To size a battery bank properly, the amperage-hour number must be equaled or exceeded in the final bank setup. Remember, batteries connected in sequence cannot accumulate amperage output, so if you put four batteries in sequence with an amp rating of 100 each, the total amperage delivered by that string will remain 100 amp-hours. If it is not possible to fine batteries with amp-hour numbers that equal the required amp-hour level of output, more than one battery string will need to be used to secure greater levels of cumulative amperage output.

To put this explanation into more concrete terms, let’s look at an imaginary example. To start out, let’s say that a home looking to go off-the-grid with a solar energy setup backed by a 48-volt battery bank is using 5,000 watt-hours of energy per day. With four backup days expected per month, we would multiply 5,000 by 4 to come up with a figure of 20,000 watt-hours. With a planned 25% limit on depth of discharge, we would divide 20,000 by .25, which gives us the 80,000 watt-hours. With a lowest average temperature of 60 degrees, we then multiply 80,000 by 1.11, giving us a value of 88,800 watt-hours. Finally, to get the amp-hour number we must divide the latter number by 48, which represents the intended voltage capacity of the system.

The answer to this final calculation is 1,850 amp-hours, and this number along with the planned voltage are the two key figures that will determine the architecture of the battery bank that will ultimately be set up. For example, if it were possible to find 2-volt batteries with an amperage rating of 650 amp-hours each, a series of three parallel battery strings, each eight batteries long, would meet and exceed the minimum amperage and voltage requirements of the imaginary off-the-grid household in question.

You Can Take it to the Bank

While properly sizing a battery bank for an off-the-grid system may at first glance seem complicated, the procedure just outlined is really straightforward, and none of the calculations described above requires advanced mathematical knowledge. But for those intimidated by even the most basic math – and there are many of us who are – fortunately there are websites available that can perform the required calculations quickly and easily. Actually installing and connecting a battery bank after the correct specifications have been determined is a relatively simple affair, and if complications should arise other members of the off-the-grid community are always standing by ready to offer guidance and advice to their fellow pioneers.

©2012 Off the Grid News

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