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Deep Cycle Batteries for Solar, Small Wind and Micro-Hydro

The following is a basic overview of batteries and their use in renewable energy (RE) applications. The subject of RE batteries is vast, and though we will touch on some characteristics of gel and absorbent glass matt - also called absorbed glass mat- (AGM) batteries, our focus is on the batteries most commonly used in RE applications, deep cycle lead-acid batteries.

1.   What is a Battery?
2.   Types of Batteries
3.   Battery Voltage
4.   Battery Sizes
5.   Golf Cart Batteries
6.   L16 Size Batteries
7.   Industrial 2 Volt Batteries
8.   Battery Storage Capacity - Amp Hour Ratings
9.   Battery Efficiency
10. What Are Battery Cycles?
11. What is Deep Cycling?
12. Battery Lifespan
13. Depth of Discharge and Battery Life
14. RE Systems - What the Batteries Do
15. Flooded Lead Acid (FLA), Gel or Absorbent Glass Mat (AGM?)
16. Battery Bank Sizing Considerations
17. How Many Batteries Do I Need?
18. Battery Strings: Putting Batteries in Series
19. Housing Your Battery Bank
20. Building a Battery Box
21. Venting Your Battery Box
22. Battery Charging
23. Charge Controlling
24. Battery Monitoring
25. Care and Maintenance of Your Battery Bank
26. Equalizing Your Deep Cycle Lead Acid Batteries
27. Adding Water to Your Batteries
28. Preventing Battery Corrosion
29. How Temperature Affects Lead Acid Batteries
30. Depth of Discharge (DOD) and Battery Life
31. The Effect of Over-Discharging On Deep Cycle Batteries
32. Your Aging Batteries
33. What Do I Do With My Old Batteries?
34. Safety Considerations

What is a Battery?

A battery is basically any device that stores energy for later use. More commonly, the term "battery" is used as an electrical term describing a device that converts chemical energy into electricity. Batteries do not make electricity, they merely store it.

Types of Batteries

Batteries are categorized in two ways; by application (what they are used for) and construction (how they are built). Three major battery applications are automotive, marine and deep cycle. Deep cycle batteries include those used in solar electric, small wind and/or micro-hydro applications, in recreational vehicles, and marine batteries. The main construction categories for these applications are flooded (wet), gel, and absorbent glass mat (AGM). AGM batteries are also knows as "dry" batteries because they contain no excess sulfuric acid.

Flooded batteries are available with removable caps (for periodically adding distilled water), or in the maintenance free variety. All gel and most AGM types are sealed, maintenance-free batteries.

Important: Do not use marine-class batteries for renewable energy applications. Though a few are true deep cycle batteries, most are hybrids and are not suitable (and can even be dangerous) when used in this type of application.

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Battery Voltage

RE batteries can be 2, 4, 6 or 12 volts. Which voltage is best for your application will depend on how many amp hours of storage you need (2 volt batteries offer the most storage capacity), and the voltage of your renewable energy system (12, 24 or 48). In order to match your batteries to your system voltage, they will be connected together in series or parallel, or a combination of the two, i.e. for a 24 volt system you will need to string twelve 2 volt cells, four 6 volt cells, or two 12 volt cells.

Battery Sizes

Golf Cart Batteries

T-105 batteries (6-volt "golf cart" batteries) are the minimum battery recommended for residential solar applications. Golf cart batteries have thick plates and are designed for hours of heavy discharge followed by a fast recharge of only a few hours. This duty cycle is similar to that of a residential solar application, because a solar battery must be able to provide long periods of deep discharge each evening and night, followed by a full recharge in only a few hours of sunlight each afternoon. Few batteries can handle daily deep discharge-recharge cycles. T-105 batteries are the least expensive and most readily available ones that can. Golf cart batteries have limited storage capacity -about 220 amp hours (Ah) at the 20 hour rate- however, so they are not the best or most cost effective choice for full-time off-grid homes or other applications requiring a generous amount of storage capacity.

L16 Size Batteries

The next step up from T-105 batteries is the larger L16 size battery. If you need more capacity than available from golf cart batteries, the larger "L16" size battery, with a higher amp hour rating, may provide what you need with fewer batteries. This battery is the same length and width as a golf cart battery, but is taller and heavier. This type of battery is perfect for solar, wind and micro-hydro applications because it can take very heavy charge-discharge cycling. Some battery manufacturers, such as Trojan, have developed L16 batteries specifically for renewable energy systems. These industrial rated batteries are usually only available from battery dealers specializing in products for RE applications.

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Industrial 2 Volt Batteries

If you need a lot of storage capacity, 2 volt industrial batteries should be your choice. They are more expensive than the other classes of batteries we’ve discussed, but in addition to their high storage capacity they also offer the longest battery life.

There are a number of manufacturers that offer this type of battery. Based on industry data and our professional and personal experience, our highest recommendation for this class of battery goes to the Solar One with patented HuP (High Utilization Positive) technology. Trojan now also offers a 2 volt battery option (their L16RE-2V).

Battery Storage Capacity - Amp Hour Ratings

Battery storage capacity is measured in amp hours. The amp hour rate is simply the number of amps of power that a battery discharges in one hour.

When reading battery specifications you may see multiple amp hour ratings listed. The accepted AH rating time period for batteries used in renewable energy systems (and for nearly all deep cycle batteries) is the “20 hour rate”. Be sure to use this number when selecting and comparing batteries for your off-grid or battery backup system. Beware of any company that quotes you the 100 hour rate for this type of application. Though useful in other situations and for other purposes, for this use the 100 hour rate is generally used only to make the battery look better than it really is.

Battery Efficiency

Batteries are not 100% efficient; some of their energy through chemical reactions that occur during charging and discharging. That is why it takes more energy than the rated capacity of a battery to charge it, and why you are not able to get 100% of the rated capacity when discharging (using) the battery. Standard lead-acid batteries are about 75-85% efficient. The energy that is lost appears as heat; which is why batteries feel warm when they’re in use. Deep cycle lead acid batteries are typically more efficient than the standard lead acid batteries; some manufacturers state their deep cycle batteries have efficiencies as high as 98%.

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What Are Battery Cycles?

A battery “cycle” is one complete discharge and recharge cycle. When selecting and comparing batteries, be sure to not only look at the number of rated cycles, but also at what depth those cycles are being discharged to. A battery may state it has a 20-year life, but read the fine print. A 20-year life at 5% depth of discharge (DOD) would only be a 5-year life at 50% DOD!

What is Deep Cycling?

Every battery is designed for a specific type of charge and discharge cycle. Car batteries have thin plates to keep their weight down and are designed for a heavy discharge lasting a few seconds, followed by a long period of slow re-charge. Although similar to ordinary car batteries, the batteries used in renewable energy systems have thicker plates and are specialized for repeated discharge ( to below 50% of their storage capacity) and recharge. This is known as "deep cycling". Choosing the wrong type of battery for your application can be costly and dangerous, so it is important that you purchase a battery designed for how it will used.

Battery Lifespan

Batteries are sometimes called the "weakest link" in green power systems due to their relatively short life expectancy (when compared to solar panels, inverters, etc.), and because most problems that happen with RE systems are battery problems. But battery troubles are nearly always the result of bad equipment choices, installation errors, and/or improper maintenance. Which means these often expensive and hazardous troubles are usually the result of human error, not a fault or limitation of the technology. How they are used, how well they are maintained and charged, and the temperature of the battery storage area all affect the lifespan of batteries.

If you invest in good quality batteries, and if they are properly installed and maintained, you can typically expect the following life from your battery bank:

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Depth of Discharge and Battery Life

Standard Deep Cycle Batteries (L16 type): 4-8 years
Premium Deep Cycle Batteries (Surrette, Trojan L16RE-A & L16RE-B): 7-15 years
Industrial Deep Cycle Batteries (Surrette 4KS & Trojan L16RE-2V): 10-15 years
Solar One Batteries with patented HuP Technology: 15-25 years

However, if abused or neglected, batteries can die in just a year or two.

RE Systems - What the Batteries Do

A battery bank, a group of batteries wired together (and also known as a string of batteries), is a key component in stand-alone (off-grid) wind, solar and micro-hydro electric systems. Without batteries, you can only use power at the time you produce it. In other words, you will not have power when the sun isn’t shining on your solar panels (or the wind isn’t turning your wind turbine, etc.) if you don’t have batteries to store the power when your renewable energy (RE) system is producing power. If your RE system is tied to your utility (grid-tie), you will only need batteries if you want to store power for use during a power outage.

Flooded Lead Acid (FLA), Gel or Absorbent Glass Mat (AGM)?

A full-time, off-grid system will typically experience 50 to 100 cycles per year at 30% to 80% depth-of-charge. Flooded lead acid batteries are always best for this type of application.

FLA batteries are typically less expensive, more efficient, and generally last longer than AGM or gel-cell technologies. The major drawback to these batteries is that they require periodic maintenance – the addition of distilled water occasionally to replenish water lost during the normal charging process, and equalization to remove build-up on the lead plates to keep them at peak efficiency.

Batteries in grid-tie emergency backup (standby) systems typically "float" at full charge for long periods, and are deep cycled only occasionally when there is an outage of the utility grid. Because flooded deep cycle batteries need to be actively charge periodically to prevent stratification of the solution, maintenance free AGM batteries may be a better option in this application. AGM batteries usually cost up to twice as much as flooded lead acid batteries, and require more careful recharging, but their design makes them the best battery type for standby applications.

Also consider AGM batteries for poorly ventilated areas, or where fumes may cause corrosion to electronics, such as repeater and cell phone sites; where shock and vibration resistance is important; where spilled acid from leaking, tipped, or broken batteries cannot be tolerated, in locations where maintenance would be difficult or expensive (such as remote communications sites), where the batteries may be subject to freezing), and/or anywhere you need a totally sealed battery for safety or environmental reasons, such as inside RV’s or in enclosed spaces in boat.

In applications where the batteries must be kept in an unheated space, gel-cell batteries can be a good choice due to their freeze-resistant qualities.

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Battery Bank Sizing Considerations

Off-grid battery banks are usually sized to keep the structure powered for between one and six cloudy days. Because on-grid structures usually consume a considerable amount of electricity, these systems are usually sized only to keep critical electrical loads operating until utility power is restored.

When sizing a battery bank for a stand-alone system, you must first calculate the number of kilowatt hours of electricity that will be consumed each day. Then you need to decide how many days of energy storage you require. This figure will vary depending on your average daily electrical consumption, seasonal variations in how much power your solar, wind or micro-hydro system(s) produces, and how often and how long you are willing to run your backup generator when your RE system is not producing sufficient energy to power your structure.

When sizing a battery bank for grid-tie emergency backup, calculate the kilowatt hours of electricity that will be consumed each day by your critical loads, and decide how long you want to be able run those loads. Because most on-grid consumers do not invest in both a battery bank and a backup generator for emergencies, it is important that the batteries be sized correctly and that you are conservative in your power consumption during grid failures.

How Many Batteries Do I Need?

Most of us are unaware of just how much power the electrical necessities and conveniences in our life consume, so it is strongly suggested that you seek professional design help before purchasing your RE system batteries. Power consumption in most homes increases over time as new loads are added, and because we tend to become less conservative in our energy use, leaving lights and small appliances on more often over time. Because of this, it is essential to consider future needs when calculating your power use.

Future planning is critical because it is unwise to add new batteries to an existing set. The reason is that battery efficiency decreases as they age, and because of how batteries work, the efficiency of any new batteries added to a string will be diminished to that of the existing set of batteries. So, if you foresee growth in your power needs it is best to start with a battery bank that is larger than what is needed at the time of system purchase. However, it is also important that you have sufficient charging capacity so that the life of your batteries is not shortened because of chronic undercharging.

These factors (and more) should to be taken into account so that your batteries are not consistently overdrawn, and so you are not left without power.

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Battery Strings: Putting Batteries in Series

Ideally, a battery bank consists of a single string of battery cells sized in series that are just the right size for the job. This is also the simplest battery bank design and minimizes maintenance and the effect of minor battery variances on your system.

Unfortunately, it is not uncommon for consumers to purchase smaller batteries than they actually need in order to save money. Doing this means that the batteries will need to be connected together in multiple strings and then those strings are connected together in parallel. The problem with this is that the amount of current going to each string is never exactly equal. Terminal corrosion or a slightly weak cell will cause a whole string of batteries to receive less charge, which means that some strings will likely degrade and fail before other strings in the battery bank. Because replacing a single string will just accentuate the variations in the strings, the only practical solution at that point is to replace the entire battery bank. So doing this may save money up front, but the cost of the system over its lifespan will be much higher than it needs to be.

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Housing Your Battery Bank

Proper battery storage is essential. The most common location for a battery bank is an area of a garage or storage building that has a concrete floor, though some choose to build a small building specifically for housing their batteries. If your batteries will be housed in a space that is used for multiple purposes, a battery enclosure can provide a neat and safe way to contain the batteries. Pre-made battery enclosures tend to be expensive though, so most folks simply build their own.

Building a Battery Box

A battery box can be built using standard 2 x 4 framing construction, pressure treated plywood, fire-resistant insulation, and non-flammable paint or other wood sealant. The top door should be hinged to accommodate periodic battery maintenance, and should include a gasket to prevent gases from entering the room. Since batteries lose capacity at lower temperatures, your batteries should not rest directly on a cold uninsulated concrete floor or directly against an uninsulated exterior wall.

Leave air gaps of about 1/2 inch between the batteries to allow for air flow so all batteries stay at the same temperature. Do not install any switches, breakers or other devices that could cause a spark inside the battery box, to reduce the risk of explosion.

If you have gel or absorbent glass matte (AGM) batteries, you will have more flexibility in locating your battery bank since these batteries do not need to be refilled and do not normally generate explosive gasses.

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Venting Your Battery Box

During normal operation, lead acid batteries expel hydrogen gas. This gas is toxic and potentially explosive, and should be vented from your battery box, preferably to the outside air away from potential ignition sources. A 2-inch PVC pipe makes a good vent, but be sure it is located at the highest point in your battery enclosure where the lighter hydrogen gas will accumulate. The pipe should include a screened vent cap to keep out rain and insects. We also recommend a battery box exhaust fan, such as the Zephyr Power Vent Battery Box Vent, to insure that all the gasses are expelled from your battery box.

Battery Charging

Proper charging is essential to preserve the storage capabilities and protect the longevity of your lead acid RE batteries. Conversely, keeping your batteries at a low state of charge or undercharging is a surefire way to reduce their usable life to only a fraction of what it should be.

There are three basic stages when your renewable energy system batteries are being charged; Bulk, Absorption and Float. Bulk charging means electrical current is sent to your batteries at the highest rate they will safely accept, until they are at 80% to 90% of full charge.

In the second phase of charging, Absorption, your charger puts out maximum voltage while the current gradually tapers off.

The main purpose of the final stage of charging, the Float charge (also called trickle charging), is to keep already charged batteries from discharging. During this phase the voltage is lowered to reduce gassing and prolong the life of the batteries.

Ideally, your battery bank should be brought to full charge at least once a week; more frequently is better. At certain times of the year, during certain weather conditions or during periods of higher power consumption, you may need to run your backup generator to "top off" or "finish charge" your batteries and bring them to full charge. Though finish charging can be an inefficient use of fuel, if you are unable to get regular, full charges from your RE system you will need to run your backup generator to do this. It is also important to use a battery charging monitor to make sure your batteries are fully charged. Turning off your generator during the absorption stage would result in only about 85% charge.

Vent caps should always be in place while your batteries are being charged to prevent water loss and splashing of acid.

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Charge Controlling

Battery-based RE systems should have a charge controller to manage the charging of your battery bank. It is important that your charge controller be programmed with the correct set points for your specific battery type in order to reduce the risk of damage from inconsistent charging.

Battery Monitoring

Battery monitors provide crucial information for managing and protecting your battery bank, yet many battery-based systems don’t have one. A good quality battery monitor is relatively inexpensive, and provides important data such as accumulated amp hours, charge status of your battery bank, and maintenance and troubleshooting information.

The battery monitor should be mounted in a central location in your home where it can be easily seen, and it needs to be properly programmed, based on the parameters of your system. Programming is generally only needed, during meter installation.

Care and Maintenance of Your Battery Bank

Equalizing Your Deep Cycle Lead Acid Batteries

Equalization is a controlled overcharge of your battery bank. It provides an aggressive and essential mixing of battery electrolytes, and equalizes cell voltages which become inconsistent during normal battery cycling. Most RE system charge controllers available today have a battery equalization function. If your solar, wind or other RE system is large enough, you can use it for equalization. Otherwise, equalization can be done with your backup generator.

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Adding Water to Your Batteries

As with your automobile battery, fluid is lost during the normal cycling of flooded RE batteries. Low fluid levels cause excessive gassing, which best is damaging to your batteries. At worst it can cause an explosion. This is why flooded batteries require periodic replacement of lost fluid. The type of battery, system usage, charge settings and battery temperature all affect the frequency, but typically you will need to add distilled water every two to six months. It is also important not to overfill your batteries since this can lead to corrosion from spatter and overflow.

A simple solution to maintaining proper fluid levels in your batteries is to install an automatic battery watering system.

Note: Unless the plates are exposed, batteries should be watered after they are fully charged.

Preventing Battery Corrosion

The gasses that escape from flooded batteries during charging tend to accumulate on the terminals and top of the batteries causing corrosion. This corrosion affects the efficiency of the batteries and can potentially be hazardous. The good news is that this corrosion is easy to prevent. Simply apply a thorough coating of a suitable sealant to the battery terminals, wire lugs, and the nuts and bolts. Products designed specifically for this use are available, but plain old petroleum jelly works just fine too.

Prevent corrosion to exposed wires at the terminal lugs by carefully applying tape or coating them thoroughly in petroleum jelly, making sure all connections are good. It is also important to keep the tops of your batteries clean, to prevent dust and acid spatter from corroding the housing. The simplest way to do this is to use distilled water and a rag to clean the tops each time you add water to your batteries. Do not use a solvent or spray cleaner on the outside of your batteries as it could result in an explosion.

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How Temperature Affects Lead Acid Batteries

The temperature of your battery storage area has a strong affect on the amount of electricity a lead acid battery can hold. The capacity of lead acid batteries temporarily diminishes to about 20% of their effective capacity when their temperature falls below 30 F, compared to their rated capacity at 77 F, the standard for battery ratings. (This is why your automotive battery may not start your car at very low outdoor temperatures.) On the other end of the scale, the rate of permanent degradation of lead acid batteries increases at higher temperatures. This is why it is important to protect your batteries from temperature extremes.

If your proposed battery location will not maintain a temperature between 50 and 80F, you will want to house them in an insulated battery box (click here for more about battery boxes). Where low temperatures cannot be avoided, you will need to increase the size of your battery bank to compensate for the reduced capacity during colder temperatures.

Battery charging voltage also changes with temperature. Selecting a charge controller that compensates for temperature variances will eliminate this issue, but only as long as the charge controller is subject to the same temperatures as the batteries. If your charge controller is mounted inside and the batteries are stored outside, it will not compensate for the affect of temperature on charging voltage. If you cannot mount your charge controller in the same location as your batteries, you will need to mount a temperature sensor at the batteries to communicate accurate temperature information to the control.

Another thing to be aware of is that if you are in an area with extremely high or low temperatures, the electrolyte (acid) strength of locally purchased batteries may be different than batteries purchased elsewhere. This difference would need to be taken into account when calculating storage capacity and charging voltage.

Depth of Discharge (DOD) and Battery Life

The life of a battery is directly related to how deep the battery is cycled each time. A battery that is discharged to 80% during each cycle will have about half the life of a battery that is cycled to 50% DOD. A battery that is cycled to 10% DOD will last about 5 times longer than one that is cycled each time to 50% DOD. Obviously there are some sensible considerations to each scenario. You would not want to have a giant bank of batteries just to reduce the depth of discharge. For renewable