Batteries

Batteries store the electrical energy generated by the modules during sunny periods, and deliver it whenever the modules cannot supply power. Normally, batteries are discharged during the night or cloudy weather. But if the load exceeds the array output during the day, the batteries can supplement the energy supplied by the modules.

The interval which includes one period of charging and one of discharging is described as a "cycle." Ideally, the batteries are recharged to 100 % capacity during the charging phase of each cycle. The batteries must not be completely discharged during each cycle.

No single component in a photovoltaic system is more affected by the size and usage of the load than storage batteries. If a charge controller is not included in the system, oversized loads or excessive use can drain the batteries' charge to the point where they are damaged and must be replaced. If a controller does not stop overcharging, the batteries can be damaged during times of low or no load usage or long periods of full sun.

For these reasons, battery systems must be sized to match the load. In addition, different types and brands of batteries have different "voltage set point windows." This refers to the range of voltage the battery has available between a fully discharged and fully charged state.

As an example, a battery may have a voltage of 14 volts when fully charged, and 11 when fully discharged. Assume the load will not operate properly below 12 volts. Therefore, there will be times when this battery cannot supply enough voltage for the load. The battery's voltage window does not match that of the load.

Performance

The performance of storage batteries is described in two ways. These are (1) the amp-hour capacity, and (2) the depth of cycling.

Amp-hour capacity

The first method, the number of amp-hours a battery can deliver, is simply the number of amps of current it can discharge, multiplied by the number of hours it can deliver that current.

System designers use amp-hour specifications to determine how long the system will operate without any significant amount of sunlight to recharge the batteries. This measure of "days of autonomy" is an important part of design procedures.

Theoretically, a 200 amp-hour battery should be able to deliver either 200 amps for one hour, 50 amps for 4 hours, 4 amps for 50 hours, or one amp for 200 hours.

This is not really the case, since some batteries, such as automotive ones, are designed for short periods of rapid discharge without damage. However, they are not designed for long time periods of low discharge. This is why automotive batteries are not appropriate for, and should not be used in, photovoltaic systems.

Other types of batteries are designed for very low rates of discharge over long periods of time. These are appropriate for photovoltaic applications. The different types are described later.

Charge and discharge rates

If the battery is charged or discharged at a different rate than specified, the available amp-hour capacity will increase or decrease. Generally, if the battery is discharged at a slower rate, its capacity will probably be slightly higher. More rapid rates will generally reduce the available capacity.

The rate of charge or discharge is defined as the total capacity divided by some number. For example, a discharge rate of C/20 means the battery is being discharged at a current equal to 1/20^th of its total capacity. In the case of a 400 amp-hour battery, this would mean a discharge rate of 20 amps.

Temperature

Another factor influencing amp-hour capacity is the temperature of the battery and its surroundings. Batteries are rated for performance at 80°F (26.7°C). Lower temperatures reduce amp-hour capacity significantly. Higher temperatures result in a slightly higher capacity, but this will increase water loss and decrease the number of cycles in the battery life.

Depth of discharge

The second description of performance is depth of discharge. This describes how much of the total amp-hour capacity of the battery is used during a charge-recharge cycle.

As an example, "shallow cycle" batteries are designed to discharge from 10% to 25% of their total amp-hour capacity during each cycle. In contrast, most "deep cycle" batteries designed for photovoltaic applications are designed to discharge up to 80% of their capacity without damage. Manufacturers of deep cycle "Ni cad" batteries claim their product can be totally discharged without damage.

Even deep cycle batteries are affected by the depth of discharge. The deeper the discharge, the smaller the number of charging cycles the battery will last. They are also affected by the rate of discharge and their temperature.

Different battery types

Vented lead acid batteries

Although automotive batteries are not appropriate for photovoltaic applications, deep cycle lead acid batteries similar to automotive types, are referred to as marine type batteries and are used more often.

These batteries are true deep cycle units. They can be discharged as much as 80%, although less discharge depth will result in more charge cycles and thus a longer battery life.

Internal construction

These batteries are made up of lead plates in a solution of sulfuric acid. The plates are a lead alloy grid with lead oxide paste dried on the grid. The sulfuric acid and water solution is normally called "electrolyte."

The grid material is an alloy of lead because pure lead is a physically weak material. Pure lead would break during transportation and service operations involving moving the battery

The lead alloy is normally lead with 2-6% antimony. The lower the antimony content, the less resistant the battery will be to charging. Less antimony also reduces the production of hydrogen and oxygen gas during charging, thereby reducing water consumption. On the other hand, more antimony allows deeper discharging without damage to the plates. This in turn means longer battery life. Lead-antimony batteries are deep cycle types.

Cadmium and strontium are used in place of antimony to strengthen the grid. These offer the same benefits and drawbacks as antimony, but also reduce the amount of self-discharge the battery has when it is not being used.

Calcium also strengthens the grid and reduces self-discharge. However, calcium reduces the recommended discharge depth to no more than 25%. Therefore, lead-calcium batteries are shallow cycle types.

Both positive and negative plates are immersed into a solution of sulfuric acid and subjected to a "forming" charge by the manufacturer. The direction of this charge causes the paste on the positive grid plates to convert to lead dioxide. The negative plates' paste converts to "sponge" lead. Both materials are highly porous, allowing the sulfuric acid solution to freely penetrate the plates.

The plates are alternated in the battery, with separators between each plate. The separators are made of porous material to allow the flow of electrolyte. They are electrically non-conductive. Typical materials include mixtures of silica and plastics or rubber. (Originally, spacers were made of thin sheets of cedar.)

Separators are either individual sheets or "envelopes." Envelopes are sleeves, open at the top, which are put on only the positive plates.

A group of negative and positive plates, with separators, makes up an "element". An element in a container immersed in electrolyte makes up a battery "cell."

Larger plates, or more of them, will increase the amp-hours the battery can deliver. Thicker plates, or less plate count per cell, will allow a greater number of cycles and longer lifetime from the battery.

Regardless of the size of the plates, a cell will only deliver a nominal 2 volts. Therefore, a battery is typically made up of several cells connected in series, internally or externally, to increase the voltage the entire battery can deliver.

This is why a six volt battery has three cells, and 12 volt batteries have six. Some batteries used in photovoltaic systems have only one cell, allowing the user to have any number of volts in the battery system, as long as it is a multiple of two.

Terminals

The internal straps which make these internal connections are brought up to the top of the battery and connected to the external terminals. The most familiar terminal is the tapered top type. The taper allows for a wide variety of cable clamp sizes. The positive terminal is slightly larger than the negative one to reduce the chance of accidentally switching the cables. Other terminal types used more often in photovoltaic battery applications include "L" terminals, wing-nut terminals and "universal" terminals. The type of terminal used may depend on the number and type of interconnections between the batteries and the balance of the system.

Interconnections can be made with short cables, #2 AWG or larger. The cables end in appropriate terminals. They can also be made with bus bars made specifically for this purpose by the battery manufacturer.

Venting

The cells of a vented lead acid battery are vented to allow the hydrogen and oxygen gas to escape during charging, and to provide an opening for adding water lost during gas production. Section 3.1.7 provides more information on battery venting requirements.

Although open caps are most common, the caps may be a flame arrester type, which prevents a flame from outside the battery from entering the cell.

"Recombinant" type caps are also available. These contain a catalyst that causes the hydrogen and oxygen gases to recombine into water, significantly reducing the water requirements of the battery.

WARNING!
Never smoke or have open flames or sparks around batteries! As the batteries charge, explosive hydrogen gas is produced.
Always make sure battery banks are adequately vented and that a No Smoking sign is posted in a highly visible place. 

Sulfation

If a lead acid battery is left in a deeply discharged condition for a long period of time, it will become "sulfated". Some of the sulfur in the acid will combine with lead from the plates to form lead sulfate. If the battery is not refilled with water periodically, part of the plates will be exposed to air, and this process will be accelerated.

Lead sulfate coats the plates so the electrolyte cannot contact it. Even the addition of new water will not reverse the permanent loss in battery capacity.

Treeing

Treeing is a short circuit between positive and negative plates caused by misalignment of the plates and separators. The problem is usually caused by a manufacturing defect, although rough handling is another cause.

Mossing

Mossing is a build-up of material on top of the battery elements. Circulating electrolyte brings small particles to the top of the battery where they are caught on the element tops. Mossing causes shorts between negative and positive plates. Heavy mossing causes a short between the element plates and the plate strap above them.

To avoid mossing, the battery should not be subjected to continuous overcharging or rough handling.

State of charge, specific gravity and voltage

The percentage of acid in the electrolyte is measured by the "specific gravity" of the fluid. This measures how much the electrolyte weighs compared to an equal quantity of water. Specific gravity is measured with a hydrometer.

The greater the state of charge, the higher the specific gravity of the electrolyte. The voltage of each cell, and thus the entire battery, is also higher. Measuring specific gravity during the discharge of a battery will be a good indicator of the state of the charge. During the charging of a flooded battery, the specific gravity will lag the state of charge because complete mixing of the electrolyte does not occur until gassing commences near the end of charge. Because of the uncertainty of the level of mixing of the electrolyte, this measurement on a fully charged battery is a better indicator of the health of the cell. Therefore, this should not be considered an absolute measurement for capacity and should be combined with other techniques.

Freezing point

Since lead acid batteries use an electrolyte which is partially water, they can freeze. The sulfuric acid in a battery acts as an antifreeze, however. The higher the percentage of acid in the water, the lower the freezing temperature. However, even a fully charged lead acid battery will freeze at some extremely low temperature.

At a 50% charge, a typical lead acid battery will freeze around -10°F (-23.3°C). Notice that as the state of charge goes down, the specific gravity goes down as well The acid is becoming weaker and weaker, and lighter and lighter, until it is only slightly denser than water.

NOTE
The information in Table 2-3 is for deep cycle lead acid batteries. Shallow cycle automotive batteries have slightly different values.

TABLE 2-3:
States of Charge, Specific Gravities, Voltages, and Freezing Points for Typical Deep Cycle Lead Acid Batteries

State of Charge	Specific Gravity	Voltage per Cell (volts)	Voltage of 12V (6 cell) Battery	Freezing Point (°F)
Fully Charged	1.265	2.12	12.70	-71 (-57.2°C)
75% Charged	1.225	2.10	12.60	-35 (37.2°C)
50% Charged	1.190	2.08	12.45	-10 (-23.3°C)
25% Charged	1.155	2.03	12.20	+3 (-16.1°C)
Fully Discharged	1.120	1.95	11.70	+17 (-8.3°C)

The charging characteristics of lead acid batteries changes with electrolyte temperature. The colder the battery, the lower the rate of charge it will accept. Higher temperatures allow higher charge rates.

If a battery will be used in a climate that will continuously be extremely hot or cold, with minimum temperature swings, it would be wise to adjust the electrolyte specific gravity for the temperature. This will help extend the life and enhance the performance of the battery under these extreme conditions. This adjustment should be done at the battery manufacturer, or through their supervision.

For example, a typical lead acid battery which is half charged will only accept two amps at 0°F (-17,8°C). At 80°F (26,7°C), it will accept over 25 amps. This is why most charge controllers equipped with temperature compensation change their voltage settings with temperature. A few measure the battery temperature, and adjust the charging rate (current flow) accordingly.

A final characteristic of lead acid batteries is their fairly high rate of self discharge. When not in service they may loose from 5% per month to 1% per day of their capacity, depending on temperature and cell chemistry. The higher the temperature, the faster the rate of self-discharge.

Sealed flooded (wet) lead acid batteries

As described before, the use of less antimony, or using calcium, cadmium, or strontium in place of antimony, results in less gassing and lower water consumption. However, these batteries should not be discharged more than 15-25%, or the life of the battery will be dramatically shortened.

Self discharge is less of a factor with sealed lead acid batteries due to the fact that these batteries are typically lead-calcium or lead-calcium/antimony hybrids. Self discharge can be minimized by storing batteries in cool areas between 5-15°C.

The rate of water loss may be so low that the vent plugs for each cell can be nearly or completely sealed . In most of these batteries, there is still some production of hydrogen gas. Therefore, a venting system is still required, but it is typically a pressure valve regulated system.

The temperature range sealed batteries can accommodate is about the same as unsealed batteries. Since the specific gravity cannot be measured with a hydrometer, many sealed batteries have a built in hydrometer.

A built-in hydrometer is a captive float in the electrolyte. If the specific gravity is high enough, the float comes up against a window at the top of the battery. If the float is visible in the window, the battery is nearly fully charged. In PV systems, sometimes this float gets stuck and the battery should be lightly tapped to ensure free movement of the hydrometer.

If the battery is not fully charged, the float will sink, and cannot be seen in the window.

The charging characteristics of sealed lead acid batteries also changes with electrolyte temperature. Charge controllers used on these batteries should include temperature compensation for battery temperatures below 70°F (21,1°C).

Captive electrolyte batteries

Batteries with a gelled (Gel) or absorbed glass mat (AGM) electrolyte are available completely sealed. These batteries are sometimes referred to as "Valve Regulated Batteries." Some of the newer batteries have catalytic recombiners internal to their battery to aid in the reduction of water loss. All sealed batteries will vent if they are overcharged to the point of excessive gassing to prevent extreme pressures from building up in the battery case. This electrolyte is then lost forever and the life of the battery may be shortened. This problem can be reduced or eliminated by properly charging the battery as recommended by the manufacturer and by using temperature compensation in the charge controller.

This type of battery is generally a lead calcium or lead calcium/antimoninal hybrid. Because the electrolyte is captive, there is no need to charge the battery high enough to gas the electrolyte. The battery can be used in any position, even upside down. Since the electrolyte does not flow away from the plates, the battery still delivers full capacity. The manufacturer should be consulted for the proper regulation voltage for their specific battery. These batteries are typically shallow cycle batteries. Discharging these batteries greater than 20% will significantly reduce the lifetime of the battery.

These batteries have shown some temperature limitations, typically ranges in excess of -20 to +50 degrees C should be avoided. Self discharge rates are very low, comparable to lead calcium batteries or better.

Nickel cadmium (Ni cad) batteries

Ni cad batteries have a physical structure similar to lead acid batteries. Instead of lead plates, they use nickel hydroxide for the positive plates and cadmium oxide for the negative plates. The electrolyte is potassium hydroxide.

The cell voltage of a typical Ni cad battery is 1.2 volts, rather than the two volts per cell of a lead battery.

Ni cad batteries can survive freezing and thawing without any effect on performance. High temperatures have less of an effect than they have on lead acid batteries. Self-discharge rates range from 3-6% per month.

Ni cad batteries are less affected by overcharging. They can be totally discharged without damage. They are not subject to sulfation. Their ability to accept charging is independent of temperature.

Although the initial cost of Ni cad batteries is higher than lead acid types, their lower maintenance costs and longer lives make them a logical choice for many photovoltaic installations. This is particularly true if the system is in a remote or dangerous location.

Since battery maintenance is a major part of all photovoltaic system maintenance, significant reductions in service time and costs can be achieved.

However, Ni cad batteries cannot be tested as accurately as a "wet" lead-acid battery. If state of charge monitoring is necessary, Ni cad may not be the best choice.

Source: Polar Power Inc.

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