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 1/20th 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,°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 |
| 75% Charged |
1.225 |
2.10 |
12.60 |
-35 |
| 50% Charged |
1.190 |
2.08 | 12.45 | -10 |
| 25% Charged |
1.155 |
2.03 |
12.20 | +3 |
| Fully Discharged |
1.120 |
1.95 |
11.70 |
+17 |
Battery Types
- Automotive batteries (regular type, used in cars, also called SLI batteries)
- very short service life, high maintenance, only shallow charging
+ very low price, good availability
- Automotive batteries (heavy- duty type, used in trucks and buses)
- short service life, medium maintenance, shallow discharging
+ low price, good availability
- Solar batteries (different designs)
- medium discharging, limited availability, medium price
+ better service life, limited maintenance
- High-quality batteries (different designs)
- expensive (4-6 x automotive battery price)
+ high service life, low maintenance, rather deep discharging
|
Characteristics of battery types Figures depend on type and operating conditions | |||
|
|
automotive |
solar |
quality |
|
Energy efficiency |
70% |
75% |
80% |
|
Self-discharge/month |
20% |
4% |
2.5% |
|
Costs per kWh |
70 € |
120 € |
300 € |
|
Maintenance |
4x/a |
2x/a |
1x/a |
Automotive batteries are unsuitable if any alternative exists. Heavy-duty ones may be used in smaller systems with severe budget constraints or if others are not available. High-quality, deep-cycle batteries are optimal for solar applications. Solar batteries are a compromise between quality and costs.
As with modules, batteries are connected in series for higher voltage levels or in parallel to increase the capacity. Batteries that are connected should be identicalin type, capacity, age and electrolyte density. Parallel connection should be avoided if possible.
Service life of batteries
The service life of batteries, which is expressed in cycles of charging and discharging periods, increases with:
- the quality of the battery,
- the quality of the charge controller,
- low temperature,
- a low depth of discharge and
- careful operation and maintenance.
Depth of discharge
The depth of discharge (DoD) is that portion of the battery capacity which is removed during discharging. The capacity is indicated in ampere hours (Ah) or sometimes in days of autonomy. This is the number of days a fully charged battery can satisfy the energy demand. The common 12 V battery includes six cells of 2 V in one housing.
Example: A 12 V, 60 Ah battery can store 12 V x 60 Ah = 720 Wh. At a 75% depth of discharge, 540 Wh have been removed and the remaining capacity is 180 Wh / 12 V = 15 Ah. If the daily energy demand is 270 Wh, the battery has 2 days of autonomy.
As a rule of thumb, reducing the DoD by half will double the number of cycles. Furthermore, batteries have to be protected against overcharging and deep discharging by a charge controller.
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