Understanding Solar PV Batteries

For most people a battery is just a “black or grey colored box” that performs silently without ever being noticed until it dies. But in reality there are significant differences in design, construction and performance among various types of batteries available in the market. Since solar power is newly emerging concept, it helps to be informed about this critical component.

Storage batteries are the heart of all stand alone solar PV electrical systems whether electrical load require DC or AC supply. Their function is to fulfill the requirements of connected electrical loads when the sun is not shining and the panels are not producing current. Batteries are already used with UPS/inverter systems to provide backup power; they are also often charged with diesel generators.

The performance of a PV system depends upon both battery design and the operational parameters of the system. A battery not designed and constructed for the operational conditions of a solar PV system will almost certainly fail prematurely. Likewise, hostile operational conditions and lack of proper maintenance will result in failure of even the more durable and robust deep-cycle batteries. Therefore, both system design and the battery construction play crucial role in determining the health of the overall PV system.

Type of Batteries

Lead Acid Batteries: Lead acid batteries are the most common type of energy storage in PV systems due to their versatility and low cost. They are based on the lead/sulphuric acid chemical reaction. They have evolved into two groups: 6V or 12V batteries in tough plastic cases with capacities up to say 200Ah and the larger capacity 2V battery bank Cells, ranging from about 100 Ah to several thousand Ah capacities.

The 6V or 12V batteries are either flooded cell type with vent for gassing and require topping-up with distilled water; or sealed/ gel types which have immobilized electrolyte. The 2V cells are generally flooded type but sealed types are also now available.

What are sealed maintenance free (SMF) lead acid batteries?

These batteries have the electrolyte in the immobilized form and can’t be replenished. They are often also referred to as valve regulated lead acid (VRLA) batteries. The two most common captive electrolyte batteries are the gelled electrolyte and absorbed glass mat designs. A benefit of immobilized electrolyte designs is that they are less susceptible to freezing compared to flooded batteries. However, the VRLA battery technology is very sensitive to charging methods, regulation voltage and temperature extremes.

These captive electrolyte lead-acid batteries are popular for PV applications because they are spill-proof and easily transported. Since they don’t require water additions, they are ideal for remote applications where regular maintenance is difficult. However, a common reason for failure of these batteries in PV systems is excessive overcharge and loss of electrolyte which is accelerated in the warm climates. For this reason, it is essential that the battery charge controllers are used with proper adjustments of set points to prevent overcharging.

Note that while charging lead-acid battery nears full state of charge, hydrogen and oxygen gasses are produced from water by the reactions at the negative and positive plates, respectively. In a flooded battery, these gasses escape from the battery through the vents, thus requiring periodic water additions. In the VRLA batteries, these gases are made to recombine to form water which eliminates the need for water addition.

Gelled Batteries

The electrolyte is ‘gelled’ by the addition of silicon dioxide to the electrolyte, which is then added to the battery in a warm liquid form and gels as it cools. In the gelled batteries hydrogen and oxygen produced due to overcharging are made to recombine which reduces water loss. The path for gas transport between the positive and negative plates is provided by the cracks and voids which develop within the gelled electrolyte during the first few cycles of charge and discharge. Drying of the gel during charging is a major cause of loss of performance for gelled lead-acid batteries.

Absorbed Glass Mat (AGM) Batteries

In an AGM battery the electrolyte is absorbed in glass mats which are sandwiched in layers between the plates rather than as a nearly solid mass as in the gelled batteries. Similar in other respects to gelled batteries, AGM batteries are also intolerant to overcharge and high operating temperatures. They also require proper charging methods like gelled batteries. In an AGM battery, the oxygen molecules from the positive plate migrate through the electrolyte suspended in the glass mats and recombine with the slowly evolving hydrogen at the negative plate to form water. Under conditions of controlled charging, the pressure relief vents in AGM batteries are designed to remain closed which prevents release gasses and water loss.

AGMs are typically used when the batteries are kept at a full state of charge as in home UPS/inverters. In cold climates, the gel type batteries are a good choice due to their freeze-resistant qualities; they are unsuitable in hot environment. AMG and Gelled batteries normally use lead-calcium plates which reduces gassing and water loss.

Nickel Cadmium Batteries: These are manufactured in many sizes. Sealed batteries are of smaller capacities. The larger ‘wet’ nicad are ideal for renewable energy storage. The main disadvantages of nickel-cadmium batteries are their high cost and limited availability compared to lead-acid designs.

A typical nickel-cadmium cell consists of positive electrodes made from nickel-hydroxide [NiO(OH)] and negative electrodes made from cadmium (Cd) and immersed in an alkaline potassium hydroxide (KOH) electrolyte solution. When a nickel-cadmium cell is discharged, the nickel hydroxide changes form [Ni(OH)₂] and the cadmium converts to cadmium hydroxide [Cd(OH)₂].

Do Nickel-Cadmium batteries offer any extra advantage?

Nickel-cadmium (Ni-Cd) batteries offer several advantages over lead-acid batteries that make them attractive for use in stand-alone PV systems. These advantages include long life, require little maintenance, can be 100% discharged, left in any state of charge without damage, do not need critical voltage regulation, withstand overcharging and temperature extremes, and have excellent low temperature capacity retention. Nickel-cadmium batteries can accept significant charging rates, and are also rather more tolerant of continuous overcharge compared to the LA batteries.

The nominal voltage for a nickel-cadmium cell is 1.2 volts, compared to about 2.1 volts for a lead-acid cell, requiring 10 nickel-cadmium cells to be configured in series for a nominal 12 volt battery. The voltage of a nickel-cadmium cell remains relatively stable until the cell is almost completely discharged, where the voltage drops off dramatically.

How to choose between the flooded and sealed lead acid batteries?

Sealed batteries are preferred in areas with poor ventilation, in remote areas or applications where maintenance is a problem or is costly, when the batteries are located where checking water level is difficult or when batteries must be mounted in non-vertical orientation.

When none of these conditions apply, flooded batteries are the obvious choice. They are nearly always cheaper and are available in various types and sizes. They are also the automatic choice when you require large battery bank; sealed batteries have rather sparse range of choices for capacities over 250 amp-hours.

What are Shallow cycle and Deep cycle Batteries?

A shallow cycle means discharging only a little, say 20% or less, and recharging back to full. Such batteries are used in the automobiles and are designed with rather thin plates which give a large surface area to provide a lot of current for a short period, as needed while starting the vehicle. At the start such a battery delivers several hundred amperes for a few seconds, then the alternator takes over and the battery is quickly recharged.

These types of batteries are not suitable for solar PV system and would not last very long. PV systems demand continuous and deep discharging and recharging of batteries. Therefore, deep cycle batteries are needed for the PV systems; they deliver fewer amperes but for longer hours between charging. Deep cycle batteries are designed with thicker electrode plates and thus, have less overall surface area. They are suitable for giving current for much longer periods.

They can normally withstand up to 80% discharge (better not to use the remaining 20%). However, it is recommended to use only 50% as the normal maximum discharge and leave 30% for emergencies.

Battery Life

This is measured usually in number of discharge-charge cycles rather than years. The more deeply the battery is discharged the lower the number of cycles it will last for. Therefore, for maximum battery life it is best to shallow-cycle the deep cycle batteries. For example, for its TORR range of low maintenance solar Tubular batteries EXIDE gives the following cycle lives:

1500 cycles at 80% DoD,

3000 cycles at 50% DoD, and

5000 cycles at 20% DoD

Battery Capacity

This is how much electrical energy the battery can deliver and is measured in ampere hours (Ah) when uniformly discharged over a given period of time. For example, a 120Ah battery rated for 10 hr discharge (C10) will be fully discharged in 10 hours by supplying uniform 12 Ampere current. If discharged at a higher rate (by a higher current) then the battery capacity is reduced (will be less than 100 Ah) and vice versa.

The maximum charge/discharge current should be no more than 10% of battery Ah capacity. Further, lower temperatures significantly reduce battery capacity and as batteries age their Ah capacity is reduced. The capacity of the battery bank needed depends upon factors like backup time or number of days of autonomy and also on operating temperature. In lower temperature conditions, larger battery banks are required since capacities are reduced.

Battery Operation

Overcharging a battery raises the temperature leading to excessive gassing, loss of distilled water and eventually damage to the plates. Therefore, use of a suitable charge regulator is necessary with any battery charging system to limit charging current as the battery voltage rises.

Excessive discharge of a battery can also lead to permanent damage. If a battery is close to its fully discharged state loads must be disconnected and it should be recharged immediately. Normally the charge-discharge circuit has a low voltage disconnection point to prevent excessive discharge. Monitoring of batteries is generally done by monitoring its voltage; it can also be done by checking the specific gravity of the electrolyte.

Equalization

During prolonged operation it often happens that individual cells of a battery acquire slightly different voltages. Thus, an equalization is recommended. It is done through a boost charging and allowing it to “gas” freely for about an hour. This helps avoid electrolyte stratification. This is not an issue with sealed batteries.

Efficiency

Like any other electrical device batteries are not 100% efficient and energy is lost during storage (self discharge), charging and discharging. With new batteries efficiencies of about 90% can be expected but then with aging sulphation and stratification take place which progressively reduce efficiency. In order to get the best from the batteries they must be kept at room temperature, sized correctly for their purpose and must not be charged or discharged too rapidly.

Note that sulphation is caused when a battery is left in a partial state of charged state for some longer period and stratification results by shallow cycling allowing the battery electrolyte to settle into layers of different densities. Both can be prevented by regular equalization and keeping the battery fully charged.

Battery Maintenance

Batteries should be ideally installed in a weather-proof and ventilated area at 25 – 30 ˚C. For optimum performance in a battery bank all batteries should all be of the same brand, age and amp-hour capacity. Connections should be tight and covered with petroleum jelly to prevent corrosion. They produce explosive hydrogen gases during charging, so no sparks or flames should be allowed near them.

Keeping an eye of batteries state of charge is useful. However, a typical battery bank will be usually charging or discharging (often at the same time) so there is a high degree of voltage variation throughout the day. Thus voltage alone can’t give accurate status of the state of charge (SOC). It can also be checked by measuring the specific gravity using a hydrometer: a reading in the range 1.25-1.28 implies full charge and 1.14-1.16 indicates discharged state.

However, with tall cells, since the electrolyte can only be sampled above the battery plates, readings can be misleading due to stratification in the electrolyte. Further, specific gravity varies inversely with temperature: it is lower in the warmer battery, and vice versa. Of course, a hydrometer cannot be used with sealed AGM or Gel batteries. The best method is to use an Ampere-hour meter; it keeps record of battery net usage.

Frequently Asked Questions on batteries