Introduction of solar system battery banks
Battery is a device that stores electrical energy. Battery banks consist of one or more batteries. The battery bank is one of the most important components of off grid solar power system. It greatly increases the usability of the PV system.
Significance of battery banks in off grid solar power plant
During the hours of sunshine the off grid solar power systems directly delivers power to load. The excess energy generated during the day will be stored in the battery to meet the load demands when sunlight is not available for example during night . Without the battery, the system would be unable to meet the load demands outside the available daylight. Hence it acts as lifeline of off grid solar photo voltaic system.
One of the problems with solar power is that the flow of electricity can’t be constant the sun sets or goes behind clouds. The battery is necessary in such a system because of the fluctuating nature of the output delivered by the off grid solar arrays to meet the load demand irrespective of the source power (photo generated power) fluctuations.
Solar batteries are really deep cycle batteries that provide energy storage for solar, wind and other renewable energy systems. Different from a car battery, a deep cycle battery is capable of surviving prolonged, repeated and deep discharges which are typical in renewable energy systems that are stand alone(off grid).
Characteristic parameters of solar batteries
Following are some of the important parameters of batteries:
Nominal capacity (qmax)
Nominal or rated capacity of a battery is the number of ampere-hours (Ah) that can maximally be extracted from the battery, under predetermined discharge conditions.
State of charge SOC
State of charge SOC is the ratio between the present capacity and the nominal capacity of the battery qmax:
SOC= q/ qmax
0 ≤ SOC ≤ 1
It is also called as Depth of Discharge (DOD) of solar battery. Being a ratio it has no units. If SOC=1 the battery is totally charged, otherwise if SOC=0 the battery is totally discharged.
Charge (or discharge) regime
It is the parameter which reflects the relationship between the nominal capacity of a battery and the current at which it is charged (or discharged). It is expressed in hours.
For example, discharge regime is 30 h for a battery 150 Ah that is discharged at 5A.
Efficiency of battery
Efficiency is the ratio of the charge extracted (Ah or energy) during discharge divided by the amount of charge (Ah or energy) needed to restore the initial state of charge. It depends on the state of charge SOC and on the charging and discharging current.
It is the number of cycles charge/discharge the battery can sustain before losing 20% of its nominal capacity.
To maintain healthy batteries and prolong battery life, most manufacturers suggest limiting the depth of discharge to about 20%. (That means the deep cycle batteries will be at 80% capacity or better.) At the very least, one should not allow the batteries to be discharged below 50% Depth of Discharge (DOD).
Types of Solar batteries
Some of the typically used deep cycle solar batteries in off grid solar power systems are
- Flooded lead acid batteries
- Sealed Gel batteries
- Sealed AGM (Absorbed Glass Mat batteries)
Principle of operation of lead acid batteries
During discharging phase the electrons flow from positive plate to negative plate due to external force field (here in PV system it will be the force generated by PV generator). In the charged state, each cell contains negative plates of elemental lead (Pb) and positive plates of lead oxide (PbO2) in a sulfuric acid (H2SO4) electrolyte.
Negative plate reaction:
PbSO4(s) + H+(aq) + 2e− → Pb(s) + HSO4− (aq)
Positive plate reaction:
PbSO4(s) + 2H2O(l) → PbO2(s) + HSO4− (aq) + 3H+(aq) + 2e−
During discharging phase the electrons flow from negative plate to positive plate. In this process both the positive and negative plates become lead sulfate and the electrolyte becomes primarily water..
Negative plate reaction:
Pb(s) + HSO4 – (aq) → PbSO4- (s) + H+(aq) + 2e−
Positive plate reaction:
PbO2(s) + HSO4− (aq) + 3H+ (aq) + 2e− → PbSO4(s) + 2H2O(l)
The total reaction can be written as
Pb(s) + PbO2(s) + 2H2SO4(aq) → 2PbSO4(s) + 2H2O(l)
Typically at the discharging voltage of 2 Volts, the energy that battery will be able to supply starting from fully discharged state is around 30–40 watt-hours per kilogram. Overcharging leads to generation of hydrogen and oxygen and hence loss of electrolyte.
Flooded Lead Acid batteries
Flooded Lead Acid type battery is one of the oldest rechargeable batteries and is invented by a French scientist Gaston plant. It consists of sulfuric acid between two identical lead sulfate plates.
This type of battery contains electrolyte in liquid in an unsealed container. This means that the battery must be kept upright and in a well ventilated area to ensure safe dispersal of the hydrogen gas produced by these batteries during overcharging.
Flooded Lead Acid (FLA) batteries require a bit of maintenance, however, they generally last longer than their sealed counterparts. Vents are provided to remove the gases produced during overcharging. Periodic inspection of the electrolyte level and replacement of any water that has been lost is must for flooded lead-acid batteries.
Flooded Lead Acid deep cycle batteries can build up sulphate crystals on their lead plates over time. Battery desulfators can be used to get rid of this problem as they shake these sulphate crystals off the surface electronically and can actually save the life of a battery that would otherwise be thrown out.
Sealed Gel and AGM batteries (VRLA)
Sealed Gel and AGM batteries belongs to a class of solar batteries known as valve regulated lead acid batteries. It is a type of lead acid rechargeable batteries. A gel battery (also known as a “gel cell”) is a VRLA battery with a jellified electrolyte; the sulfuric acid is mixed with silica fume, which makes the resulting mass gel-like and immobile. AGM batteries differ from flooded lead acid batteries in that the electrolyte is held in the glass mats, as opposed to freely flooding the plates. The principle of operation of sealed AGM and Gel type batteries is same as that of flooded lead batteries.
Advantages of valve regulated lead acid batteries
- Reduced Maintenance
- Without the need to add water, most routine maintenance required on flooded batteries is eliminated.
- Elimination of Gassing
- No gases are evolved from the battery because they are recombined inside the container. Thus, there is no need to ventilate the battery compartment or room.
- Higher safety
- These batteries are intrinsically safer than flooded types. The acidic spray emitted from vents of flooded batteries during charging that can cause corrosion of battery terminals and affect adjacent electronic circuitry does not occur.
- Operation in Any Orientation
- Electrolyte immobilization prevents acid spills. These batteries can be used on their side, a big advantage in installations where space is limited.
Disadvantages of valve regulated lead acid batteries
VRLA batteries has some disadvantages compared to flooded type acid batteries such as
- Less reliable compared to Flooded Type A lead acid batteries
- Overcharging leads to damage of batteries
- Less battery life
Sizing of battery banks(number of batteries in battery bank)
Each deep cycle battery can be 2, 4, 6, or 12 volts DC (Vdc). The choice of individual solar battery voltage and number of batteries in the battery bank depends on two factors
- Photo generated voltage (12, 24 or 48 Vdc)
- Desired amp hours of storage.
For example: if you are using 2 V deep cycle batteries you need up to 24 of these huge deep cycle batteries, if your system requires 48 volts.