Pb battery Chemical equation: discharge> Pb+2 H 2 SO 4 + PbO 2 <charge 2 PbSO 4 +2 H 2 O
Acid density: Acid density (ad) determines: Voltage (rule of thumb: Vo=ad+0,84) => state of charge can be determined by the acid density when the battery is in a `rest state` Capacity => maximum acid density is at 1,28 kg/l. Above this value reactions without external circuit will take place. It also tends to increase corrosion
Processes and problems during operation: At the moment Lead Acid Batteries are the cheapest and most practical solution for photovoltaic systems. But: Lead Acid Batteries are sensitive to overcharging and deep discharge Other chemical reactions will lead to problems and damage
Gassing: When the battery is charging, the charge current will lead to a electrolytic breakdown of the water in the electrolyte. => Electrolyte gets lost, the acid concentration increases => During high gassing, some particles will be blasted off the plates. => The particles will settle on the bottom of the battery cell. In extreme cases this can lead to a short circuit between the plates. Result: capacity loss as particles get blasted off the plate maintenance is required e.g. replacing the water in the electroyte Destruction of cells caused by short circuit
Remedy to high gassing: Limiting gassing Voltage regulation The intensity of gassing is dependant on the temperature and the voltage. Therefore the regulation voltage should be automatically adjusted to the temperature. Maintenance work can be limited by: catalytic recombination Larger amount of acid in the cell Reduction of short circuit occurance Pocket separator => Particles are kept in a pocket around the plate Large distance between plate and bottom of the cell (Footroom)
Acid density layers: Dense acid, created in the plates during charging `flows` along the plates towards the bottom of the cell. => The acid density at the bottom part of the plate is higher than on the upper part. => There s a voltage drop between the upper and lower part of the battery plates. => The voltage difference leads to higher discharge of the lower part of the plates. This results in: Sulfation of the lower parts of the plates capacity loss Remedy: Circulation of acid with a pump or by gassing.
Corrosion: The structure which holds the battery plates together is often made of Lead which has been oxidized on the surface. These `holders will be attacked by the acid and slowly destroyed. The speed at which this destruction takes place is dependant on the acid density and the temperature. Remedy: restriction of charge voltage lowering the nominal acid density (particularily at high ambient temperatures) In solar systems the consequences of corrosion are often overestimated.
Acid concentration gradient: During discharge water will be developed, during charging sulphuric acid. This will lead to a difference in the acid concentration between the inside of the Pb-plate and the electrolyte around it. =>Open circuit voltage adjusts itself according to the inside acid density, the voltage of the cell will be shifted accordingly. =>In operation it is inaccurate to determine the state of charge via the terminal voltage of the cell. However it is a rough guide.
Sulfation: Main problem in stand alone solar systems!!!!!! Pb sulphate which develops during discharging is crystalline. After discharing the crystalls are small but have a large surface area. Lead sulfate disolves in the electrolyte and tends to recrystallize in areas which already contain Lead sulfate crystalls. => crystalline growth => smaller crystalline (reaction) surface => Pores of the plate will get closed
Consequences of sulfation: => Higher internal resistance caused by smaller reaction surface => Higher acid concentration differential caused by closed pores. This results in: Cells develop a high resistance and stop working.
Precautions against sulfation and remedy: Prevent a low state of charge using deep charge protection. (speed of sulfation is dependent on the amount of Lead sulphate) Higher concentration of acid in the cell as the solubility of Lead sulphate will increase with lower acid density. Fully charge the battery as often as possible using => Hybrid systems with additional sources (e.g. Diesel generator) => Intelligent (but restrictive) Charging technology
Movement caused by mass change: During discharging of the Battery PbSO 4 ( lead sulphate) will be developed by the reaction of Pb (lead) and PbO 2 ( lead oxide). The volume of PbSO 4 is 1,5-times higher than the volume of PbO 2 and 3-times higher than the volume of Pb. => Each discharge cycle causes a mechanical movement of the plate. => With time the structure of the plates will be destroyed Remedy: Discharge protection to restrict the discharging Selection of a stable plate
Different plate types for Lead Acid Batteries: Plate with a large surface area: (positive plates) Consists of pure solid Lead. The surface is rippled to create a large surface area for the chemical reactions to occur on. Characteristics: High cycle life, small capacity, expensive box plate: (positive and negative plates) Lead box holds the active mass together Characteristic: high cycle life, expensive, very rare
Most common plate type: Grid plate:(+)(-) The active mass will be pasted into the grid made of pure Lead. Characteristics: Extremely efficient structure, susceptible to corrosion and movement caused by change of mass, high peak current output. Tubular plate:(+) The Lead active mass will be pressed into a porous plastic tube. Characteristics: very resistant to mass movement; high cyle life
Common Battery types and their use in solar systems There are many different types of Lead Acid Batteries available on the market. Each has advantages in different applcations. For every application you have to consider the following criteria Capacity Field conditions (Industrial usage, rural electrification, leisure...) Ease of maintenance and maintenance cost The choice of which Battery will be used influences the cost structure of the whole system significantly. In most cases the user has to make a compromise between the different battery characteristics and the cost.
Lead accid battery types: Starter battery Starter batteries are designed to start engines. They predominantly use grid plates. Characteristics short cycle life (about 50 complete cycles) susceptible to sulfation and mass movement high self discharge very cheap To sum up: Starter batteries are unsuitable for solar systems!!! In cases where only starter batteries are available Oversize the Solar generator and the battery to minimise depth of discharge.
Lead acid battery types: Solar batteries Solar batteries are modified Starter batteries. Ideally the following measures have been implemented: Thicker, more solid plates (-> higher cycle life) Less antimony in the grid (less self discharge) More electrolyte with less acid density (less sulfation, maintenance and corrosion) Cost: about 30-50% more than starter batteries for similar capacity Use: Leisure market and rural electrification Size of the systems 30-500Wp
Lead Acid Batteries: Sealed Batteries In sealed Batteries the acid will be held in an Absorbed Glas Mat (AGM) or in Gel. The plate mainly consists of grid plates. In rare cases the negative plate in Gel Batteries is a tubular plate. Only limited gassing is allowed in Sealed Batteries. Maintenance is not possible. Small self discharge. relatively deep discharge tolerated.
Lead Acid Type Batteries: Sealed Batteries Difference between AGM and Gel Gel Batteries are resistant to sulphation. => They can be stored for up to 1,5 years without charging minor acid stratification will develop in AGM Batteries => Danger of sulfation exists, limitted warehouse shelf life Cost: about 200-300% more than Starter Batteries Use: Solar systems (Solar gel Batteries are available in sizes from 60Ah and above) High grade solar systems Systemsize: from 5Wp up to about 1000 Wp
Lead Acid Battery Types: Stationary Batteries Stationary Batteries are designed to have high life time, high cycle life and are reliable in use. Some have tubular plates. In rare cases special high surface area plates are used. Some stationary Batteries are available as Gel batteries. Characteristics: small self discharge high cycle life => Suitable for big solar systems Cost: about 200-300% more than Starter Batteries Use: Big industial solar systems, telecom systems Systemsize 200Wp to 50KWp
Lead Acid Battery Types: Cycle Life of different types
Sizing of Lead Acid Batteries: A common mistake is to design the system with too little battery capacity. => The batteries tend to sulfate Rule of thumb Not more than 1Ah Per Peak Watt of array capacity, (northern China); Closer to the equator 2 Ah per Peak Watt is possible.
Interconnecting Pb-Batteries: Connecting Batteries together in parallel will cause equalising currents to flow between them. Avoid parallel connections when possible Galvanically isolated Batteries can be connected in parallel using special regulation technology. When stringing many Batteries together in Series the voltage can be monitored and controlled in blocks of 12V or 24V.
LiFePO Batteries Anode (Grafit): discharge Li 1 C 6 C 6 + Li + + e - Chemical Equation charge Cathode: Li + + e - +FePO4 LiFePO 4
LiFePO Batteries Structure of LiFePO Battery
Ageing mechanisms of LiFePO High Temperature > Binding material in graphite get destructed > high resistance > loss of capacity if single graphite sectors get disconnected Consequence: > prevention of high temperature
Ageing mechanisms of LiFePO High Voltage > Oxidation of nanostructure > loss of capacity Consequence: > limitation of charge voltage
Ageing mechanisms of LiFePO Deep Discharge > Copper-Dendrite growth > Short circuit of electrolyte Consequence: > Prevention of low voltages
Ageing mechanisms of LiFePO Charging at negative temperature > Li-Dendrite growth > Short circuit of electrolyte Consequence: > Reduced charge current