HOW TO STORE ELECTRICITY
Most small system electricity generating systems will require a bank of storage batteries to store the energy generated. This article will examine how a battery works, different types of batteries and how it fits in with the rest of the system.
Cells
A cell is essentially a self contained device to convert stored chemical energy into electrical energy. A battery can be one cell or a series of cells. Cells may either be primary (non rechargeable) or secondary (rechargeable). Primary cells include dry mercury cells, zinc-carbon cells, simple cells and silver oxide and manganese cells all of which the chemical reactions cannot be reversed. Secondary cells include nickel-cadmium cells and car batteries (lead-acid accumulators). In secondary cells the energy can be converted back to chemical energy.
All cells contain two basic components:
1. Electrodes: These are the conductors which connect with the non-metallic part of a circuit (in a battery this is the electrolyte). In cells the electrodes are either the anode or the cathode. In a primary cell is this fixed, the anode is always the negative and the cathode always the positive, however in a secondary cell this switches dependent upon the voltage.
2. Electrolytes: In an electrochemical cell the electrolyte is the non-metallic part that closes the circuit. For example in a simple cell the electrolyte is a dilute sulphuric acid. Electrolytes are typically a solution but may be solid, a gel or in the case of dry cells, a paste.
When a voltage is applied to the electrolyte it conducts electricity, as the electrons cannot themselves pass through the electrolyte they will move to the cathode which essentially consumes electrons from the anode causing a negative charge to develop around the cathode and a positive charge around the anode. The ions in the electrolyte move to neutralize these charges and the electrons can continue to flow.
The Simple Cell
In a simple voltaic cell like the one developed in 1800 by Allesandro Volta, the cathode is a rod of copper while the anode is zinc. The electrolyte is electrolyte such as ammonium chloride into which both metals lose electrons to form ions. The positive ions (an electron that has lost or gained an electron) in the electrolyte pick up electrons from the copper plate which then gains a positive charge (remember positive = deficit), the electrolyte transfers to the electrons to the zinc plate and it gains a negative charge (negative = surplus). Connect the two rods with a conductor and because there is a difference in potential between the two rods a current will flow. With time the zinc rod is ‘eaten away’ as electrons are transferred to the copper rod (via the conductor). It dissolves because the zinc atoms are changed into a zinc ion when it loses an electron. The zinc ions dissolve in solutions
Car Batteries
Car batteries are the most common secondary cell used. It is comprised of 6 identical cells and the electrodes are made of lead and lead oxide while the electrolyte is dilute sulphuric acid. They are known as Lead Acid cells. Each electrode basically converts to the other as the battery is used or charged thus it is a rechargeable battery. Each cell has a voltage of approximately 2V, giving an overall voltage of 12V.
Deep Cycle Batteries
For the purposes of storing generated electricity in a stand alone system it will be necessary to use a deep cycle battery. These are batteries that are specifically designed to provide a stable amount of current over a long period of time. Contrast this with a car battery which is designed to provide a very high amount of current over a very short period of time (to start the car). A deep cycle battery is also designed to withstand being deeply discharged repetitively (it is not recommended to discharge even these batteries below 20% of their charge capacity as it will shorten their lifespan). There are many types of deep cycle batteries available these include: flooded (car batteries are typical); gel batteries and AGM (Absorbed Glass Matt). Deep cycle batteries can be rated either by volts or amps.
Lithium Re-chargeable Battery
A lithium rechargeable battery actually uses a solid electrolyte comprised of a lithium salt impregnated polymer. The electrical charge comes from the flow of lithium ions and in many lithium batteries the lithium compound frequently acts as both electrolyte and cathode. One of the most common lithium batteries is LiMn ‘CR’ in which the cathode is heat treated manganese dioxide and the electrolyte is lithium in carbonate which generates about 3.7V.
Gel Batteries
This type of battery uses a gel electrolyte. It is a sealed battery which is valve regulated. These are very low maintenance due to being sealed and can be used in any position (do not need to be kept upright). They can cope with a higher temperature range and more shock and vibration than other battery types thus making them suitable for transport vehicles.
Flooded Lead Acid Batteries
This is the traditional rechargeable battery. As they are not sealed they must be maintained in well ventilated and upright position and will require periodic maintenance. See car battery above.
AGM Batteries
In this type of battery the electrolyte is actually absorbed into the glass mat. This offers several advantages: cope with higher temperatures, self-discharge slower; totally sealed; do not require topping up; safe for transport; do not need to be kept upright; can full charge at a lower voltage; recharge quickly; can be discharged deeper than other batteries; very robust.
Nickel Based Batteries
This range of batteries includes the NiCd and the NiMH (nickel –metal-hydride). They are frequency used as car batteries and are considered superior to the traditional lead acid types. They are capable of greater charge cycles and generally hold more charge than lead acid. Because of this they are also used in many other areas such as cameras, traditional small batteries and space applications.
