Battery
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A battery is a device consisting of one or more electrochemical cells, which store chemical energy and make it available in an electrical form.
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Types
Wet Cells
Wet cells are your common car battery and come in both sealed maintenance-free configurations and those that need to have their electrolyte level checked often. If you are using sealed batteries you will not have to worry about explosive or acidic gases being expelled from your batteries; these types of batteries can be placed anywhere and require low maintenance. For unsealed batteries you will need to keep them outside or in an area kept away from ignition sources. If you place them inside the bus they will need to be placed into a sealed container that is vented to the outside.
This type of battery is typically used as the engine battery to power the lights, starter, and ignition for the bus.
Gel Cells
Unlike a traditional wet-cell lead-acid battery, Gel batteries do not need to be kept upright (though they cannot be charged inverted). In addition, gel batteries virtually eliminate the electrolyte evaporation, spillage (and subsequent corrosion issues) common to the wet-cell battery, and boast greater resistance to extreme temperatures, shock, and vibration. At high currents, electrolysis of water occurs, expelling Hydrogen and Oxygen gas through the battery's valves. Care must be taken to prevent short circuits and rapid charging.
Absorbed Glass Mat (AGM)
Absorbed glass mat (AGM) is a class of VRLA lead-acid battery in which the electrolyte is absorbed into a fiberglass mat. The plates in an AGM battery may be flat like wetcell lead-acid battery, or they may be wound in a tight spiral. Their unique (for lead acid chemistries) construction also allows for the lead in their plates to be purer as they no longer need to support their own weight as in traditional cells.
This type of battery is probably the most ideal for construction of a battery bank, they can be mounted in any direction, do not require maintenance, do not spill acid, and can be found locally in many sizes.
Uses
Car batteries have different uses and various other elements are alloyed with the lead such as calcium, cadmium or strontium to change density, hardness, or porosity of the plates and to make the plates easier to manufacture.
- The starting (cranking) or shallow cycle type is designed to deliver quick bursts of energy, usually to start an engine. They usually have a greater plate count in order to have a larger surface area that provides high electric current for short period of time. Once the engine is started, they are being continuously recharged. See Jump start (vehicle).
- The deep cycle (or motive) type is designed to continuously provide power for long periods of time (for example in a trolling motor for a small boat, a golf cart or other battery electric vehicle). They can also be used to store energy from a photovoltaic array or a small wind turbine. They usually have thicker plates in order to have a greater capacity and survive a higher number of charge/discharge cycles. See battery pack.
Sulfation
Sulfation refers to the process whereby a lead-acid battery (such as a car battery) loses its ability to hold a charge after it is kept in a discharged state too long due to the crystallization of lead sulfate. Lead-acid batteries generate electricity through a double sulfate chemical reaction. Lead and lead oxide, which are the active materials on the battery's plates, react with sulfuric acid in the electrolyte to form lead sulfate. When formed, the lead sulfate is in a finely divided, amorphous form, which is easily converted back to lead, lead oxide and sulfuric acid when the battery is recharged. Over time, lead sulfate converts to the more stable crystalline form, coating the battery's plates. Crystalline lead sulfate does not conduct electricity and cannot be converted back into lead and lead oxide under normal charging conditions. As batteries are "cycled" through numerous discharge and charge sequences, lead sulfate that forms during normal discharge is slowly converted to a very stable crystalline form. This process is known as sulfation.
Sulfation is a natural, normal process that occurs in all lead-acid batteries during normal operation. Sulfation clogs grids, impedes recharging and ultimately can expand and crack the plates as it accumulates, destroying the battery. Crystalline lead sulfate is resistant to normal charging current, and does not re-dissolve completely. Thus, not all the lead is returned to the battery plates, and the amount of usable active material necessary for electricity generation declines over time. In addition, the sulfate portion (of the lead sulfate) is not returned to the electrolyte as sulfuric acid.
Sulfation also affects the charging cycle, resulting in longer charging times, less efficient and incomplete charging, excessive heat generation (higher battery temperatures). Higher battery temperatures cause longer cool-down times and can accelerate corrosion.
Exploding batteries
Maintenance free (MF) Batteries rely on valves fitted to each cell which can vent hydrogen if over-pressurisation occurs. Generally however, oxygen and hydrogen recombine in the space above the electrolyte, so that over-pressurisation rarely occurs. However, should such a condition occur, and the valves fail to operate (through being blocked for example), then there is a possibility of an internal explosion if the oxy-hydrogen mixture is ignited. Just a slight jolt can cause a spark to jump between the posts, and the gas explodes. Personal injuries can result. The condition can be assessed if any swelling in the cell walls of the battery is visible. The swelling from internal pressurisation varies from cell to cell, that at the battery ends being most obvious, because the plastic is unsupported by cells at either side. It is surprising how powerful an explosion can be caused in the small air space above the electrolyte can occur, but when one cell explodes, it sets off a chain reaction in the rest. Such batteries should be isolated and discarded, taking great care using protective personal equipment (goggles, overalls, gloves etc) during the handling.
Terms and ratings
- Ampere-hours (A·h) is the product of the time that a battery can deliver a certain amount of current (in hours) times that current (in amps), for a particular discharge period. This is one indication of the total amount of charge a battery is able to store and deliver at its rated voltage. This rating is rarely stated for automotive batteries.
- Cranking amps (CA), also sometimes referred to as marine cranking amps (MCA), is the amount of current a battery can provide at 32°F (0°C). The rating is defined as the number of amperes a lead-acid battery at that temperature can deliver for 30 seconds and maintain at least 1.2 volts per cell (7.2 volts for a 12 volt battery).
- Cold cranking amps (CCA) is the amount of current a battery can provide at 0°F (−18°C). The rating is defined as the amperage a lead-acid battery at that temperature can deliver for 30 seconds and maintain at least 1.2 volts per cell (7.2 volts for a 12-volt battery). It is a more demanding test than those at higher temperatures.
- Hot cranking amps (HCA) is the amount of current a battery can provide at 80°F (26.7°C). The rating is defined as the amperage a lead-acid battery at that temperature can deliver for 30 seconds and maintain at least 1.2 volts per cell (7.2 volts for a 12-volt battery).
- Reserve capacity minutes (RCM), also referred to as reserve capacity (RC), is a battery's ability to sustain a minimum stated electrical load; it is defined as the time (in minutes) that a lead-acid battery at 80°F (27°C) will continuously deliver 25 amperes before its voltage drops below 10.5 volts.
- Peukert's Law expresses the fact that the capacity available from a battery varies according to how rapidly it is discharged. A battery discharged at high rate will give fewer amperehours than one discharged more slowly.
- The hydrometer measures the density, and therefore indirectly the amount of sulfuric acid in the electrolyte. A low reading means that sulfate is bound to the battery plates and that the battery is discharged. Upon recharge of the battery, the sulfate returns to the electrolyte.
- The open circuit voltage, measured when the engine is off. It can be approximately related to the charge of the battery by:
Open Circuit Voltage ~ State-of-charge 12.65 V 100% 12.45 V 75% 12.24 V 50% 12.06 V 25% 11.89 V 0%
Calculations
There are several calculations that can be useful when determining how many batteries will be required or how long an existing bank will last. NOTE: These calculations are for an ideal case with 100% discharge, you may want to take a factor of 20% when considering the results of these calculations.
Required Battery Power
First you will need to add up the required amount of power by the devices you wish to run this will be TotPower(Watts). The other value needed is how long you would like to run these devices while running on only battery power Time(Hours). The value BankTot is the total number of Amp-Hours required to run the devices for the required amount of time. Several batteries can be connected in series to achieve this amount.
TotPower(Watts) / 120(Volts) = TotCurrent(Amps)
TotCurrent(Amps) * Time(Hours) = BankTot(Amp-Hours)
