Float and Boost Charging of Batteries

Float charging is used where the battery rarely gets discharged.  A typical application where float charging can be used would consist of the float charger, battery and the load in parallel.  During normal operation, the load draws the power from the charger.  When the supply to the charger is interrupted, thebattery steps in.

Float charging of a battery involves charging the battery at a reduced voltage.  This reduced voltage reduces the possibility of overcharging.The Float charger ensures that the battery is always in the charged condition and is therefore considered "floating".  The Float charger starts by applying a charging volltage to the battery.  As the battery gets charged, its charging current reduces gradually.  The float charger senses the reduction in charging current and reduces the charging voltage.

If the battery gets drained, the float charger will again increase the charging voltage and process continues.  Float chargers can be connected indefinitely to the batteries.

Boost charging involves a high current for short period of time to charge the battery.  It is generally if the battery has been discharged heavily.  Boost charge enables the quick charging of depleted batteries.

For instance, a two volt lead acid battery which has been discharged will initially be boost charged with a charging voltage of around 2.35-2.4 volts.  However, as the battery voltage rises, the charger will switch over to the float charge mode with a float voltage of 2.25 volts.

Most battery chargers come equipped with provisions for both boost and float charging.

Single Phasing - its causes and effects

Single phasing is a condition in three phase motors and transformers wherein the supply to one of the phases is cut off. Single phasing causes negative phase sequence components in the voltage. Since, motors generally have low impedances for negative phase sequence voltage. The distortion in terms of negative phase sequence current will be substantial.

Negative phase sequence currents cause heating of the motor and consequently motor failure.

Single phasing is caused by the use of single-phase protection devices such as fuses and circuit breakers. Three phase loads should be protected by devices which cause the interruption of power to all three phases simultaneously when a fault occurs.

Defective contacts in three phase breakers can also cause single phasing.

Single phasing can be identified by special protective relays which can identify and isolate the connected loads. Smaller motors rely on overcurrent and negative phase sequence relays. Motor protection relays for larger motors come readily fitted with protection against single phasing.

Single phasing can sometimes cause excessive noise and vibration in motors.

Peukert's Law and discharge of lead acid batteries

In 1897, the German Physicist Peukert proposed that as the rate of a lead-acid battery's discharge increases its available capacity.This is known as Peukert's law.  It can be Mathematically represented as

C_p=I^kt

where Cp is the capacity of the battery in ampere hours
k is the Peukert's constant
I is the current and
t is the time

The Peukert's Constant k is specified by the manufacturer of the battery and is usually in the range of 1.2 to 2.

Thus the time a battery can provide sustain a certain current without any appreciable drop in voltage would be given by 

t=C_p/I^k

Thus a 100Ah capacity with a Peukert's constant of 1.2 will be able to supply a current of 5A for 14.5 hours.

Adding water to the electrolyte level of unsealed batteries.

The electrolyte in the battery is a mixture of sulphuric acid and water.  The amount of water in a battery can fall due to electrolysis or evaporation.  This may cause in a drop in the level of the electrolyte and consequently a drop in the battery output. 

Hence, it is necessary to periodically inspect the level of electrolyte in the battery. If the level of the electrolyte falls below the minimum level, it can be topped up by adding water.  Only distilled water should be added as ordinary water may contain a lot of impurities and ions which may contaminate the electrolyte.

The level of electrolyte in the battery tends to fall as the battery gets discharged and tends to rise as the battery gets charged.  Hence, water should be added to the electrolyte only when the battery is fully charged.  If the water is added to the battery when it is in the discharged condition, the level can increase beyond the limit when the battery is fully charged and may overflow

The acid used as the electrolyte is extremely corrosive and should be handled with extreme care.  Proper protective outfits should be worn while handling them.  Water can be added to a container of acid.  However,  acid can never be added to container of water as the heat generated can cause splashing. 

Specific Gravity Measurement in Batteries

Specific Gravity of electrolyte refers to the its relative density.  Specific gravity is the ratio of the density of a liquid to the density of water.  The specific gravity is measured by means of a hydrometer.  The specific gravity gives an indication of the amount of charge in a battery.

When a lead acid battery is charged, the sulphuric acid which is the electrolyte is transformed into water.  The specific gravity of the electrolyte varies between 1.1 and 1.3.

The specific gravity should be periodically checked.  If the specific gravity becomes more than 1.3, the electrolyte may be overly acidic and can damage the plates.  If the specific gravity is less than 1.1, the plates can become hydrated.

The specific gravity is directly linked with the open circuit voltage (OCV) of the battery.  The open circuit voltage rises and falls with the specific gravity of the electrolyte.

Individual manufacturers give a graph or a table describe the exact relationship between the open circuit voltage and specific gravity.

Specific gravity of the electrolyte also varies in accordance with temperature, it decreases with increase in temperature and increases in colder conditions.

Back FlashOvers - An Introduction

Back Flashovers generally occur in transmission lines during lightning strikes when the potential of the tower rises vis-a-vis the conductor.  This causes the voltage across the insulators to increase beyond the limits resulting in a flashover.

Lightning strokes have the ability to discharge thousands of amperes of current in very short time. This high current needs to be discharged quickly into the earth to prevent the potential of the tower from rising.

Back flashover occurs when the lightning which has struck the tower is unable to get discharged to the earth.  This can occur due to high impulse resistance of the ground.

When a tower is struck by lightning, a travelling voltage is induced which moves many times between the top and the bottom of the tower , the potential of the tower is thus raised.  The elevated voltage also appears on the cross arms of the towers.  This can cause back flashovers if the insulators are unable to withstand the voltage surge.

Back flashovers can be avoided by improving the impulse resistance of the earth point of the transmission towers and improving critical flashover limits of the insulators.

Back flashovers are identified as line to earth faults.

U.S. says wind could power 20 percent of eastern grid

For the 20 percent wind scenario to work, billions must be spent on installing wind towers on land and sea and about 22,000 miles of new high-tech power lines to carry the electricity to cities, according to the study from the Energy Department's National Renewable Energy Laboratory.

"Twenty percent wind is an ambitious goal," said David Corbus, the project manager for the study. "We can bring more wind power online, but if we don't have the proper infrastructure to move that power around, it's like buying a hybrid car and leaving it in the garage,"

The private sector cannot fund all the needed spending, so a big chunk would have to come from the federal government through programs such as loan guarantees, Corbus said.

The Obama administration is already dedicating billions of dollars to double the amount of electricity produced by wind and other renewables energy sources by January 2012.

The Interior Department will decide this spring whether to approve the Cape Wind project off Cape Cod, Massachusetts. That project, long delayed because of local opposition, would provide electricity to about 400,000 homes.

The amount of U.S. electricity generated by wind was up 29 percent during January-October of last year compared to the same period is 2008, according to the Energy Department.

Reaching the 20 percent threshold for wind by 2024 in the eastern electric grid would require 225,000 megawatts of wind generation capacity in the region, about a 10-fold increase from current levels, the study said.

One megawatt of electricity can provide power to about 1,000 homes.

Wind turbines would be scattered throughout the eastern grid, which extends from the Plains states to the Atlantic Coast and south to the Gulf of Mexico.

Most of the big wind farms would be concentrated off the Atlantic Coast in federal waters from Massachusetts to North Carolina and on land in Midwest states from North Dakota to Nebraska and into Kansas.

Many states already require utilities to produce a portion of their electricity from renewable energy sources, but a federal mandate covering all utilities nationwide would help create the 20 percent wind scenario, Corbus said.

Sen. Byron Dorgan said on Tuesday he thought the Senate would forgo dealing with climate change legislation this year after going through the contentious health care debate and instead focus on passing an energy bill that, in part, requires U.S. utilities to generate 15 percent of their electricity from renewables by 2021.

source: reuters.com