Electrical Terms Defined for Solar & Batteries

AC Alternating current; a flow of electrons which regularly reverses its direction of flow. 60 cycles per second is the US standard. One cycle per second is called one Hertz ( Hz ). Because of how generators work the switching of peak positive current to peak negative current takes place gradually in a sine wave pattern. Can not be stored like DC in a battery.

DC Direct current; a flow of electrons that flows only in one direction, negative to positive. PV's produce DC power. DC can be stored electrochemically in a battery.

Electron A negatively charged particle which orbits the positively charged nucleus of an atom. Electrons have 1/1837 the mass of hydrogen (the lightest atom ), therefore electrons are subatomic!

Electricity The movement of electrons through a conductor. In conductors, such as gold, copper, and aluminum electrons can be easily forced to break orbit and flow to a new orbit around at a neighboring atom. Each electron leaving an atom is replaced by another in a musical chairs like dance. Electrons can only flow in a closed circuit.

Electromotive Force (emf)  The force that causes electrons to flow because of a difference in electrical potential (measured in volts ).

Volts ( V ) or ( E ) The electromotive force which will cause current to flow. A standard definition of the volt is: an emf of 1 Volt is necessary to move a current of 1 Amp through a 1 Ohm resistor. A voltmeter measures the difference in potential between two points. It may be helpful to think of voltage to electricity flow as pressure is to water flow. Named after Allesandro Volta the Italian physicist who by 1800 was making the first batteries.

Ampere, or Amp (A) or ( I)  Unit of measurement of electric flow, or current, like volume of water in a stream. A count of how many electrons pass a given point in one second. A closed circuit is necessary for current flow. Named after Andre Ampere, the French mathematician and physicist who quantified electric flow around about the year 1820.

Resistance Opposition to the flow of current. Unit of measurement is called an Ohm, stated as R, or by the Greek letter omega. All conductors have some resistance, this is necessary because if a conductor had no resistance the connection would be a short circuit with excessive current flow. Insulators have high resistance and conductors have low resistance. Bad connections ( loose, corroded, or dirty ) have high resistance and can result in little current flow.

Ohm's Law Named after the German physicist Georg Ohm who in 1827 described resistance to electrical flow. Expresses the fundamental relationship between voltage, current, and resistance. Current (Amps, or I ) in a circuit equals Voltage ( E ) divided by Resistance ( Ohms, or R)
I = E / R or Amps = Volts / Ohms
By using simple algebra Ohm's law can be rearranged as: E = I x R or Volts = Amps x Resistance And also be rearranged a third way: R = E / I or Resistance = Volts / Amps-  So to recap: The current flow in a circuit depends on both the voltage of the source ( battery, or PV ) and the resistance of the circuit.

Power The amount of work being done. The unit measurement of power is called a Watt (W). It is named after James Watt a Scottish engineer, 1736-1819. Electric power is simply the product of voltage times current.
Watts = Volts x Amps
Can be rearranged as Volts = Watts / Amps and also as Amps = Watts / Volts

Inverter A device that converts DC to AC. There are modified sine wave (MSW) inverters, which are in reality closer to square wave than a sine wave. MSW inverters have from 30 to 40% Total Harmonic Distortion ( THD ), or distortion from a pure sine wave. The utility grid averages, about, 3% THD. A Trace SW inverter has from 3 to 5%, and a Exeltech inverter has 1 to 2% THD.

Photovoltaic or PV - Photoelectric, light electric, or solar electric. Principal was discovered by the French physicist Edmund Becquerel in 1839; interestingly one of his relatives discovered the principal of atomic energy some years later. So PV predates nuclear power! It was not until scientists at Bell Labs in 1954 were working on silicon rectifiers ( diodes ) that the real potential of PV started to become clear. The space race of the late 50's forward gave PV's a niche that they have filled ever since. Because space has unlimited sunlight and PV's are autonomous power sources and are lightweight they have powered all of America's satellites, from the tiny Vanguard to the very large Skylab. Unfortunately the government spent very little on research and development to make better and cheaper PV's, though it spent billions to develop nuclear power.

Silicon PV Cell A device that converts sunlight directly into electricity. "When illuminated, the PV cell produces a voltage between front and back. This voltage is developed across a junction that is built into the cell structure. This voltage can be used to produce a current, just like from a battery, but the amount of current is limited by the amount of light falling on the cell ... Richard Komp, Ph.D. ( in his book Practical Photovoltaics ). PV cells have, about, .5 volts (one-half volt) of electrical potential.

Open-Circuit Voltage (Voc) The voltage of a PV cell, or module, when measured without a load being connected. It depends on the amount of sunlight hitting the PV but this voltage reading is always higher than the peak voltage (voltage at the maximum power point, or knee, of a I-V curve).

Short-Circuit Current ( Isc) The maximum current a PV cell can deliver into a short circuit. This number of Amps is directly proportional to the area of the cell and the intensity of the sunlight. This can be measured with an appropriate meter, or meter - shunt combination.

PV, or Solar Module A series string of 32 to 36 cells, producing an open circuit voltage in bright sunlight of about 20 volts, or about 17 volts when producing maximum power when connected to a load ( battery, water pump, etc... ). Total current of a series string is the same as a single cell.

PV, or Solar Array   A group of modules combined on a mounting structure. May be wired in series for increased voltage, or in parallel for increased current, or a combination called series-parallel.

PV Charge Controller, or Regulator A device used to prevent the overcharging of battery's by PV's.

Shunt   A low value resistor used in parallel with a meter to increase the amount of current the meter can measure.

Battery    A group of interconnected electrochemical cells. Single cells are considered to be a battery if they are used alone. A battery cell contains an anode, a cathode, and the electrolyte. The nominal voltage of a lead-acid cell is 2 volts.

Deep-Cycle Battery   A battery specifically made to have up to 80% of its energy capacity removed and replaced repeatedly for many cycles. The plates of this type of battery are much thicker than are starting battery's plates.

Anode Is the electrode ( plate ) within the cell which undergoes the chemical process of oxidation. Electrically, the anode is the cell's positive terminal.

Cathode The electrode (plate ) which undergoes the chemical process of reduction. The negative terminal of the cell.

Electrolyte The medium of ion ( electrically charged particle or molecule transport within the cell. The electrolyte provides a path for electron transfer between the anode and the cathode.

Active Material The materials which chemically react within the cell to release free electrons. One active material is metal or metallic compound which is oxidized. The other active material, often a metallic oxide, is reduced.
Series A connection from one cell or battery to another (or from one PV module to another), negative to positive, which results in twice the voltage. Note that the current stays the same when series interconnecting.

Parallel   A connection between cells or batteries (also PV's) to increase the current capacity. The voltage stays the same when we wire positive to positive and negative to negative, but the capacity in battery amp-hrs is the sum of the batteries wired in parallel. With PV arrays parallel wired you add up the amps available from each paralleled unit, or set of series wired modules, to get your total current output.

Ampere-Hours, or amp-hrs   An amp hour is a current of one amp flowing for one hour. A term used to tell the amount of energy a battery has before it needs to be recharged. Capacity of a battery. You can convert amp-hrs to waft hours by multiplying amp-hrs by the systems battery voltage.

Cycle One complete charge / discharge sequence of the battery. Deep cycle batteries are rated to last for so many cycles. Such as a Trojan T-105 golf-cart battery will last for 700 deep cycles, and a Surrette CH-375 will last for 1,175 deep cycles.

State Of Charge A ratio, expressed in percent, of the energy remaining in a battery in relation to its capacity when full.

Specific Gravity, or SG Is the ratio of a liquid's density to the density of water. Sulfuric acid is denser than water so therefore you can measure it with a battery hydrometer (tool that measures SG) and you can tell at what state of charge your cells are at.

Self Discharge The tendency of all electrochemical cells to lose energy due to internal chemical reactions within the cells. Also called local action.

Charge Rate The amount of energy per unit of time which is being added to the battery. Commonly expressed as a ratio of the battery's rated capacity to charge in relation to the time of charge duration. If you have a battery with a storage capacity of 1,000 amp-hrs and your PV array is rated at 50 amps then your charge rate is expressed as C/20 (1,000 / 50 ).

Equalize Charge The controlled overcharge of a seemingly fully charged battery (many cells within the battery already are fully charged) to bring the weaker cell(s) up to full charge. Should be accomplished every 30 days, or as needed. You can determine unequal cells with a hydrometer (should be within 15 points lowest to highest reading), or if you have the large individual cells with outside terminals you can check voltage readings across each cell and compare them (should be within .05 volts lowest to highest readings

How a Lead Acid-Battery Works
The positive plates (anodes) are made of lead dioxide (PbO2). The negative plates (cathodes) are made of lead (Pb). The electrolyte is a dilute solution of 35% sulfuric acid (H2SO4) and 65% distilled water. In the charged state the electrolyte exists as ions. Both electrodes are completely immersed in the electrolyte. The reversible chemical reaction between the plates and the electrolyte allows the storage and retrieval of energy from the battery. The voltage produced across a single cell (2 V) is a function of the electrochemical reaction between the active constituents of the cell. If more voltage is needed then more cells will have to be added in series. The physical size of the cell is variable and determines the amount of current available from the battery. The larger the cell the greater its capacity in amp-hours, but no matter how large the cell is its voltage is still, nominally, 2 volts.

When a battery is being discharged, the active materials of both electrodes are changed into lead sulfate (PbSO4). The sulfuric acid is gradually consumed from the electrolyte. As the battery is discharged, all the electrodes gradually become plated with lead sulfate which is an electrical insulator. The sulfate (SO4) ions are gradually consumed from the electrolyte and are bondedto the plates to form PbS04 (lead sulfate). This reaction releases two electrons at the cathode (negative) for every sulfate radical which is bonded to the plates. This release of free electrons at the cathode from the electrochemical reaction is the source of the battery's power.

During discharge the area of the plates available for reaction decreases as the surface becomes covered with the insulative lead sulfate. This decrease in the cells active area results in a sharp rise of the cell's internal resistance and a sharp drop in the cells voltage. It becomes impossible to remove any more energy from the battery and the battery is said to be fully discharged.

The charging process is the reverse of the discharging process. A current flow of electrons is forced through the battery in the opposite direction by the application of voltage across the battery's terminals. The chemical bond between the lead and the sulfate ions is broken , and the sulfate ion is released back into the electrolyte solution. When all the sulfate ions have been removed from the plates are in solution with the electrolyte the battery is said to be fully charged.

If the lead sulfate ions stay on the plates long enough (about three to four weeks) the sulfate becomes hardened crystals and is difficult to remove. This is called sulfation. Storage capacity of the battery is reduced when the plates are sufated. Sulfation can even render an otherwise good battery useless.

Practical Photovoltaics by Richard Komp, Ph.D. Aatec>
Electricity and Basic Electronics by Stephen R. Matt Good heart-Willcox
The Complete Battery Book by Richard A. Perez Tab Electricity and Basic Electronics by Stephen R. Matt Good heart-Willcox