Four-terminal sensing (4T sensing), 4-wire sensing, or 4-point probes method is an electrical impedance measuring technique that uses separate pairs of current-carrying and voltage-sensing electrodes to make more accurate measurements than traditional two-terminal (2T) sensing. 4T sensing is used in some ohmmeters and impedance analyzers, and in precision wiring configurations for strain gauges and resistance thermometers. 4-point probes are also used to measure sheet resistance of thin films.

The key advantage of four-terminal sensing is that the separation of current and voltage electrodes eliminates the impedance contribution of the wiring and contact resistances.

Four-terminal sensing is also known as Kelvin sensing, after William Thomson, Lord Kelvin, who invented the Kelvin bridge in 1861 to measure very low resistances. Each two-wire connection can be called a Kelvin connection. A pair of contacts that is designed to connect a force-and-sense pair to a single terminal or lead simultaneously is called a Kelvin contact. A clip, often a crocodile clip, that connects a force-and-sense pair when it closes or slides onto a conductor is called a Kelvin clip.

source: http://en.wikipedia.org/wiki/Four-terminal_sensing

When a photon with sufficient energy impinges upon a semiconductor it can transfer enough energy to a electron to free it from the bonds of the semiconductor's valence band so that it is free to move and thus carry an electric current. The junction in a semiconductor diode provides the necessary electric field to cause the current to flow in an external circuit.

The typical output voltage of a PV cell is between 0.5 and 0.6 Volts and the energy conversion efficiency ranges from less than 10% to over 20%. An array of cells can therefore generate about 200 Watts of electrical power per square meter when illuminated by solar radiation of 1000 Watts per square meter. The corresponding current density will be about 400 Amps/m2. Because of climatic conditions the intensity of the insolation rarely reaches 1000 W/m2.

Practical cells are also much smaller than one square meter with actual sizes of commercially available cells ranging from about one centimeter square to 15 centimeters square. The corresponding output Wattage for these cells range from 20 milliWatts to about 4 Watts.

Determining battery lifetimes is beset with difficulties. Performance data are not generally available and costly to generate since large numbers of batteries must be tested to destruction. Furthermore, the required test period to verify the predictions is often greater than commercial decision lead time. Charge – discharge times for high capacity batteries are very long and using accelerated life testing to determine battery lifetime is most likely to lead to misleading results since battery life depends on temperature, rate and depth of discharge and the test conditions used to accelerate the occurrence of failures are quite likely to introduce new and unrepresentative failure modes.

Analogue circuit, also known as Analog circuit, is an electronic circuit in which an electrical value (usually voltage or current, but sometimes frequency, phase) represents something in the physical world.The magnitude of the electrical value varies with with the intensity of an external physical quantity.

Also, an electrical circuit which provides a continuous quantitative output ( as opposed to a digital output which may be a series of pulses or numbers) in response to its input.

Ampere (Amp) is the unit of current flow equal to one coulomb per second. Ampere hours(Ah), also known as Amphours, is the unit of measure used for comparing the capacity or energy content of a batteries with the same output voltage. For most batteries it defines the battery's C rate. For automotive (Lead Acid) batteries the SAE defines the Amphour capacity as 20 times the current delivered for a period of 20 hours when the battery is discharged at 1 twentieth of the C rate until the cell voltage drops to 1.75 Volts.

Strictly, 1 Ampere hour is the charge transferred by one amp flowing for one hour, namely, 1Ah = 3600 Coulombs.

The true capacity of any battery is its energy content and this is measured in WattHours (Wh). It is the battery's Amphour capacity multiplied by the battery voltage