NICKEL-METAL HYDRIDE (NiMH) accumulators for cameras
Nowadays, digital cameras are electronic products endowed with a blitz, powerful processors, memory sticks and electric motors that run the precision mechanism, the zoom or the aperture, an LCD screen which allows you to see the subject or the captured picture etc, all of them being energy consumers. Lately, accumulators have been introduced for exploitation. They give higher amperage and they have the great advantage of recharging. Even if the initial investment in these batteries and in the recharger is bigger, on long term they prove to be lucrative.
NiMH accumulators
Energy density is approximately 30-50% higher than that of NiCd accumulators, and correspondingly, the size and weight are less. They “prefer” slight discharge (not deep one) and its service life is directly connected to the depth of discharge.
NiMH accumulators dissipate much more heat during the charge as compared with NiCd and require a more complicated algorithm of complete charge control.
NiMH accumulators can’t charge so quickly as NiCd do. The charging time is usually two times more than that of NiCd. The recommended charging current is one fifth to one half of the rated capacity.
The functioning principle is based on the capacity of some metallic alloys to capture (forming hydrides) and release oxygen. The most suitable alloys were acknowledged to be those with nickel and “rare earth” (lanthanum, zirconium), so that the processes take place at the environment temperature.
When charging, the following reactions occur in the NiMH batteries:
At the negative pole, water is decomposed by applying an electrical potential, as indicated below:
Alloy + H20 = Alloy (H) + OH+
At the positive pole, the oxidation of nickel hydroxide takes place as indicated below:
Ni(OH)2 + OH+ = NiOOH + H2O + e-
When discharging (in exploitation) the processes take place the other way round, the reactions being reversible.
Charge then discharge is the common method used in pulse charge, i.e. charge for 5 seconds, then discharge for 1 second, thus the most oxygen generated during charge is reverted to electrolyte at pulse discharge. Not only the gasification quantity of inner electrolyte is limited, but also the old batteries which have been seriously polarize can recover to or approach to their primary capacities after using this method for 5-10 times .
The relationship between the positive and negative electrodes is regulated so that it protects the battery: when overcharging, the positive electrode will be saturated first; at this moment the water electrolysis begins and the oxygen is released, it spreads and it is fixed at the level of the negative electrode. The battery electrolyte is a potassium hydroxide solution. The mechanical realization of the battery is almost identical with that of a NiCd one. The external package is metallic and it serves as negative pole, and the axial electrode is the positive pole. At one of the extremities - where the positive electrode goes out, there is an isolating disk plate, fastened by some producers by a safety valve, which opens if the hydrogen or the oxygen are generated in too large quantities (marked overcharge).
The main parameter of accumulators is the discharge (functioning) period at a certain current drained by the consumer. The current evaluation of batteries is abbreviated “C” (capacity) and it is the result of the measurement of the discharge of a new battery, but well “conditioned”, recently and completely charged.
A recently charged battery provides a 1.4 volt tension to the terminals, at 20ºC. At the usual charge of 0.2 C - for example 400 mH for a 2000 mAh - there is a rapid collapse of the tension to 1.25 Volts, and then tension falls slowly (to 1.2 Volts for 50% C) up to 85% C, followed by a rapid fall in the terminals.
The environment temperature has a significant influence on the discharge capacity of the NiMH cells; thus, between 10 and 40º C, the battery provides over 95% of the capacity; in return, at 0ºC the capacity falls to 80% and it reaches only 20% at - 10ºC; the good news is that batteries can recover their charging capacity completely, if they are brought again to positive temperatures. This happens as a consequence of the decrease of the reaction rate at low temperatures.
Life expectancy
Currently, NiMH cells have a life expectancy (charge-discharge cycles) similar to that of NiCd cells, meaning 500 - 1000 cycles, if correctly maintained. Cells deteriorate gradually as a result of the negative electrode oxidation - which induces tension fall at terminals and the positive electrode oxidation - which induces a capacity reduction. The capacity reduction generates precocious charging; nevertheless, the tension reduction at terminals may block the consumer’s functioning.
In order to have maximum life expectancy, the consumer must control the charging process as far as time and rhythm are concerned, and to avoid overcharging. A small overcharging degree is, however, useful because it ensures the complete charging of the battery, but maintaining the charge at a high current for a longtime reduces the exploitation period of the cell.
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Using flash as fill in light
Flash may be used not only in conditions of dim ambient light, but also as supplementary light in the daytime. Many times we have the opportunity to take contre-jour pictures [against the light, the sunlight is behind the subject], and in most of the cases the subjects can be so dark in the picture that they show little or no detail. If the shadow covers a large part of the subject, the effect can be distracting and unattractive. In this mode the flash fires even when there is enough available light to take the picture.
Electronic flash improves the color balance of the image, improves color saturation and may help increase sharpness. Fill-In Flash is a setting on most digital cameras that will produce a short burst or bursts of light as you snap your photograph. This will provide lighting to avoid your objects in the foreground from being shadowed and the colors will appear brighter and more vibrant.
Many of the modern cameras and the advanced flashes can automatically calculate the fill in light given by the flash, but in order to fully control the final image, a combination of TTL flash and a measurement device for the indoors will provide the finest images.
There is one basic principle that should be understood, namely that flash exposure compensation actually represents a simultaneous double exposure. One exposure is suitable for the ambient light, a second exposure being given by the flash light. If the ambient exposure is given by the relationship between shutter speed and aperture, the flash exposure is controlled by the flash power, the light interval given by the flash, the manual or auto exposure, and finally by the distance to the subject, or the employed diaphragm aperture. Shutter speed has no influence on this illuminating mode as long as this is the one indicated for the flash synchronization.
You should pay more attention especially in the open air, because the flash power is much smaller than in a studio or in a closed space. Since there are no objects outside to reflect the flash light in the direction of the subject, the flash power should be negatively reconsidered. Given that there is no mathematical formula for it, any user should test his flash under these conditions.
Finally, this exposure technique gives you more details in the shadows than you may obtain using overexposure or other exposure automatisms. With a little experimenting, everyone can use it once they are familiar with the efficiency of their flash in the open air.
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