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.