The ratchet effect is a physicochemical phenomenon affecting the performances of the electric fencer .

Technologies concerned

The ratchet effect relates to mainly technologies Ni-Cd (cadmium-nickel) or Nor-MH (nickel-metal hydride or metal nickel-hydride).

However, the accumulators Nor-MH are less sensitive there, because the latter are, in general, out of use before one cannot observe a significant ratchet effect.

The batteries lead-acid or containing Lithium (as the batteries Li-ion) are not sensitive to the ratchet effect; one can consequently reload these battery without to have discharged them beforehand.

Manifestation of the ratchet effect

The ratchet effect involves a reduction in the quantity of energy which the accumulator can exchange with outside with for consequence a reduction of the rating of the storage battery.

The accumulator cannot discharge any more as in the beginning. It gives the impression to be able to store less energy, but actually, it is more the restitution which poses problem: energy is not simply accessible any more in the same way because of the ratchet effect.

One can make the analogy with the tank of a vehicle of which a part would be stopped but would lock up still fuel. With such a tank and the same quantity of fuel, one will not be able to traverse any more the same distance but with one reserve in good state.

Discovered ratchet effect

Historically, this effect was discovered by NASA with the satellite .

The satellites reload their accumulators using solar collectors. certain satellites pass from a place lit to a dark place with very regular intervals. Their accumulators take care and discharge always in the same way. After certain numbers of these regular cycles, they cannot discharge any more beyond the value to which they were accustomed. The accumulator “recorded” this level of discharge, from where the name “ratchet effect”.

In the everyday life, this situation practically never meets, except for some rare automated systems, because this effect appears only when the successive discharges are stopped exactly on the same level of capacity, cases rather not very current.

According to the commonly allowed direction name “ratchet effect” corresponds rather to an abuse language. The English terms voltage depression or lazy battery effect , if they had French equivalents, would be more suitable.

Description of the ratchet effect

The problem appears during the use of accumulators in apparatuses provided with a detection with the supply voltage. This detection is intended to cause the stopping of the apparatus when the accumulators are empty (i.e. when the tension on their terminals reaches a minimum threshold). This safety is necessary to the good performance of the apparatus itself and avoids the irreversible destruction of the accumulator by major discharge.

An accumulator of battery Ni-Cd has a nickel electrode, made up of small crystals. Under certain conditions, one observes an increase in the size of the crystals. This enlarging decreases the surface of contact between the electrode and the electrolyte what causes a fall of tension of the degraded part and increases the internal resistance of the accumulator.

Whereas the nominal voltage of Ni-Cd is of 1,2 V/élément, this degraded structure presents a lower nominal voltage, of approximately 1,08 V/élément.

During the use, all occurs as if each element had two parts, a normal part with 1,2V and a part with 1,08V degraded. Energy will be initially taken in the part presenting the most raised tension and all will appear normal. When this part is exhausted, one will then come to draw from the degraded part, but the threshold of tension will pass then brutally from 1,2V/élément to 1,08V/élément.

This change creates a discontinuity in the curve of discharge of the accumulator. The nominal voltage will approach the minimum threshold of operation of the apparatus, will pass in lower part and will cause the stop.

Seen of the user, one can accept a loss of capacity, makes the capacity of it is always available but with a lower tension.

One observes in particular this phenomenon in 2 cases:

  • during the discharges partial of the accumulator (it appears a difference in structure between the part charged and the discharged part)
  • at the time of the load of a battery containing nickel.
    • Once the finished load the battery receives a current of maintenance supposed to compensate for the car-discharge. Unfortunately, after a certain time, this current of maintenance generates the progressive degradation of the storage cells by localized Cristallisation of the electrolyte. After that, the tendency is to give the battery on the charger, which by again injecting the current of maintenance at the end of the load deteriorates couple Ni-Cd more.
The apparatus never empties the accumulator until its minimal threshold and there is a successive increase in the degraded part. Actually this faded structure is normal and fact part of the operation of the accumulator. Fortunately this phenomenon is not irreversible.

To recover a battery which has a ratchet effect

By connecting an affected battery of the problem on a suitable system of discharge (unloader), intended to empty it until the minimum threshold of 1V/élément, the part of the capacity which has “a bad” nominal voltage, will be restored. It could then be proceeded to the refill of the battery which will find most of its performance.

These operations should not be carried out without material adapted , nor precautions . All the electric fencers are dangerous : they can warm up , to take fire , even to explode and cause serious physical injuries . It is to better buy an unloader of battery adapted rather than to take the risk to stupidly destroy the battery while reducing its tension in lower part from 1V.

Adapted chargers

The modern chargers and of quality, specific to technology Ni-Cd, thanks to their functions of “intelligent” pre-discharge and load like to their detection of end of correct load, allow to free themselves from the effect “says” memory.

See too

Internal bonds

External bonds

  • the ratchet effect on ni-cd.net
  • Canadian Site on the batteries

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