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Bateries Charger/Cycler


Something on the batteries NiCD and NiMH


A battery is made up in series of a set of individual elements (or cells) connected, each one of which has a nominal voltage, in loaded state, of 1,2 V. In the market elements with capacities between 50 are commercialized mAh and 3300 mAh. A 1000 battery mAh is able to give a 1000 current mA (1A) during one hour, or 10 To during 1/10 of hour, etc; this value that brings form is the “C” to which we make reference ahead but.
The reloadable batteries are able to release a very high current, because the Maxima current is limited by its internal resistance. The batteries of NiMH, of greater capacity than those of NiCd (a 70% more) have however an internal resistance something superior to those of NiCd.
In a battery of elements in series, the internal resistance by element is multiplied by the number of these. When the electrical circuit to traverse is closed a load, the resistance of the battery produces a fall of tension due to its internal resistance: the provided voltage is inferior to which it is moderate in open circuit. This means that it leaves from the energy of the battery it is consumed and it dissipated inside the battery in heat form. A battery of 7 elements just loaded provides, in emptiness, a voltage of almost 10 V; when the circuit through a load is closed (motor) with a current of, we say, 1,5 To, the voltage by element falls to 1,2 V, giving a total of 8,4 V for the battery. The rest 1.6V 1,5 xs A= 2,4 w dissipate in heat form.
The heat is the great enemy of the elements of NiCd and still more of those of NiMH. If a battery with a high current is loaded and once reached the fully factored load it is continued providing current, this one will dissipate inside the battery in heat form, being able to deteriorate it or to even destroy it.

Ciclado memory and of batteries

The call “effect of memory” in the batteries of NiCd (that does not exist in those of NiMH) is still reason for controversy. Whereas the experience of much people assures that the partial unloading of a battery of NiCd, followed of a load and another partial unloading etc. are little by little reducing the capacity of the battery, there are detractors that assure that the memory effect is fictitious, and which only occurs if the partial unloading happens in all the occasions until he himself point, is very improbable that it happens in the reality. The supposed effect of memory can be suppressed ALMOST completely by means of the ciclado one of the batteries, unloading them and returning them to load in its totality several times.
What totally it is demonstrated is that the batteries just acquired are “sluggish” and they do not acquire its Maxima capacity nor they give its Maxima current until them has ciclado some times. This effect also is observed in batteries that have been loaded with a current not very elevated but during a long time (days or weeks): this produces the growth of crystals in the electrolyte and reduces the lifting capacity drastically, but it is possible to be solved ciclando the batteries some times.
Otra certeza es que NUNCA SE DEBE DESCARGAR COMPLETAMENTE una batería, bajo riesgo de producir la llamada "inversión de polaridad": una batería nunca está compuesta de n elementos exactamente iguales, en capacidad y prestaciones, por lo que, en una descarga total, inevitablemente se descargará antes el elemento más débil, y al seguir circulando corriente impulsada por los elementos más fuertes, se producirá la carga parcial del elemento débil con polaridad contraria. Esto es nefasto para la vida útil de ese elemento, pues en su interior se crean dendritas metálicas que pueden llegar a cortocircuitar los dos electrodos, destruyendo la celda.

Baterías de NiCd frente a NiMH

Como ventajas fundamentales, las baterías de NiMH tienen una mayor densidad de carga (capacidad/peso superior, aprox. 40%-70% más capacidad); no contienen Cd (tóxico) y aparentemente no tienen efectos de pérdida de capacidad por mal uso, o de formación de dendritas. Como inconvenientes, tienen una resistencia interna superior que limita su uso en aplicaciones de alta potencia. Es cierto que han aparecido nuevos tipos en el mercado (Sanyo RC 3000 HV, RC330 HV) que prácticamente igualan en capacidad de descarga a las celdas del mismo tamaño de NiCd, RC2400. Otro inconveniente es que no admiten una carga tan rápida como las de NiCd, bajo riesgo de deteriorarlas. Es buena medida no cargarlas a regímenes  superiores a C: una celda RC3000 HV se podrá cargar sin problemas a 3 A. Aún más que en el caso de NiCd, los elementos de NiMH son sensible al calor: un sobrecalentamiento puede producir gases internos y sobrepresiones que dan lugar a escapes de electrolito y pérdidas de estanqueidad, reduciendo la vida útil de las celdas. El estado de carga total es también más difícil de detectar en las NiMH, por lo que se recomienda el uso de cargadores que especifiquen su aptitud para cargar baterías de NiMH, evitando así sobrecalentamientos indeseados.

Because an intelligent charger?

An intelligent charger is called thus because he has the total control of the state of the battery to load. He himself is programmed to execute controls on the tension, current and temperature of pack of batteries connected, causing that the security is the factor but important to fulfill.

When a battery is loaded, there are many factors that take part, from the materials of his manufacture to the room temperature. Each one of these factors deberian to be considered like so that the charger realize of the best way the load and cut of load.

The main problem that has a nonintelligent charger is that “the intelligent” part provides the operator, that is to say, we. She is the person who puts to load the batteries the one that the time controls that estan under load, the temperature, and with luck another parameter like the tension and/or the current.  This is very dangerous because any forgetfulness, omission or any other factor that make distract us of the control can cause from simply losing the batteries to a fire.

On the other hand, an intelligent charger has the great disadvantage of the price, since he is much more expensive in relation to one fact with 3 transistors or single a CI dedicated. In which to it concerns, the value is left place setting by the life utility of first packs which we have.


Load curves of batteries of NiMH for different values from C
to see the detail of - dV/dt (tip of V at the end of the load)

Recommendations for an intelligent charger

Since I have found as much information on the matter, and so much is said on the chargers in Internet, we can get to say that these are the requirements that deberia to have an intelligent charger:

  1. That it detects when the battery or battery this connected
  2. That it allows to select the type of load, amount of elements
  3. That it detects the state of the batteries just connected by means of a tension analysis and current, short circuits, discharges tensions, etc.
  4. That at the outset min of reading to hope that lets 5 the tension becomes stabilized. This is in case the battery finishes unloading and we thought to recharge it right away or comes from a cycle of the ciclador.
  5. To have a total control of the tension, current and temperature of the elements to produce a cut
  6. That it loaded to 0.3C whereas the tension does not reach 0.8V/elem. This is for preventing a high load when the battery this very unloaded.
  7. When this loading to regime, must stop by:
    1. Exeso of temperature dT/dt
    2. Variation of tension - dV/dt (tip of tension in the load curve)
    3. Maximum tension
    4. Maximum time
  8. To detect and to declare maximum temperature when the temperature reaches a value I criticize or when the increase of temperature is much for a given time.
  9. To detect a tip in the tension of the battery during the load (- dV/dt), but this must be measured single after 30min of load because false tips at the beginning of the load can take place.
  10. That it produces the cut when the tension of the battery arrives at 1,78v by element.
  11. That it produces the cut when is arrived at the same time maximum from load, based on the amount of elements, the regime of load.
  12. That after detecting a cut, it leaves the battery in load by dripping (0.1C) measuring that the temperature never surpasses 35º
Many of these recommendations are those that Maxim gives, respect to the use of microcontroller in chargers of batteries.

Ciclador of batteries

Like second part, after the load of the elements of the battery took place; if these are not going to be used in a period of reasonable time, it is recommended to ciclar them before storing them months. Also deberia to be applied to the ciclado process of after the battery to state a time without use since this way he is as better “rejuvenece” and allows to the best autonomy and life utility.

A ciclador is not another thing that a circuit of unloading associated to the charger of batteries, which does is to unload pack and later to warn that it has unloaded so that the charger returns to load it again. If this circuit of unloading also has a circuit of control like the one of the charger, we have the best combination, an intelligent ciclador charger/.


Curves of unloading for different values from C

Recommendations for an intelligent ciclador

Although not encontre much information on the matter, the ciclador by logic deberia:
  1. To allow to select the unloading regime, at the most to 1C
  2. To have the control on the current of unloading and the tension of the battery.
  3. To never unload the battery below 0.8V by element
  4. To stop the unloading if the battery reaches a greater temperature to 35º

Bibliography



Microchip, PICREF-2 Specifications of an intelligent charger











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