Magazine article American Cinematographer

Cinema Workshop

Magazine article American Cinematographer

Cinema Workshop

Article excerpt


The most attractive feature of the Mi-Cad battery is obviously its ability to be recharged. The average Ni-Cad sealed cell is capable of 300 to over 1000 charge/discharge cycles during a lifetime. Moreover, the Ni-Cad cell is capable of a very wide variety of charge rates.

Ni-Cad battery chargers can be broken down into three basic charge rates. These are slow charge (overnight), quick charge and fast charge and these are defined in FIGURE 1. However, before discussing the types of chargers, we must first look at the Ni-Cad cell itself. The Ni-Cad cell can accept a charge rate from about .05C (30 hours) to over 1OC (6 minutes). The .05C rate is usually considered the lowest practical charge rate due to non-recoverable energy losses. This means that during the charging of any Ni-Cad, not all the charge current is being converted to power that can be used later. By design, a small amount of current is absorbed in side reactions that cannot be recovered. During normal charge currents (C/10 and higher) these losses are negligible. However, as the charge current gets smaller than C/20, most of the current goes into this side reaction, and very little, if any, will actually go to charge the battery.

The Ni-Cad cell may appear extremely versatile; accepting charge rates from as little as C/20 to over 1OC. There is a catch, however. Once the cell is fully charged, all the current that formerly was used to charge the cell is now channeled into a side reaction, namely the production of oxygen at the positive electrode. As the cell continues into overcharge, the pressure and temperature begin to rise very rapidly. At extremely high charge rates, the temperature and pressure can build up so fast, that the cell will vent and be destroyed within minutes of reaching full charge. Thus, a Ni-Cad cell can accept fast charges. However, it will be destroyed within minutes if this charge current continues past the point of full charge.

The invention of the sealed cell Ni-Cad brought a simple solution to this problem. We know that once the cell reaches full charge, the charge current produces oxygen at the positive electrode. It is this oxygen that eventually builds up the pressure to a point at which the cell "explodes" or vents. In the sealed cell, the negative electrode is designed to absorb oxygen. Thus, when the cell goes into overcharge, the excessive oxygen that is produced migrates to the negative electrode where it is absorbed. Simple as that- not quite; there is one little catch. There is a limit to how much oxygen the negative electrode can absorb. If the positive electrode produces more oxygen than the cathode can absorb, we are back to the same old problem: pressure will build up, temperature skyrockets and the cell will be destroyed.

The ultimate solution is very simple. Keep the charge current low enough that the anode will never produce more oxygen than the cathode can absorb. This charge current is almost always the C/10 rate or "overnight charge." If the Ni-Cad battery is charged with a C/10 charger, it will reach full charge in 14-16 hours and then enter a "safe" overcharge region indefinitely. A Ni-Cad can actually be left on a C/10 charge for days, weeks or even months without <. any permanent damage to the cells. This is because the charge current is producing oxygen at a rate well within its capacity to reabsorb it. Once the charge current rises much past the C/10 level, the cell produces oxygen faster than it can absorb it and the cell will eventually "explode" or vent. …

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