Charging versus Discharging

The previous discussion was about what happens when a NiCad cell or battery is discharging. Well, what happens when a cell or battery charges? In effect, charging is the reverse of discharging. During the charging cycle current is being forced back into the cell (opposite of discharge current). Here, electrons are being taken out of the positive terminal and forced into the negative terminal. This means that the material at the positive terminal is being oxidized (hence it is now the anode -- confusing, eh?) and the material at the negative terminal is being reduced (now the cathode). In the NiCad system the cadmium hydroxide is being re-converted into cadmium, and the nickelous hydroxide is being re-converted to nickelic hydroxide.

The easy part of charging is reconverting the spent material on the plates to the charged condition. The hard part comes in knowing when to stop. Let us take a moment to think about what happens when we overcharge the battery. Once all the nickelous hydroxide is converted into nickelic hydroxide, and in theory all the cadmium hydroxide is converted into cadmium, the charging current has to go somewhere. As the energy of the charging current cannot go into more chemical energy, it goes into splitting water molecules (water is still the major constituent of the electrolyte). Just like the age old chemistry experiment of splitting water into hydrogen and oxygen, a fully charged NiCad cell does the same thing. You are forcing oxidation at the positive terminal and reduction at the negative. When one oxidizes water (actually the OH- ion), one produces oxygen. Likewise, at the negative terminal (now the cathode), one produces hydrogen.

This of course is bad because oxygen + hydrogen = BOOM!!!

Cell manufacturers attempt to prevent this from happening. During manufacture, they deliberately oversize the negative plate, and then partially discharge it. That is, they put a fully charged positive plate, but put a slightly discharged, but bigger plate of cadmium in. The amount of free cadmium in the oversized plate is matched to discharge in step with the amount of nickelic hydroxide provided in the positive plate.

Now consider what happens as full charge is achieved. Oxidation of water starts at the anode, but since the cathode is oversized, and has excess hydroxide, the current continues to produce cadmium metal instead of hydrogen. At the same time, the separator (the material used to prevent the plates from shorting) is designed to allow oxygen gas to diffuse through, from the positive to the negative plate. The free oxygen then oxidizes the cadmium metal to form more cadmium hydroxide to prevent hydrogen from being formed. The outcome is a safe battery.

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