Ni/Cd cells and Ni/MH cells are usually charged by constant current. When Ni/Cd cell is being charged or discharged, a chemical reaction occurs in the positive and negative electrode and the active material is transformed. If the Ni/Cd cell is overcharged, the water is decomposed. If the negative active material is more than the positive active material, oxygen gas comes out from the positive electrode and is absorbed into the negative electrode. In this case, the decrease of cell voltage (-¡âV)is showed and the full charging(100 %) can be possible by detecting it.

In Li-ion cell, an electric energy occurs through the redox reaction when Li-ion moves between positive and negative electrods. If cell voltage rises up after full charging, the solvent begins to be decomposed. However, it is necessary that you detect the upper voltage during charging and avoid charging above the upper voltage. The Li-ion cell is charged by a constant current till the upper voltage is reached. Then, it is charged by the constant voltage.

In cases of Ni/Cd and Ni/MH cells, (-¡âV) detecting method is used because it is possible that we sense the full charging by detecting the dropping point of voltage. It is the way of observing the voltage variation per unit time and detecting the time when the variation of voltage becomes negative. In this case, the voltage is decided by approximate values. On the other hand, the voltage of Li-ion cell is decided by absolute quantity because it follows the way of constant-voltage charge method.

Here are some values below, which are used as control voltage of each battery.

Ni/Cd cell : (-¡âV) -10 ~ -20mV
Ni/MH cell : (-¡âV) 0mV
Li-ion cell: 4.1V/cell or 4.2 V/cell

In the case of Ni/MH cell, (-¡âV)=0mV is used. Ni/Cd cell absorbs heat during charge through chemical reactions, But the temperature raise of Ni/MH cell become very rapid, so, we decide the point in which voltage stops rising under the full charge.

At the present time, two different sorts of voltage of charge control, that is, 4.1 V/cell and 4.2 V/cell(excluding the parts connected in a line) are used for Li-ion cell, which depending on anode materials(cokes or graphite). About the level of precision when voltage is relatively measured, the minimum analysis ability can be a problem and, in terms of the difference of the battery amount connected in a series, there is no effect with the relative estimation. In the case of measuring the absolute quantity, both the range of measurement and minimum analysis ability can be a problem. Therefore, it is necessary for the charge control circuit of the Li-ion cell to have a high precision circuit with constant-voltage charge which corresponds to cell voltage.