Overcharging

The performance of a battery is often designed to be limited by one electrode ia order to achieve special performance characteristics, such as overcharge protection and safety. The coulombic efficiency of the active mass is of particular iaterest ia battery design and performance. 

Charge acceptance of the silver—2inc system is normally on the order of 95—100% efficient based on coulombic (ampere-hour output over input) values. This is tme of any of the charging methods when carried out in the proper manner. Thus overcharge is rarely necessary in charging silver—2inc cells and batteries. 

Fig. 17. 50 A-h nickel—hydrogen performance showing (a) pressure and (b) voltage curves where region A represents charging at 10 A, region B represents overcharge at 10 A, region C represents discharge at 25 A, and region D represents reversal at 25 A. To convert MPa to psi, multiply by 145.

The overcharge reactions for the cell are the same as for nickel—cadmium and nickel—hydrogen cells. The oxygen generated on the nickel electrode at the end of charge and overcharge finds its way to the anode and reacts to form water in the Ni—H2 case and Cd(OH)2 in the Ni—Cd case. 

Spontaneous low resistance internal short circuits can develop in silver—zinc and nickel—cadmium batteries. In high capacity cells heat generated by such short circuits can result in electrolyte boiling, cell case melting, and cell fires. Therefore cells that exhibit high resistance internal short circuits should not continue to be used. Excessive overcharge that can lead to dry out and short circuits should be avoided. 

Overcharge Reactions. Water electrolysis during overcharge is an irreversible process. Oxygen forms at the positive electrode ... 

When these half-reactions are summed, there is no net reaction. Thus the material balance of the cell is not altered by overcharge. At open circuit, equation 19 at the negative electrode is the sum of a two-step process, represented by equation 15 and... 

These equations are based on the thermodynamically stable species. Further research is needed to clarify the actual intermediate formed during overcharge. In reahty, the oxygen cycle can not be fully balanced because of other side reactions, that include gtid corrosion, formation of residual lead oxides in the positive electrode, and oxidation of organic materials in the cell. As a result, some gases, primarily hydrogen and carbon dioxide (53), are vented. 

Other alloying ingredients in lead, eg, arsenic (0.5—0.7%) and silver [7440-22-4] (0.1—0.15%), inhibit grid growth on overcharge and reduce positive grid corrosion. Tin added to a lead alloy produces well-defined castings that are readily adapted to mass production techniques (84). 

Overcharge/overfeed of reactants Possibility of overfilling vessel, or initiating runaway reaction. 

Overcharge of catalyst or initiator, too much or too fast. Possibility of runaway reaction. 

In a rapid changeover to make another product the meter was not recalibrated for the new monomer, monomer B. This led to a large overcharge of monomer B and the subsequent runaway reaction. 

The charge state of the cell must be maintained in operation to have a cell voltage of 0.9 to 1.2 V [6]. Overcharging the cell is to be avoided due to electrolytic decomposition of water and evolution of gas. The cell voltage should therefore not exceed 1.4 V. Cathodic protection stations should be operated so that the cell voltage lies in the desired range. 

To avoid this problem for lithium-ion batteries consisting out of non-overcharge-able cells, computer-controlled charging systems regulate the voltage for each single cell. 

The system can prevent explosion, fire, and venting with fire under conditions of abuse. These batteries have a unique battery chemistry based on LiAsF6/l,3-di-oxolane/tributylamine electrolyte solutions which provide internal safety mechanism that protect the batteries from short-circuit, overcharge and thermal runaway upon heating to 135 °C. This behavior is due to the fact that the electrolyte solution is stable at low-to-medium temperatures but polymerizes at a temperature over 125 °C... 

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