GB1046801A - A coulometric device - Google Patents

Abstract

1,046,801. Charging batteries. CANADA, MINISTER OF NATIONAL DEFENCE. April 16, 1963 [April 30, 1962], No. 14878/63. Heading H2H. [Also in Division G1] An electro-chemical cell comprises a pair of reversible electrodes in contact with an electrolyte, active electrode material being present in reduced and oxidized states, the voltage between the electrodes being substantially zero while current is flowing through the cell causing a complementary oxidation-reduction reaction to occur at the electrodes, the said voltage exhibiting an abrupt change from zero when more than a predetermined quantity of electricity has passed in a single direction through the cell. This latter voltage may be used to indicate the passage of the said quantity, or to control a circuit, e.g. the charging circuit of an accumulator. Such a cell may also be used to determine the state of charge of nickel-cadmium batteries, and to produce an indication when a primary cell of the zincmercuric oxide type has expended a large fraction of its useful life. Cell construction.-In Fig. 1 (not shown), a cell 10 contains two sintered nickel plates 11, 12 impregnated with cadmium 13 and cadmium hydroxide 14 respectively, in an electrolyte 15 of potassium hydroxide solution. Current flow in the appropriate direction oxidizes the cadmium 13 and reduces the cadmium hydroxide 14; when this process is completed further current flow in the same direction causes oxygen evolution from the oxidized plate, whose potential increases abruptly and hydrogen evolution from the reduced plate, whose potential falls abruptly. In Fig. 2 (not shown), two sets X, Y of sintered nickel plates 21 carrying Cd(OH) 2 are interleaved in a rectangular container 26 of plastic, nylon, nickel, or stainless steel, adjacent plates being separated by a woven nylon cloth 20. The plates of each set X, Y are interconnected by a metal tab 23 and the electrolyte is 20-30% KOH solution. The cell is formed by the passage of a current which reduces the Cd (OH) 2 on plates Y and evolves from plates X oxygen which escapes through a vent 28. When all the material on plates Y has been reduced to metallic Cd, the cell is short-circuited or the current reversed to reoxidize 5-10% of this Cd. The vent 28 is then closed. In another embodiment (not illustrated) the electrodes consist of platinum, on one of which copper has been deposited, and the electrolyte is a solution containing copper ions. While copper is being dissolved from the one electrode, and plated on the other, no substantial voltage occurs between the electrodes, but when this process is complete oxygen is evolved and the voltage rises steeply. The Specification tabulates a number of further examples of active electrode materials and electrolytes. Control of battery charging current.-The cell is connected in series with the battery, and the inherent electrical capacity of the cell may be used to determine the charge; alternatively, the cell may be " calibrated " during discharge of the cell, so that during charge it permits the return to the cell of a quantity of electricity equal to that discharged. In a constant potential charging arrangement, the cell voltage aids the battery voltage in backing off the charging voltage; more than one cell may be used, according to the range of voltage regulation in use. For constant current charging the circuit shown in Fig. 5 is used. When the predetermined charge is complete, the voltage between plates X, Y of the cell rises and is amlpified by a transistor T 1 whose output actuates a second transistor T 2 to shunt the battery circuit. In practice a safe overcharge current is continued through a resistor R B and a diode D B A simpler alternative circuit, Fig. 6 (not shown) uses a single transistor. Determining state of charge.-A cell connected in series with a Ni-Cd battery has electrodes of highly porous Ni carrying electro-chemically active manganese oxides, and a centre-zero voltmeter is connected between the electrodes. Fig. 7 (not shown) depicts how the state of charge of the battery may be determined from the magnitude and polarity of the voltage between the electrodes. To prevent the actual evolution of H 2 , two diodes are shunted across the cell in opposite directions; when the voltage across the cell exceeds the forward voltage of the diodes, the overcharging or overdischarging current is diverted from the cell. Primary cell monitoring.-A cell placed in series with a primary battery of the zincmercuric oxide type has, on one electrode, a quantity of metal such as copper representing, say, 75% of the capacity of the battery. During battery discharge this copper is transferred to the other electrode; thereafter the cell voltage rises as O 2 is evolved at the first electrode. The cell may then be short-circuited pending replacement of the battery.