Patent GB-1175424-A (GB)
Title: Battery Discharge Control
Inventor: CARSON WILLIAM NEWTON JR
IPC: G01R31/36
Filed: 1967-03-22
Abstract: 1,175,424. Battery discharge arrangements. GENERAL ELECTRIC CO. 22 March, 1967 [19 May, 1966], No. 13555/67. Headings H2H. [Also in Division G1] An arrangement to ensure that a rechargeable battery 10 is only discharged to a predetermined extent into a load 14, comprises a coulometer cell 12 which carries the battery discharge current and undergoes an abrupt voltage change after the predetermined discharge, this voltage change being used to actuate a switch 16 and alarm 22. The cell 12 comprises cadmium hydroxide electrodes 18, 20 immersed in potassium hydroxide electrolyte and the electrode 20 is initially charged (by converting a portion of its cadmium hydroxide to free cadmium) to the desired capacity to be drawn from the battery 10 before the cell 12 develops its " end of discharge " voltage signal, i.e. when all the free cadmium has been reconverted to cadmium hydroxide. When the predetermined discharge point has been reached, linked switches 26, 27 are respectively closed and opened to recharge the battery 10 and cell 12, whose setting is not disturbed by overcharge with respect to the initial charge setting or by over-discharge past the predetermined discharge point. The desired initial charge setting of the electrode 20 may be obtained by charging the cell 12 beyond this value, removing all gases and sealing, determining the capacity (by measuring the total charge required to discharge the cadmium charged electrode), admitting a predetermined amount of oxygen to remove the excess capacity, and finally resealing. In the arrangement of Fig. 2, simultaneously adjustable resistors 34, 36 determine the proportion of battery current passing through the cell 12<SP>1</SP> which controls a switch 30 for opening the battery-load circuit when the predetermined discharge point has been reached. The cell 12<SP>1</SP> has an auxiliary electrode 32 connected to the initially-charged electrode 20 by low value resistor 33, which arrangement promotes the recombination of oxygen generated in the cell on overcharging. A method of setting the cell 12<SP>1</SP> is as follows: (1) The unsealed cell is placed in an oxygen atmosphere with electrode 32 connected to electrodes 18, 20. (2) When electrodes 18, 20 are completely converted to cadmium hydroxide, the electrodes are disconnected, the oxygen is pumped off and the cell is filled with hydrogen. (3) With the electrode 32 connected to the positive terminal of a charging source and the electrode 20 connected to the negative terminal, a desired charge is taken from the source corresponding to the required setting of electrode 20, after which the excess hydrogen is pumped off and the cell is sealed. Another method of setting uses an auxiliary electrode 32 which does not self-discharge and acts as a cathode for the charged cadmium electrode, e.g. a mercuric oxide mercury auxiliary electrode is stated to be suitable. This method is as follows: (1) The cell is sealed with all three electrodes substantially discharged. (2) With the electrode 32 connected to the positive terminal of a charging source and the electrode 20 connected to the negative terminal, a desired charge is taken from the source corresponding to the required setting of the electrode 20. This second method of setting may be used to adjust a cell which has been initially charged to value slightly above or below the desired final value, the polarity connection of the electrodes to the charging source being made according to whether it is required to add or substract charge. Other materials suitable for the auxiliary electrode 32 are cupric oxide, silver peroxide and ferric oxide. If desired, the coulometer cell may be combined with the battery it controls, e.g. its cadmium electrodes may share the same electrolyte as the electrodes of a nickel cadmium battery.