Patent GB-1309394-A (GB)
Title: Battery chargers
IPC: H02J7/10
Filed: 1970-01-27
Abstract: 1309394 Charging batteries J A MACHARG 18 Jan 1971 [27 Jan 1970] 4001/70 Heading H2H A battery charger comprises means to compare a reference voltage with the actual voltage of the battery being charged to derive a difference which is applied to sensing means for producing either a zero signal or one of given value according to whether the difference is above or below a predetermined value, the sensing means controlling integrator means whose output controls the magnitude of the charging current between limits and the reference voltage being varied between predetermined upper and lower limits as an integrated function of the output of the sensing means. The embodiment shown in Fig. 1 includes members 12, 14, 16, 18 and Rf1 which are the same as the corresponding parts of Fig. 4 of Specification 1,225,802, but the relay contacts of the latter Figure are replaced by a power stage 10 whose output is determined by the potential across a resistor Rf2. In a typical charging operation the Schmitt trigger 18 switches on or off according to whether the difference signal from the comparator 12 is below or above a predetermined value corresponding to an amount by which the battery voltage exceeds the reference voltage Rf1. When the trigger switches on, its output signal is applied to integrators 22, 24 of which the latter has the shorter time constant, the outputs of the integrators being applied to voltage to current converters 26, 28. The former controls the current source 16 to augment the current from the source 14 and so progressively raise the reference voltage from a lower to an upper limit. The converter 28 feeds a constant current device 30 which builds up a voltage across the resistor Rf2 through contacts 32 which are closed by a relay driver 20 when the latter receives an output signal from the trigger 18. When the trigger switches off, members 20, 22 and 24 are de-energized and the battery and reference voltages then progressively reduce from the maximum values achieved at the end of the previous "trigger on" period. Cycling continues in this manner to produce variations of battery and reference voltages, which variations alter as the battery becomes fully charged. Typical characteristics are discussed, Figs. 5A, 5B and 5C (not shown), in which the battery and reference voltage variations become almost identical when the battery is nearly charged, for which condition the reference voltage varies over a limited range between the upper and lower limits. In a modification, Fig. 2 (not shown), the relay driver 20 and contacts 32 are omitted and the resistor Rf2 is in series with the second transistor of a Darlington pair, which forms the voltage to current converter controlled by the integrator 24. The latter and the integrator 22 comprise series RC circuits. Charging current flows all the time since the Darlington pair always maintains a current through the resistor Rf2, which current is varied by the integrator 24. The converter 26 and a voltage stabilizer (44) for the whole circuit may be the same as corresponding parts of Fig. 4 of Specification 1,225,802. The integrator 22 may be omitted, in which case the converter 26 is controlled from the tap of a potentiometer (R56) in parallel with a Zener diode (Z51) and resistor (R51) connected in series with the Darlington pair, Fig. 3A (not shown). In another embodiment omitting the integrator 22, Fig. 3B (not shown), a parallel-connected Zener diode-potentiometer circuit (Z57, R58) has a tapping connected to the Darlington pair, the tap of whose associated potentiometer again supplies the converter 26. Adjustment of the gain of the last two modifications is discussed to show how to produce an effect equivalent to the different time constants of the integrators 22, 24. In Fig. 4 the augmenting reference current source 17 is switched in and out by a transistor TR38 controlled by the output of a comparator 34, a first input p of which receives from terminal 36 a voltage proportional to the charging current and a second input q of which receives the voltage across a capacitor C26. The latter voltage is produced by closure of a switch 38 to charge the capacitor from the terminal 36 and by opening of the switch 38 to allow the capacitor to discharge linearly through a constant current device 40. The switch 38 may be operated mechanically by a driven rotary switch or electronically by a multi-vibrator. The comparator 34 operates to switch on or switch off the transistor TR38 according to whether the rate of reduction of voltage at input p is greater or less than that at the input q, thus producing a high reference voltage when the battery is nearly discharged and a low reference voltage when nearly charged. The power stage 10 may be in the form of one or more thyristors whose firing angle is determined by the voltage across the resistor Rf2. The various problems associated with this type of power stage and the consequential modifications to the control circuit are discussed. Application of the invention to charge a large number of cells is described, Fig. 6 (not shown).