This application is a continuation of copending application Ser. No. 825,478, filed on Aug. 17, 1977. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a charger using one or more solar batteries and useful for small-size apparatus such as an electronic wristwatch. 
     It is well known that a nickel-cadmium (Ni-Cd) battery can be used as a power source for electronic wristwatches in combination with one or more solar batteries (SB). However, the Ni-Cd battery is not preferable because cubic efficiency of battery capacity is poor and the amount of self discharging is large. For instance, when the Ni-Cd battery has a diameter of 11.6 mm and a height of 5.1 mm, the battery capacity is about 20 mAH and the self discharging amounts to nearly 30% for 90 days. In contrast, a sealed type silver oxide battery, which consists of a positive electrode made of Ag, AgO or Ag 2  O and a negative electrode made of Zn or Cd, has excellent properties with respect to cubic efficiency, the self discharging, etc. For instance, in the case of a silver oxide battery of substantially the same size as the above, that is, a diameter of 11.56 mm and a height of 5.33 mm, the battery capacity is about 190 mAH and the amount of self discharging is about 10% per year when the electrolyte is NaOH and about 20% per year when the same is KOH. Moreover, the silver oxide battery shows charging efficiency up to about 90% while the Ni-Cd battery has a charging efficiency of only about 70%. Nonetheless, in the event that the silver oxide battery maintains its charge after full charging, the battery will swell out and the output voltage of the battery will swing. The former is deemed to occur due to the fact that gas is generated within the battery by the over-charging. The latter is deemed to occur because Ag oxide is dissolved into an alkali electrolyte and then shifted into the Zn negative electrode or Zn is deposited in a dendrite or spongy fashion to cause short circuiting. 
     Even in the prior art Ni-Cd battery, the battery might swell out by gas accumulation within the battery when more than 0.1 CA charging current flows under the full charged condition. To this end, a current limiting resistor or the like is usually connected to suppress the charging current in the order of less than 0.1 CA. However, unlike the Ni-Cd battery, the above-mentioned silver oxide battery will result in expansion of the battery or swinging of the output voltage even when the charging current is limited. A sealed type mercury battery consisting of a positive electrode made of Hg, HgO or Hg 2  O and a negative electrode made of Zn or Cd, must undergo the same circumstances. 
     Therefore, it is an object of the present invention to provide a charger which overcomes the above-mentioned disadvantages. According to the present invention, a charging current limiting means and a charging voltage limiting means are provided simultaneously in a charger unit including one or more solar batteries and a secondary battery such as a silver oxide battery. In one preferred form, the charging current limiting means and the charging voltage limiting means are enabled only when the solar batteries deliver the output voltage. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and novel features of the present invention are set forth in the appended claims and the present invention as to its organization and its mode of operation will best be understood from a consideration of the following detailed description of the preferred embodiment taken in connection with the accompanying drawings, wherein: 
     FIG. 1 is a characteristic chart showing relationship between permissible battery voltage and charging current of a silver oxide battery; 
     FIG. 2 is a circuit diagram showing one preferred form of the present invention; 
     FIG. 3 is a characteristic chart showing relationship between charging voltage and charging current; 
     FIG. 4 is a characteristic chart showing a charging property of the silver oxide battery; 
     FIG. 5 is a circuit diagram showing another embodiment of the present invention; 
     FIG. 6 is a circuit diagram showing still another embodiment of the present invention; 
     FIG. 7 is a circuit diagram showing another embodiment of the present invention; and 
     FIG. 8 is a characteristic curve showing relationship between charging voltage and charging circuit. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Although the present invention is equally applicable to the silver oxide battery, the mercury battery or the like, details of the present invention will be set forth by way of example of the silver oxide battery. 
     FIG. 1 illustrates a battery voltage vs. charging current characteristic of the sealed type silver oxide battery with the numbers on the abscissa indicating the battery voltage E(V) and the numbers on the ordinate indicating the charging current I o  (m A ). The battery will be operable well without any expansion or circuit-shorting provided that neither the battery voltage nor the charging current extends beyond a range surrounded by the slant lines. It is obvious from FIG. 1 that the battery voltage E keeps increasing with the development of charging and, if the charging current exceeds such permissible range, then the battery will swell out and short-circuiting will occur. In other words, it is not possible to overcome the swelling out and the short-circuiting of the battery even if the charging current is limited as was typical with the prior art Ni-Cd battery, because of further increasing of the battery voltage E. To solve this problem, it is required that the charging current be suppressed with respect to the battery voltage and therefore not only the charging current but also the battery voltage will be suppressed within the permissible range. Since the battery voltage never increases above the charging voltage, a limitation on the charging voltage means equally a limitation on the battery voltage. 
     FIG. 2 is a circuit diagram of a charging unit of a solar battery powered electronic wristwatch, wherein the charging current and the charging voltage are both properly limited. Reference 1 represents solar batteries, reference 2 represents a current limiting resistor, reference 3 represents a reverse current preventing diode, reference 4 represents the silver oxide batteries set forth above, and 5 represents a timekeeping circuit. In front of the current limiting resistor 2 and the reverse current preventing diode 3 a voltage switching circuit 6 is provided in parallel with the solar batteries 1. The circuit further comprises a voltage detecting transistor Tr 1 , switching characteristic improving transistors Tr 2  and Tr 3 , a constant voltage element Z such as a Zener diode available for shifting the operating point of the transistor Tr 1  upward and improving a temperature characteristic of the voltage switching circuit 6, a variable resistor R available for adjusting a bias voltage for the transistor Tr 1 , and temperature compensating diodes D 1  -D 3  available for accommodating temperature-depending variations in the permissible range of the battery voltage vs. charging current characteristic. 
     A voltage (Vo) vs. current (Io) characteristic of the charging unit of FIG. 2 is illustrated in FIG. 3. The silver oxide battery will be charged in proportion to the intensity of incident light onto the solar batteries; the curve (a) showing a process of charging under 15,000 lux, the curve (b) under 30,000 lux and the curve (c) under 50,000 lux. However, it is understood that the charging process is restricted as shown by the curve (d) upon arrival of the full charged condition. The curve (d) is determined mainly by the current limiting resistor 2, the reverse current preventing diode 3, and the voltage switching circuit 6. The leading voltage Vo 1  on the curve (d) is equal to a voltage turning on the voltage switching circuit 6 and the transistors Tr 1 , Tr 2  and Tr 3  minus the forward voltage of the reverse current preventing diode 3 and the incline of the curve (d) is determined by the resistance value of the current limiting resistor 2. FIG. 3 is plotted with 12 serially connected solar batteries each having a working area of 40 mm 2 . If a pair of the silver oxide batteries having substantially the same capacity is serially connected, the battery voltage vs. charging current characteristic assumes the same battery voltage as indicated within the parentheses of FIG. 1. Therefore, the voltage-current characteristic of FIG. 3 is settled within the slant line marked range shown by FIG. 1 to prevent swelling and short-circuiting of the batteries. 
     FIG. 4 illustrates a charging characteristic of the silver oxide battery at room temperature with the charging current of 3 mA indicating that charging can be accomplished with the charging voltage of 1.65 V. In the case where the two silver oxide batteries are serially connected, the charging current will flow sufficiently with 1.65×2=3.30 (V) as is clear from FIG. 3 so that the utilization of the circuit of FIG. 2 will not disturb the charging process. If there is any difference in the material which constitutes the silver oxide battery or in the characteristics of the serially connected plurality of silver oxide batteries, the battery voltage charging current characteristic will vary. In this instance, the variable resistor R is adjusted to change the on voltage of the voltage switching circuit 6. Alternatively, the resistance value of the current limiting resistor 2 may be varied. 
     FIGS. 5 and 6 show other embodiments wherein the voltage switching circuit 6 consists of a Zener diode ZD in case of FIG. 5 and consists of a series circuit of two GaP light emitting diodes D&#39;0 and a Si diode D. The former utilizes the avalanche effect of the Zener diode ZD while the latter utilizes the forward voltage addition effects of the diodes. Although the Zener diodes manifest variations in the characteristics and particularly poor trailing properties, they can be overcome by combinations with Si diodes or light emitting diodes of GaP, GaAlAs, GaAs, etc. 
     Meanwhile, since the voltage switching circuit 6 is connected in front of the reverse current preventing diode 3, current never flows from the silver oxide batteries through the voltage switching circuit. In other words, the voltage switching circuit 6 operates only when incident light is applied to the solar batteries 1. 
     In case where the two silver oxide batteries are serially connected as stated above, the charging voltage is nearly 3.30 V for example. Assume now that current consumption by the voltage switching circuit 6 mainly the variable resistor R and the diodes D 1  -D 3  is 10 μA. When the solar batteries 1 are exposed to incident light for one (1) hour per day, the current consumption amounts to 10 μA hour/day. However, considering a case where the voltage switching circuit 6 is connected directly to the silver oxide batteries 4, the current consumption will be calculated as 24 hours×10 μA=240 μA hour/day. This shortens the operating life of the batteries more particularly for use in extremely small current operating appliances such as electronic wristwatches. In the event the voltage switching circuit 6 is connected directly to the silver oxide batteries 4 in FIGS. 5 and 6, current flows at all times due to the leading edge and trailing edge characteristics. 
     Contrarily, according to the above given examples, the circuit operates only when the solar batteries enjoy incident light, and thus enables a 1/24 reduction of the current consumption. 
     FIG. 7 shows a modification in the voltage switching circuit 6 which is connected in parallel with a series circuit of the solar batteries 1 and the current limiting resistor 2. When using a Schottkey barrier diode as the reverse-current preventing diode 3, the forward voltage may be reduced in comparison with the conventional PN junction diodes. In other words, the utilization of the Schottkey barrier diode permits the charging process to complete with a relatively small photovoltage of the solar battery 1. The voltage switching circuit defined by the dotted line is implemented with semiconductor integrated circuit technology together with the reverse-current preventing diode 3. 
     The voltage switching circuit 6 includes a voltage detecting transistor Tr 1  &#39;, modified Darlington connected transistors Tr 2  &#39; and Tr 3  &#39; available for improving characteristics and Darlington connected switching transistors Tr 4  &#39; and Tr 5  &#39; and bias resistor R 1  and R 2  provided for the transistor Tr 1  &#39;, which is biased adjustably with the resistor R 2 . A transistor Tr 6  &#39; connected between the resistors R 1  and R 2  operates as a diode to improve the temperature characteristic of the voltage switching circuit in union with a base-to-emitter diode characteristic of the transistor Tr 1  &#39;. 
     The relationship between the voltage Vo and the current Io of the charging assembly of FIG. 2 is plotted in FIG. 8. The charging process is advanced as shown by the curve (a) under 9.6×10 4  lux, the curve (b) under 6.4×10 4  lux and the curve (c) under 3.2×10 4  lux. However, it should be understood that the charging process is restricted under the fully-charged condition as shown by the curve (d). The curve (d)is determined by respective characteristics of the reverse-current diode 3 and the voltage switching circuit 6. In other words, the leading voltage Vo 1  on the curve (d) is equal to the switching voltage of the voltage switching circuit 6 minus the forward voltage of the reverse-current preventing diode 3. The characteristics shown in FIG. 8 are obtained with eight (8) serially connected solar batteries each having working surface area of 12 mm 2 . 
     In case where the voltage-current characteristic (switching characteristic) of the switching circuit 6 is dull, or in case where an amount of incident light is varied within a wide range as compared with the voltage-current characteristics of the switching circuit, all the charging current Io will not be able to bias in other switching circuit 6 with the same voltage. This results in that the voltage Vo-current Io characteristics will vary for each of amounts of incident light as shown by the dotted lines (a)&#39;, (b)&#39; in FIG. 8. When this occurs, the permissible range of the battery voltage-charging current is no longer expected at high intensity for example more than 9.6×10 4  lux. 
     The switching circuit 6 of FIG. 7 is adapted to exhibit a sharp switching characteristic. When the charging voltage Vo is increased, the voltage detecting transistor Tr 1  &#39; and the characteristic improving transistors Tr 2  &#39; and Tr 3  &#39; are turned on. Within the last stage switching transistors Tr 4  &#39; and Tr 5  &#39;, the Darlington connection increases the current gain and decreases the on resistor of the transistor Tr 5  &#39;. This makes the switching characteristic sharp together with operation of the characteristic improving transistors Tr 2  &#39; and Tr 3  &#39;. Therefore, with such an arrangement, the characteristic is not varied upon variations in the amount of incident light so that the charging process is developed in accordance with the solid line characteristic without departing from the permissible range of the battery voltage-charging current shown in FIG. 1. 
     While only certain embodiments of the present invention have been described, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit and scope of the invention as claimed.
A charger, which is useful to power a variety of compact type electronic appliances such as an electronic wristwatch, includes one or more solar batteries as a primary battery, and a sealed type silver oxide battery as a secondary battery. Both the amount of charging current amplitude and of charging voltage are limited to protect the sealed silver oxide battery from its own expanse phenomenon.