Abstract:
A device for a battery charger, includes connection elements  14, 16  connected to the output lines of the charger, connection elements  16, 24  for connection to the terminals of a battery to be charged. The device includes elements  80  for detecting a voltage over the output lines of the charger, elements  80  for detecting a positive voltage over the terminals of a connected battery, switch elements  60  for connecting at least one of the output lines to the connection elements to the terminals of the battery, and a switch activating element  70  arranged to the voltage detection elements and designed and arranged such that it is capable of activating the switch when a voltage is detected over the output lines of the charger and a positive voltage is detected over the terminals of the connected battery, thereby connecting the charger to the battery.

Description:
TECHNICAL AREA  
       [0001]     The present invention relates to a device for a battery charger intended for preventing the occurrence of sparks when connected to a battery, and preventing the battery from being drained during interruptions in the power supply. The invention also relates a device which is capable of preventing short-circuiting and that can handle switching of the polarity during connection.  
       BACKGROUND OF THE INVENTION  
       [0002]     Battery chargers are used in a variety of applications for charging wet or dry batteries. When charging wet batteries there is a risk for explosions when connecting the battery charger to the battery since hydrogen gas generated by the electrolyte can explode if ignited by sparks from the connection. This may of course be solved by having the battery charger switched-off during connection, but a user may forget this, and for some applications there is a desire to have an automatic start of the charging process when the battery is connected.  
         [0003]     The patent document U.S. Pat. No. 5,045,769 describes a battery charging system where sparking is prevented in that there is a time delay of applying charging current to the battery to allow the battery terminals to be secured. The voltage over the battery terminals is measured and if there is stated that there is a voltage present a delay circuit is activated, which after a certain time, e.g. 2 seconds, closes a switch connecting the charger to the battery. However, should the power supply fail for some reason, there is a risk that the battery is drained because the energy for controlling the spark protection circuit, and also other functions of the charger according to the document is taken from the battery.  
       SHORT DESCRIPTION OF THE INVENTION  
       [0004]     The aim of the invention is to provide a device for a battery charger that prevents the occurrence of sparks when a battery is connected, even if it is done against regulations stating that the battery should be connected before connecting the charger to the power supply, and at the same time prevents the battery from being drained if the power supply is disconnected or is out. In addition the present invention provides additional features due to its design.  
         [0005]     The aim is solved according to the characterising part of claim  1 . Further advantageous developments of the present invention are put forward by the dependent claims.  
         [0006]     According to one aspect of the invention it is characterised by a device for a battery charger, comprising connection means connected to the output lines of the charger, connection means for connection to the terminals of a battery to be charged, comprising means for detecting a voltage over the output lines of the charger, means for detecting a positive voltage over the terminals of a connected battery, switching means for connecting at least one of the output lines to the connection means to the terminals of the battery, and a switch activating means arranged to said voltage detection means and designed and arranged such that it is capable of activating the switch when a voltage is detected over the output lines of the charger and a positive voltage is detected over the terminals of the connected battery, thereby connecting the charger to the battery.  
         [0007]     According to another aspect of the present invention it is characterised in that said switch activating means comprises a relay and an AND-circuit, which AND-circuit forms a logical condition for activating said relay. Further it comprises a driver for said relay, which driver obtains its driving power from a secondary circuit of the charger.  
         [0008]     According to yet an aspect of the invention it is characterised in that it further comprises a support circuit capable of providing auxiliary energy should the battery voltage decrease below a certain threshold value and preferably also that it comprises a support circuit capable of generating and stabilising said auxiliary energy.  
         [0009]     The advantages of the present invention are several. Because of the fact that the charger is connected to the battery only if it detects both a voltage from the power supply and a positive voltage from the battery, the charger can never be switched on, ie have a voltage over the connections to the battery, when a battery is connected, even if the charger is already connected to the power supply when the battery is connected. This safely prevents any sparks from occurring when the connections are attached to the battery thereby preventing any sparks during the connection to the battery. Thereby, there is no risk for explosions. Further, since the condition states that the power supply also should be connected, there is no risk of draining the battery if the power supply is cut off.  
         [0010]     The design of the device according to the present invention provides additional features and advantages. It prevents short-circuiting of the connections to the battery since the condition that there should be a positive voltage from the battery is not fulfilled, because the terminal voltage becomes close to zero, thereby breaking the connection of the switch. This design also enables protection against switching of terminals since the condition that there should be a positive voltage from the battery is not fulfilled, thereby preventing the switch from closing.  
         [0011]     If the battery voltage should decrease to such an extent that the voltage of the spool of the relay is in danger of becoming lower than the holding voltage, the support circuit increases the voltage to the spool. With this design the driving capability of the relay spool is maintained theoretically down to 0 V of the battery.  
         [0012]     In all an effective and yet cost-effective device is obtained in comparison with known devices of this type as regards functions and number of components.  
         [0013]     These and other advantages of, and aspects of, the present invention will become apparent from the following detailed description and from the accompanying drawings. 
     
    
     SHORT DESCRIPTION OF THE DRAWINGS  
       [0014]     In the following description of an embodiment of the invention, reference will be made to the accompanying drawings of which:  
         [0015]      FIG. 1  shows schematically the design of a battery charger according to the invention, and  
         [0016]      FIG. 2  shows the circuitry for a spark protection with additional functions according to the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]     An example of a battery charger comprising the present invention is shown in  FIG. 1 . It comprises a primary switched battery charger  10  of fly-back type, which is known as such for a man skilled in the art and will not be explained in detail.  
         [0018]     It is arranged with power supply connection means  12  for power supply and is provided with two output lines  14 ,  16 . One of the lines  14  is connected to a rectifier  18 , which in turn is connected to a spark protection circuit  20  via a line  22 . The spark protection circuit will be described in more detail below. The spark protection circuit is arranged with an output line  24  for connection to one of the terminals of a battery  26 . The other output line  16  is arranged for connection to the other of the terminals of the battery.  
         [0019]     The output line  14  from the battery charger is arranged with a branch line  28  and separately connected to the spark protection circuit  20 . A voltage line  29  is connected between the spark protection unit  20  and the voltage feed of control and regulation circuits  34 . An output line  36  from the voltage feed circuit is connected to a control circuit  38 . Two input lines  40  and  42  to the control circuit  38  are respectively connected to the battery connection lines  16  and  24  for voltage feedback. An input line  43  for current feedback is arranged between the battery connection line  16  and the control circuit.  
         [0020]     The line  17  for the internal supply voltage is connected to the circuit for maintaining voltage is branched and connected to a circuit for maintaining voltage  50 , which is a voltage comparator for supervising the internal supply voltage on line  17 . Preferably, the internal voltage is not allowed to drop below 7V. This circuit is in turn arranged with a signal line  52  connected to a feedback circuit  54 . The control circuit  38  is also connected to the feedback circuit  54  via a line  56 . The feedback circuit is in turn connected to the battery charger  10  via a line  58 . By signal line  52  to the feedback circuit  54  the main fly-back switch power supply will be pushed to operate long enough to maintain the internal voltage level. This function is necessary to maintain current feed to relay coil, LED&#39;s and control circuits during pulse maintenance charging of the battery.  
         [0021]      FIG. 2  shows the spark protection circuit  20  in more detail. It can be divided into the following parts. Part  60  comprises a switch  62  driven by a relay  64  connected in parallel with a switch diode  66 . The relay is of normally-open contact frequently used in automotive applications. The switch diode  66  is arranged in order to take care of the inductive current during coil switch off and to protect a relay driver circuit  70 .  
         [0022]     The output line  24  is arranged with a secondary fuse  68  for emergency protection of the charger in case of for example fire, melting of the insulation of the transformer and such. The driver  70  for the relay, comprising a switching PNP transistor  72  and two base resistors  74 ,  76  in series, is arranged to the relay, and forms a relay coil driver. The driver obtains its driving power from the secondary circuit of the charger. When the lower end of resistor  76  is driven to a low potential the transistor source current is fed into the coil and the relay will activate.  
         [0023]     An AND-circuit  80  with two analogue input is also arranged to the relay for forming the logical condition for the relay to be activated as will be described below, comprising two resistors  82 ,  84  connected in series between the positive and negative input lines to the battery, where one resistor is connected in parallel with a capacitor  86 , one transistor  88  connected with its base to a line  90 , which in turn is connected between the resistors. The emitter of the transistor  88  is connected to the negative battery line  16 . The collector of the transistor  88  is connected to the emitter of a second transistor  92 , where the collector of the transistor  92  is connected to the series of resistors of the driver  70 . The base of the transistor  92  is connected over a resistor  93  to the line  17  via a line  98 . A further resistor  97  is connected between the base of transistor  92  and the line  16 . The line  98  is in turn connected between line  28  and line  16  over a capacitor  94  and a diode  95 , where the connection of line  17  and the line from the base of transistor  92  is arranged between the capacitor  94  and the diode  95 . Further, a diode  96  is connected to line  98  between the capacitor  94  and the diode  95  and connected to the line  22 .  
         [0024]     The resistors  82  and  84  sense the battery voltage level. The capacitor  86  is added to filter noise. If the voltage level of the connection point exceeds the base-emitter voltage of the transistor  88 , the logical condition for a connected battery is fulfilled. The switch diodes  95  and  96 , together with the capacitor  94 , work like a simple mains power supply detector. When the main flyback switching power starts to operate as a result of the mains being connected to the wall, the voltage will rise over the capacitor  94 . If the mains supply is disconnected this voltage will drop rapidly, independent of the battery voltage and will force to deactivate the relay coil. If the voltage level on the base line of transistor  92  exceeds the base-emitter voltage and the collector-emitter voltage of transistor  88 , the transistor  88  starts to drain current from the resistor  74  in the relay driver circuit  70 . The voltage from the mains detector also operates as an internal supply voltage on line  17 .  
         [0025]     A support circuit  100  is arranged for connection/de-connection of auxiliary energy comprising a transistor  102  connected with its emitter to the emitter of the driver transistor  72 . The voltage line  29  is connected to the connection between the transistors  72  and  102 . The base of the transistor is connected to a resistor  104 . The transistor  102  and resistor  104  form a serial voltage regulator and will start to feed current to the relay driver if the emitter voltage level on the transistor  102  drops below the reference voltage level over a zener diode  122  connected between the line  16  and the resistor  104 . This function will guarantee that the relay coil always has enough power to operate properly. The output from the serial regulator is connected to line  29  and is used to feed the control and regulation power supply.  
         [0026]     A support circuit  110  for generating and stabilising the auxiliary energy is connected to the support circuit for connection/de-connection comprising a resistor  112  connected in series with the resistor of the support circuit  100 , a switch diode  1   14  and a pulse capacitor  116 , and then to the charger. The positive line  108  coming from the charger is connected to the switch  62 , and is in the support circuit connected with a switch diode  118  between its switch diode  114  and pulse capacitor  116 , a pulse capacitor  120  between the diode  114  and the resistor  112 , and the zener diode  122 . A branch  124  is connected between the connection of the capacitor  120  and the collector of the transistor  102 . The pulse capacitors  116  and  120  together with the switch diodes  118  and  114  form a charge pump circuit that will create a voltage of 15V to 90V depending on the current source from the circuit and the main fly-back switch power working level. The selection of the capacitor value is carefully done in a way to introduce enough impedance into the circuit to not overload the support switch  100  but still deliver enough power at any time. A reference voltage is created with the resistor  112  and the zener diode  122 . For a nominal 12V relay this reference voltage is chosen to 10V.  
         [0027]     The function of the device according to the invention is as follows. The spark protection functions to prevent the occurrence of sparks by the AND-circuit. If there is power from the power supply and from the battery, in that the device detects a voltage from the power supply and a positive voltage over a certain threshold value from the battery then the driver activates the relay and the switch is closed, connecting the positive line with the output line  24 , thereby connecting the charger to the battery. The threshold value of the voltage from the battery can be chosen by choosing different values of the components in the AND-circuit, for example a minimum voltage of 1,5 V. The value of course depends on the nominal value of the battery and the application. By changing the ratio between resistor  82  and  84  the minimum threshold voltage level can be altered.  
         [0028]     If the power supply should be broken, the condition that there is a voltage from the power supply is not present for the AND-circuit, with the result that the relay breaks the connection of the switch. Thereby the battery is prevented from being drained by the charger.  
         [0029]     If the battery voltage should decrease to such an extent that the voltage of the spool of the relay is in danger of becoming lower than the holding voltage, the auxiliary energy is switched in by the transistor  102  into the support circuit  100 . The minimum voltage level for the relay coil is set by the zener diode  122 . With this design the driving capability of the relay spool is maintained theoretically down to 0 V of the battery.  
         [0030]     The spark protection is obtained in that the relay closes only if both the battery and the power supply are connected to the charger. This design also enables protection against switching of terminals since the condition that there should be a positive voltage from the battery is not fulfilled, thereby preventing the switch from closing. As an extra safety precaution the positive line is provided with a fuse. This does not activate due to switching of terminals, since the device so quickly detects this and breaks the switch, but is merely an emergency protection should the charger malfunction due to fire, damaged insulation of the transformer and such.  
         [0031]     The design also prevents short-circuiting of the connections to the battery since the condition that there should be a positive voltage from the battery is not fulfilled, because the terminal voltage becomes close to zero, thereby breaking the connection of the switch.  
         [0032]     It is to be understood that the embodiment described above and shown in the drawing is to be regarded as a non-limiting example of the invention and that it could be modified in several ways within the scope of the appended claims.  
         [0033]     As an example, many of the functions described above may be obtained and carried out by suitable software comprised in a micro-chip or the like data carrier.