Battery circuit in an emergency light

0001The present invention relates to an emergency lighting device according to the preamble of claim 1, which is provided for operating a light source, in particular an LED.

0002Emergency lighting devices are essential to implement lighting systems in larger buildings or complexes, which provide sufficient lighting even in the event of a failure of the general power supply. Only if certain relevant premises or areas are still illuminated in the event of a power failure or other emergency, can evacuation or relief measures be carried out safely. Accordingly, such emergency lighting devices are used in particular for illuminating escape routes and the like.

0003Accordingly, emergency lighting devices of the generic type have an energy storage unit, in particular a battery or an accumulator, as the central element, which is charged by the general mains supply voltage during normal or charging operation. For this purpose, a charging circuit is provided which is connected on the input side to the mains supply voltage and, during the charging operation, continuously supplies the energy storage unit with energy, which stores it. Only in the event of an emergency occurring, which is usually automatically recognized by such devices by monitoring the mains supply voltage, is it switched to an emergency lighting mode in which the light source is activated and operated, for which purpose - if necessary - the energy provided by the energy storage unit is being used. Since the storage capacity of the energy storage unit is of course limited, light sources are preferably used which consume relatively little energy. Accordingly, such emergency lighting devices are preferably equipped with gas discharge lamps, in particular fluorescent tubes. However, light sources in the form of light-emitting semiconductors, in particular LEDs, are also being used increasingly, since these light sources also have a high degree of efficiency and can accordingly be used in an energy-saving manner.

0004The monitoring of the mains supply voltage, which is carried out in order to be able to initiate emergency lighting operation in good time, is usually carried out by a control unit which evaluates the signals supplied to it and uses these signals to assess the state of the mains supply voltage. In known circuits, the mains supply voltage itself is measured in an obvious manner for this purpose. This then results in the problem, however, that the sensor system for determining the state of the mains supply voltage is at mains potential and must be galvanically isolated from the other circuit areas of the emergency lighting device, which are at the voltage level of the energy storage unit or LED. The electrical isolation required for this is not only expensive, but also takes up a relatively large amount of space in the emergency lighting device.

0005The present invention is based on the object of specifying an improved concept for an emergency lighting device in which the disadvantages described above are avoided.

0006This task is solved by the characteristics of the independent claims. Advantageous developments of the invention are the subject of the dependent claims.

0007In contrast to known solutions, the mains supply voltage can be monitored indirectly in order to possibly initiate emergency lighting operation. For this purpose, only measured values ​​on the potential of the energy storage unit or LED are recorded and, on the basis of these measured values, conclusions are drawn on the state of the mains supply voltage with the aid of further information.

0008According to a first aspect of the invention, an emergency lighting device for operating a light source, in particular an LED, has:<ul id="ul0001" list-style="bullet" compact="compact"><li>a battery,</li><li>a charging circuit to be supplied with a mains supply voltage for charging the battery, and</li><li>one during battery emergency operation powered driver circuit for operating the light source. A switch is connected in series with the battery, via which the battery can be selectively disconnected.</li></ul>

0009The switch in series with the battery can be a transistor that can be operated as a linear regulator by the control unit.

0010If a deep discharge of the battery is detected, the switch can be driven in series with the battery in a pulsed mode.

0011A discrete battery voltage detection circuit can be provided to detect the deep discharge of the battery.

0012A measuring resistor can be connected in series with the switch which is connected in series with the battery (4).

0013A further aspect of the invention relates to an emergency lighting device for operating a light source, in particular an LED, the emergency lighting device (1) having:<ul id="ul0002" list-style="bullet" compact="compact"><li>a battery,</li><li>a charging circuit to be supplied with a mains supply voltage for charging the battery during a charging operation, and</li><li>a driver circuit supplied by the energy storage unit (4) during an emergency lighting operation for operating the light source.</li></ul>

0014A measuring resistor is connected in series with the battery, by means of which both the battery charging current and the battery discharge current are recorded.

0015An offset can be applied to the battery current signal, so that both the battery charging current and the battery discharge current can be evaluated as signals of the same polarity but different amplitude.

0016An operational amplifier can be used to offset the battery current.

0017The charging circuit preferably has a controllable switch and a transformer, it being possible for the charging circuit to be formed in particular by a so-called flyback converter. The controllable switch is controlled by the control unit of the emergency lighting device, wherein the control can take place here in particular via an optocoupler.

0018The state of the mains supply voltage is now determined, in particular, taking into account the duty cycle selected by the control unit for controlling the switch of the flyback converter or the charging circuit. Furthermore, the secondary voltage of the flyback converter or, in the case of other switched-mode power supply topologies, the charging power for the energy storage unit is taken into account. This charging power can be determined in a simple manner by the control unit during charging operation, that is to say when the light source is switched off, since the values ​​to be measured for this purpose, namely voltage and current of the energy storage circuit, relate to the same basic potential on which the other components of the emergency lighting device are based lie. The same applies to the secondary voltage to be measured if the flyback converter is used. In both cases, the galvanic separation between the measuring devices and the control unit required in the prior art can therefore be omitted.

0019Another advantageous development of the present invention relates to the measures for operating the light source, in particular the LED, during emergency lighting operation. For this purpose, the driver circuit is preferably designed as a switching regulator and accordingly has a further controllable switch, which in turn is controlled by the control unit. The control of the switch is such that the light source can be operated with a constant power or constant current regardless of the state of charge of the energy storage unit. This measure is of particular importance since the power of the energy storage unit naturally drops over time, which should not, however, affect the light intensity of the emergency lighting device.

0020In the case of using an LED as a light source, it would in turn be appropriate to detect the diode current itself, which determines the light output, in order to enable the desired power regulation. According to a particularly advantageous development of the present invention, it is provided that the measurement of the diode current is omitted and instead the current or the power of the light source is measured or estimated indirectly using other parameters. In particular, preferably only the voltage applied to the light source is determined and then the diode current is derived from further information, the power loss of the driver circuit in particular being taken into account. Using previously determined comparison tables stored in the control unit, the power of the light source can then be determined without a direct measurement of the diode current, so that an almost constant light output can be set by the control unit. The light output is regulated here in particular by appropriate clocking of the controllable switch of the driver circuit, since in this way the output with which the light source is operated can be set in a very simple and elegant manner.

0021This special measure for operating the light source at an almost constant power, whereby a direct measurement of the current is dispensed with, can also be used independently of the inventive idea of ​​indirect monitoring of the mains supply described at the beginning. Accordingly, this idea is also the subject of a further independent claim.

0022The invention will be explained in more detail below with reference to the accompanying drawing. Show it:<dl id="dl0001"><dt>Fig. 1</dt><dd>schematically the circuit diagram of a first embodiment of an emergency lighting device according to the invention;</dd><dt>Fig. 2</dt><dd>a second embodiment of an emergency lighting device;</dd><dt>Fig. 3</dt><dd>a graphic for determining the mains supply voltage on the basis of operating parameters measured on the output side of the charging circuit,</dd><dt>Fig. 4</dt><dd>another graphic for determining the secondary power of the charging circuit, which is taken into account for the indirect determination of the light source power, and</dd><dt>Fig. 5</dt><dd>an illustration of an inventive battery circuit.</dd></dl>

0023This in <figref idref="f0001">Fig. 1</figref> The emergency light device according to the invention, shown in a simplified manner and generally provided with the reference number 1, is provided in the exemplary embodiment shown for operating an LED as an emergency light source. The emergency lighting device 1 is connected on the input side to a power supply network, which has a power supply voltage U_in provides and has as essential components a control unit 2, a charging circuit 3, an energy storage unit 4 in the form of a battery or an accumulator and a driver circuit 5.

0024In the first exemplary embodiment shown, the charging circuit 3 is formed by a so-called flyback converter, which on the one hand has a transformer T with a primary winding n1 and a secondary winding n2 and on the other hand a controllable switch Si. In a known manner, a corresponding alternating opening and closing of the switch S1, the from the mains supply voltage U_in The energy made available is transferred to the secondary side of the flyback converter 3 and used to charge the energy storage unit 4. The energy transfer takes place in the open state of the switch S1, with a diode D on the output side of the flyback converter 3₁ is provided. Such flyback circuits are widely used in such emergency lighting devices due to their simple structure and reliable function.

0025The controllable switch S1 is activated by the control unit 2 of the emergency lighting device, the activation being carried out, in particular, in an electrically isolated manner via an optocoupler 6. The control unit 2 controls the switch S1 alternately, the so-called duty cycle D1 for the switching operation of the switch S1 being calculated as follows:<maths id="math0001"><math display="block"><mi>D</mi><mo>⁢</mo><mn>1</mn><mo>=</mo><msub><mi>t</mi><mrow><mi mathvariant="italic">on</mi><mo>⁢</mo><mn>1</mn></mrow></msub><mo>/</mo><mfenced><mi>T</mi><mo>-</mo><msub><mi>t</mi><mrow><mi mathvariant="italic">on</mi><mo>⁢</mo><mn>1</mn></mrow></msub></mfenced></math><img file="EP2242163A2_D0001.tif" /></maths>t_on1 corresponds to the switch-on time, while T denotes the total duration of a complete switching cycle for switch S1.

0026It is also conceivable that the control of S1 is a 'cantilever' and that the natural frequency of the 'cantilever' is influenced by the optocoupler 6 via the duty cycle.

0027During a charging operation of the emergency lighting device 1, usually only the charging circuit 3 is active in order to charge the battery 4 permanently. Only in the event that there is an emergency which is caused in particular by deviations in the mains supply voltage U_in is characterized by predetermined target values, an emergency lighting operation is initiated in which the driver circuit 5 is used to control the LED. For this purpose, the driver circuit 5 designed as a switching regulator has a further controllable switch S2, an inductance L and a diode D.₂ on. By alternately actuating the switch S2 by the control unit 2, a current is made available to the LED, via which the LED is operated. The duty cycle with which the switch S2 is controlled by the control unit 2 can be varied in order to set the level of the current supplied to the LED and thus the power at which the LED is operated. In this way it can be ensured in a very elegant way that the LED is operated with constant brightness despite everything, even when the battery power fluctuates.

0028A first essential function of the emergency lighting device 1 thus consists in assessing the mains supply voltage U_in to recognize whether there is an emergency, in order to initiate emergency lighting operation if necessary. Up to now it was known to directly determine the value of the input voltage U_in to determine for the charging circuit 3, which, however, is associated with disadvantages for the reasons mentioned above.

0029It is preferred to measure the supply voltage U directly_in waived. Instead, it is planned to determine these indirectly. In particular, it is provided that only the sizes of operating parameters of the emergency lighting device 1 are measured on the secondary side of the charging circuit 3.

0030In the first embodiment according to <figref idref="f0001">Fig. 1</figref> the voltage U present on the secondary side of the flyback converter or flyback converter 3_flb2 measured after the diode D1, for which in particular no electrical isolation is required, since this variable is at the same reference potential as the control unit 2, which evaluates the measured value.

0031Alternatively, the voltage of the secondary winding of the flyback converter can also be measured.

0032Now is the level of this secondary voltage U_flb2 known, it can be based on the level of the input voltage U_in be inferred. When switch S1 of flyback converter 3 is switched on, there is a connection between input voltage U_in and secondary voltage U_flb2, which is dependent in particular on the winding ratio between the two windings n1 and n2 of the transformer T and on the duty cycle of the switch D1. This relationship between the easy-to-measure secondary voltage U_flb2 and the input voltage U to be monitored_in is now stored in the form of a table of values ​​in the control unit 2, so that after measuring the secondary voltage U_flb2 in a simple manner the level of the input voltage U_in can determine without having to measure them directly. Now the control unit 2 determines that the input voltage U_in is outside certain target value ranges, this indicates an emergency state, which in turn will cause the control unit 2 to initiate an emergency operation.

0033The solution described thus enables very simple but effective monitoring of the status of the general power supply. There is also a particular advantage of the in<figref idref="f0001">Fig. 1</figref> illustrated embodiment in that the level of the input voltage U_in regardless of whether the emergency light LED is switched on or not. The diode D₁ causes a separation between the secondary voltage U by its blocking effect_fl2 and battery voltage U_Bat, so that the activity of the driver circuit 3 is not based on the above-described process of determining the input voltage U_in affects.

0034A second, somewhat more general embodiment of an emergency lighting device according to the invention is shown in <figref idref="f0001">Fig. 2</figref> shown. The structure of this corresponds essentially to that in<figref idref="f0001">Fig. 1</figref> shown emergency lighting device 1, but now the charging circuit 3 is not formed by a flyback converter but generally by a circuit arrangement which has a potential separation and a switch S1 controlled by the control unit 2.

0035In this more general embodiment, there is not necessarily a known relationship between the input voltage U_in and the voltage on the output side of the charging circuit 3. Nevertheless, the level of the input voltage U_in can be determined indirectly, but now other farm sizes are measured.

0036On the one hand, this is the battery voltage U_asked as well as the battery current I_asked. Both variables can in turn be determined relatively easily, that is to say without galvanic isolation, since they, like the control unit 2, which evaluates these measured variables, are at the same reference potential.

0037On the basis of these two measured variables U_asked and I_asked and the known duty cycle D1 for the switch S1 of the charging circuit 3 can then the input voltage U_in can be determined, since this is related to the three known quantities. This illustrates the graphic of<figref idref="f0001">Fig. 3</figref>which different characteristics of the secondary power P_flb2 shows the charging circuit 3 as a function of the duty cycle D1 for the switch S1. These characteristic curves are determined, for example, during the manufacture of the emergency lighting device and are in turn stored in the form of a table in the control unit 2. It can be seen that these characteristics in particular also depend on the input voltage U_in are dependent. So now the duty cycle D1 and the secondary power P_flb2 the charging circuit 3 known, as in the example of <figref idref="f0001">Fig. 1</figref> to the level of the input voltage U_in be closed back.

0038In the example shown by <figref idref="f0001">Fig. 3</figref> is a lso determined, with which characteristic, the known combination of duty cycle D1 and secondary power P_flb2 the charging circuit 3 which, when the driver circuit 5 is deactivated, the product of the battery voltage U_asked and battery power I_asked corresponds, agrees. In the measured values ​​shown, for example, this value lies on the characteristic curve for an input voltage U_in of 220 volts, which corresponds to a proper condition of the general mains supply. However, the determined value would lie on a characteristic curve, for example an input voltage U_in corresponds to 140 volts or 280 volts, this would in turn be interpreted by the control unit 2 in such a way that there is an error in the mains supply and accordingly initiate an emergency.

0039In both of the exemplary embodiments shown, it can therefore be carried out reliably and without the need for a direct measurement of the input voltage U_in determine whether the mains supply is OK or not. A limitation in the example of<figref idref="f0001">Fig. 2</figref> consists, however, in that the described determination of the input voltage U_in is only possible when the driver circuit 5 is switched off. In the example of<figref idref="f0001">Fig. 1</figref> however, as already mentioned, this restriction does not exist. Basically, however, the control unit 2 will activate the driver circuit 5 and accordingly switch the LED on if a fault is detected.

0040After the driver circuit 5 has been activated, the switch S2 can then be controlled at high frequency in the manner described above in order to operate the LED with a desired power. In this case, to ensure that the power of the LED is constant, it would be necessary to use both the LED voltage U_led as well as the LED current I_led to know to enable regulation. According to a particularly advantageous development, however, only the LED voltage U_led measured and the current I_led or the resulting power P_led indirectly determined by the control unit 2, which will be explained in more detail below.

0041To the diode current I_led To be able to determine indirectly, at least the quantities “battery voltage” U_asked "Battery power" I_asked and - case of the example of <figref idref="f0001">Fig. 1</figref> - secondary voltage U_flb2 as well as the LED voltage U_led measured. From this, the further information that is used to regulate the LED current I_led are required, are calculated, for which, however, further information is required, which is not recorded by actual value measurements, but rather is stored in the control unit 2 as value tables.

0042The first table of values ​​is the information already mentioned above with regard to the relationship between duty cycle D1, secondary voltage U_flb2 and input voltage U_in in the example of <figref idref="f0001">Fig. 1</figref> or between duty cycle D1, battery voltage U_asked, Battery current I_asked and input voltage U_in in the more general example of <figref idref="f0001">Fig. 2</figref>. Furthermore, in order to determine the diode current, the power loss P_lost the driver circuit 5, which is known from the difference between the measured LED voltage U_led and the battery voltage U also measured_asked is dependent, so that: <maths id="math0002"><math display="block"><msub><mi>P</mi><mi mathvariant="italic">lost</mi></msub><mo>=</mo><mi>f</mi><mo>⁢</mo><mfenced><msub><mi mathvariant="italic">U</mi><mi mathvariant="italic">led</mi></msub><mo mathvariant="italic">-</mo><msub><mi mathvariant="italic">U</mi><mi mathvariant="italic">asked</mi></msub></mfenced></math><img file="EP2242163A2_D0002.tif" /></maths>

0043Finally, the third information required is the secondary power P present in the case of an activated driver circuit_flb2 the charging circuit 3, which is a function of the duty cycle D1, the input voltage U_in and the battery voltage U_asked is. The relationship between these quantities is in<figref idref="f0001">Fig. 4</figref> shown, wherein it can be seen that the secondary power P_flb2 the charging circuit 3 primarily from the input voltage U_in and is dependent on the duty cycle D1 for the switch S1, but also because of the battery voltage U._asked can vary.

0044Is now due to the measures described above, the input voltage U_in (possibly in the example of <figref idref="f0001">Fig. 2</figref> were first determined with the driver circuit 5) switched off, then the relationship in <figref idref="f0001">Fig. 4</figref> the secondary power P even when the driver circuit 5 is activated_flb2 the charging circuit 3 can be determined. The measured quantities of battery voltage U are then known_asked, Battery current I_asked, possibly secondary voltage U_flb2 and LED voltage U_led, the duty cycle D1 specified by the control unit for the switch S1, and the further variables input voltage U determined on the basis of the stored value tables_in, Secondary power P_flb2 the charging circuit 3 and power loss P_lost the driver circuit 5.

0045Knowing these variables in turn enables the current I to be determined in accordance with the following relationship_flb2 on the secondary side of the charging circuit 3: <maths id="math0003"><math display="block"><msub><mi>P</mi><mrow><mi mathvariant="italic">fib</mi><mo>⁢</mo><mn>2</mn></mrow></msub><mo>=</mo><msub><mi mathvariant="italic">P</mi><mrow><mi mathvariant="italic">fib</mi><mo>⁢</mo><mn>2</mn></mrow></msub><mo>/</mo><msub><mi mathvariant="italic">U</mi><mi mathvariant="italic">asked</mi></msub></math><img file="EP2242163A2_D0003.tif" /></maths>

0046Finally, the LED current I_led and the LED power P_led can be calculated according to the following equations: <maths id="math0004"><math display="block"><msub><mi>I.</mi><mi mathvariant="italic">led</mi></msub><mo>=</mo><mfenced><msub><mi mathvariant="italic">P</mi><mrow><mi mathvariant="italic">fib</mi><mo>⁢</mo><mn>2</mn></mrow></msub><mo mathvariant="italic">-</mo><msub><mi mathvariant="italic">U</mi><mi mathvariant="italic">asked</mi></msub><mo mathvariant="italic">⋅</mo><msub><mi mathvariant="italic">I.</mi><mi mathvariant="italic">asked</mi></msub><mo mathvariant="italic">-</mo><msub><mi mathvariant="italic">P</mi><mi mathvariant="italic">lost</mi></msub></mfenced><mo>/</mo><msub><mi>U</mi><mi mathvariant="italic">led</mi></msub></math><img file="EP2242163A2_D0004.tif" /></maths><maths id="math0005"><math display="block"><msub><mi>P</mi><mi mathvariant="italic">led</mi></msub><mo>=</mo><msub><mi mathvariant="italic">P</mi><mrow><mi mathvariant="italic">fib</mi><mo>⁢</mo><mn>2</mn></mrow></msub><mo mathvariant="italic">-</mo><msub><mi mathvariant="italic">U</mi><mi mathvariant="italic">asked</mi></msub><mo mathvariant="italic">⋅</mo><msub><mi mathvariant="italic">I.</mi><mi mathvariant="italic">asked</mi></msub><mo mathvariant="italic">-</mo><msub><mi mathvariant="italic">P</mi><mi mathvariant="italic">lost</mi></msub></math><img file="EP2242163A2_D0005.tif" /></maths>

0047Both equations apply in the event that the charging circuit 3 is still active, that is to say at least a certain power supply is still available. This can be the case, for example, if an emergency lighting operation has been initiated otherwise. However, in the event that the power supply fails completely and the emergency lighting operation is only maintained by the battery, the simplified equations apply:<maths id="math0006"><math display="block"><msub><mi>I.</mi><mi mathvariant="italic">led</mi></msub><mo>=</mo><mfenced><msub><mi mathvariant="italic">U</mi><mi mathvariant="italic">asked</mi></msub><mo mathvariant="italic">⋅</mo><msub><mi mathvariant="italic">I.</mi><mi mathvariant="italic">asked</mi></msub><mo mathvariant="italic">-</mo><msub><mi mathvariant="italic">P</mi><mi mathvariant="italic">lost</mi></msub></mfenced><mo>/</mo><msub><mi>U</mi><mi mathvariant="italic">led</mi></msub></math><img file="EP2242163A2_D0006.tif" /></maths><maths id="math0007"><math display="block"><msub><mi>P</mi><mi mathvariant="italic">led</mi></msub><mo>=</mo><msub><mi mathvariant="italic">U</mi><mi mathvariant="italic">asked</mi></msub><mo mathvariant="italic">⋅</mo><msub><mi mathvariant="italic">I.</mi><mi mathvariant="italic">asked</mi></msub><mo mathvariant="italic">-</mo><msub><mi mathvariant="italic">P</mi><mi mathvariant="italic">lost</mi></msub></math><img file="EP2242163A2_D0007.tif" /></maths>

0048Ultimately, the current actual value of the LED current I_led as well as the current power P_led indirectly determined and used for regulation. The corresponding calculations are carried out by the control unit 2.

0049The advantage of these indirect determinations of the actual values ​​required for regulation is that there is no need to measure an additional operating parameter of the emergency lighting device, which in turn leads to a further simplification of the device as a whole. Since this is partly based on the input voltage that has already been determined or measured parameters can be used, this indirect determination of the LED current thus represents a particularly advantageous development of the idea of ​​indirect determination of the input voltage described at the beginning. However, a corresponding indirect current and power determination for the light source could also be used with other devices, in which the first idea according to the invention has not been realized. For example, this procedure is among others useful for emergency lighting devices in which an emergency state can be signaled alternatively or in addition to monitoring the power supply. For example, in the emergency lighting device according to the invention, an emergency signal, for example from a fire alarm or another control device, could also be transmitted via a separate control input in order to initiate emergency lighting operation by means of an external signal.

0050Another development finally relates to the value tables mentioned above and stored in the control unit, which are required for the indirect determination of the input voltage and the diode current. As already mentioned, these value tables can already be saved in the control unit when the emergency lighting device is manufactured. Alternatively or in addition, it would also be conceivable to subsequently write this information or update at a later date. For this purpose, for example, a digital interface provided in the emergency lighting device could be used, which is usually used for error signaling and monitoring. With the help of this interface, new value tables could now be written into the control unit using an extended command set. This subsequent registration of new information is useful, for example, in order to Specify tolerance values ​​for the input voltage or also adapt the information for determining the quantities that are not directly measured to the connected battery. As a result, the behavior of the emergency lighting device can be adapted to new circumstances at any time.

0051Referring to <figref idref="f0002">Figure 5</figref> Details of the connection of the battery 4 will now be explained.

0052As in <figref idref="f0002">Figure 5</figref> can be seen, an element 12 is connected in parallel to the battery 4, which can have the function of a linear regulator or switching regulator and / or a switch. For example, this element 12 can be a transistor. A measuring resistor (shunt) 16 is also connected in parallel with the battery 4, so that the voltage drop across the shunt 16 is representative of the battery current.

0053The measurement signal tapped at the shunt 16 is fed to a current detection unit 13, which is preferably constructed as a discrete circuit and can have a comparator 14. The comparator 14 is only one example of how an offset to the measurement signal can be applied by the shunt 16. The application of the offset serves to be able to evaluate signals with different polarities that represent the battery current in a simplified manner by selecting the offset in such a way that the signal levels are shifted in such a way that both signal polarities now have the same polarity and therefore different amplitudes.

0054Thus, both the battery charging current and the battery discharge current, which are known to have different polarities, can be measured in a relatively simple manner, for example by the control circuit 2. The control circuit 2 is thus preferably supplied with a measurement signal 15 with a uniform polarity.

0055Otherwise, a polarity reversal of the connections of the battery 4 can also be detected. In this case it can be provided that the battery is disconnected. Operation can thus continue via the mains voltage. This polarity reversal of the battery is preferably indicated optically, acoustically or via an error signal via a bus line.

0056If the transistor 12 is in the form of a linear regulator, the battery discharge current and / or the battery charging current can be regulated to a predetermined target value by means of the current detection unit 13 and the control of the linear regulator 12. In a simplified manner, this regulation can of course also be implemented as a protective circuit, so that the switch 12 is opened in the event of an excessively high battery charging current or battery discharge current in order to protect the battery 4.

0057Instead of the linear regulator, a switching regulator can also be provided.

0058In the event of a very long failure of the mains voltage, deep discharge of the battery 4 can occur. If the battery 4 is deeply discharged, it has a voltage of, for example, 1.3 volts, which is below the permissible voltage of, for example, 1.5 volts. Even if there is now a proper mains voltage on the charging circuit 3 on the input side and the charging circuit 3 is operated properly in the manner described above, the deeply discharged battery 4 will pull the secondary side of the charging circuit to an inadmissibly low value.

0059According to the invention, this deep discharge can be detected by detecting the voltage of the battery 4. When such a deep discharge is detected, the switch 12 is preferably operated in a clocked manner. In this case, the switch 12 is preferably closed only for a relatively short period of time, during which the battery 4 is charged. Thereafter, the switch 12 is opened again for a longer period of time, so that the battery 4 is disconnected from the secondary side of the charging circuit 3 and the charging circuit 3 can again provide the proper voltage of, for example, 1.5 volts on the secondary side. This means that there is a proper tension ratio on the secondary side for a much longer period of time.

0060There is therefore a pulsed charge of the deeply discharged battery.

0061While the switch 12 is open, the mains voltage that is present again supplies the connected LED driver circuit and LEDs correctly by means of the charging circuit. On the other hand, if the switch is briefly closed, the battery is gently recharged. For example, the pulse duty factor for the switch 12 can be chosen such that it is only closed for 10% of the total time period and correspondingly open for 90%, so that the battery can recover in this 90% time period.

0062When deep discharge of the battery is detected by means of the detection of the battery voltage, switch 12 can be automatically switched over in this pulsed operation. The battery voltage is preferably monitored via a discrete circuit and thus independently of the microcontroller 2 (see<figref idref="f0001">Figures 1, 2</figref>), which may not be fully functional if the battery voltage is too low.

0063For the rest, the current detection circuit 13 with the comparator 14 is preferably selected as a discrete circuit and thus independently of the microcontroller 2 and its proper operation.