System having a battery and method of controlling the same

Abstract

A system includes a battery, a load, and a power converter for converting battery power from the battery into load power for application to the load. A system controller is capable of selecting between a continuous discharge mode and a discontinuous discharge mode of the battery. In this way, the battery can be operated in different ways according to the most suitable battery discharge characteristics.

Description

Technical Field

[0001] The invention relates to a system comprising a battery. Background Art

[0002] Rechargeable batteries are used in many applications. They are used, for example, as temporary energy storage devices in systems with a time-varying power output and a time-varying energy input. For example, lighting fixtures are increasingly equipped with integrated batteries. These can be charged by solar energy, which is usually available when lighting is not needed, and lighting is needed when solar energy is not available. They are therefore used as energy storage elements to account for the time delay between the availability of charging energy and the need for output power.

[0003] For outdoor luminaires in particular, but more generally for outdoor systems, the battery is often exposed to ambient temperatures.

[0004] The performance of rechargeable batteries such as lithium-ion (LiFePo4) batteries depends on stress parameters such as the maximum charge voltage of each battery cell, the charge and discharge current, the depth of discharge (DOD), and the operating temperature of the battery during use. As the charge and discharge current, DOD, and the maximum charge voltage of each battery cell increase, the cycle life of the battery will decrease. If the battery is not operated at room temperature, the operating temperature of the battery will also show a significant impact on the life.

[0005] For example, low temperature battery charging (e.g. less than 10 degrees Celsius) will seriously affect the battery cycle life. From the experiment, it can be seen that if LiFePo4 is cycled at 25 degrees Celsius and provides a service life of 5500 cycles, then if the same battery is cycled at 10 degrees Celsius, it will provide 3750 cycles, and if the same battery is cycled at -10 degrees Celsius, it will only provide 146 cycles.

[0006] The following table shows the number of cycles N of battery life at different temperatures (charge depth is 100%) f .

[0007] temperature <![CDATA[N f ]]> -10℃ 146±15 10℃ 3750±60 25℃ 5550±500 35℃ 4930±1000 50℃ 1950±350 .

[0008] In the case of outdoor luminaires with integrated batteries, the batteries will be subject to extremely low temperatures in many areas. Even for indoor luminaires, such as office spaces, the batteries will be placed above the luminaire and therefore in a less insulated part of the building. Again, the batteries may be subject to extremely low temperatures or elevated temperatures.

[0009] There is often a need to improve the utilization efficiency of batteries, especially where the batteries are exposed to significant temperature changes, especially low temperatures. Summary of the invention

[0010] The present invention provides a system having a battery and a method of controlling the system.

[0011] According to an example of the present invention, a system is provided, the system comprising:

[0012] Battery;

[0013] load;

[0014] a power converter for converting battery power from a battery into load power for application to a load; and

[0015] A system controller, wherein the system controller is adapted to enable selection between a continuous discharge mode and a discontinuous discharge mode of the battery.

[0016] The discontinuous discharge mode of the battery can be used to improve the utilization of the battery, especially the utilization of available energy storage, cycle life, specific energy and specific power. Discontinuity leads to faster relaxation, which helps to optimize battery performance. For example, this may be particularly meaningful for low temperature operation.

[0017] The power converter may comprise a switch mode power converter comprising an energy storage and a switching arrangement for controlling switching of battery power to the energy storage and from the energy storage to a load.

[0018] In a first example, the controller is adapted to control the switching frequency of the switching device. By controlling the switching frequency, the available time for the charging and discharging cycles is changed. As a result, the current waveform can be switched between a continuous mode and a discontinuous mode.

[0019] In a second example, the controller may be adapted to control the impedance of the energy storage.

[0020] By controlling the impedance, the charge and discharge time constants can be changed, which has the effect of changing the slope of the battery current waveform. This can be used to control the overall current level so that the average (or rms) current delivered remains constant when switching between operating modes.

[0021] The energy storage may include a first inductor and a second inductor connected in parallel, and a switch for selectively connecting or disconnecting one of the inductors. The energy storage may thus be configured as a single inductor or a pair of inductors connected in parallel. This provides a method of controlling the impedance of the energy storage.

[0022] The controller may alternatively (or additionally) be adapted to control the coupling of the smoothing output capacitor to the power converter.By controlling the output smoothing, it is again possible to select between continuous mode and discontinuous mode.

[0023] When switching between continuous and discontinuous discharge modes, the controller can adapt to achieve equal rms output currents. In this way, the load sees the same rms current, so the control transition is not noticeable to the system user.

[0024] The system may include an input for connecting to an external power source, and the controller is adapted to provide continuous power to the load during the discontinuous discharge mode by using the external power source during the discontinuous period. This ensures that continuous power is provided to the load while ensuring discontinuous battery discharge. The external power source is, for example, a mains supply.

[0025] The system may include a plurality of batteries, and the controller is adapted to select at least two batteries to alternately deliver output power to the load, thereby achieving a discontinuous discharge mode of the at least two batteries. This provides an alternative method to achieve discontinuous battery discharge, but can continuously power the load.

[0026] The power converter includes, for example, a buck converter or a boost converter.

[0027] The load is, for example, a lighting device, and the system includes an LED lamp.

[0028] The system is thus a battery-integrated luminaire. The battery can for example be charged by solar energy, which is usually available when lighting is not needed, and lighting is needed when solar energy is not available. The battery thus acts as an energy storage element to account for the time delay between the availability of charging energy and the need for output power. Discontinuous mode may be of particular interest during cold periods, such as winter nights. The battery can alternatively be charged by the mains and have the purpose of time-shifting according to the level of demand and thus according to the pricing level of the mains.

[0029] The present invention also provides a method for controlling a system, the system comprising a battery, a load and a power converter, the power converter being used to convert battery power from the battery into load power for application to the load, wherein the method comprises:

[0030] Select between continuous and discontinuous discharge mode of the battery.

[0031] The power converter may include a switch mode power converter including an energy storage and a switch arrangement for controlling switching of battery power to the energy storage and from the energy storage to a load, wherein the method includes achieving the selection by:

[0032] controlling the impedance of an energy storage of a switch mode power converter; and / or

[0033] controlling a switching frequency of a switching device of a switch mode power converter; and / or

[0034] Controls the coupling of a smoothing output capacitor to a switch mode power converter.

[0035] The method may include providing continuous power to the load during the discontinuous discharge mode by using an external power source during the discontinuous period. The method may include selecting at least two batteries from a plurality of batteries to alternately deliver output power, thereby achieving a discontinuous discharge mode of the at least two batteries.

[0036] The method is preferably used to control an LED lamp.

[0037] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Examples of the present invention will now be described in detail with reference to the accompanying drawings, in which:

[0039] Figure 1 A system 10 including a battery, a load 14 and a power converter is shown;

[0040] Figure 2 One example of a possible power converter is shown;

[0041] Figure 3 Shows Figure 2 The waveform of the continuous operation mode of the power converter;

[0042] Figure 4 Shows Figure 2 Waveform of discontinuous operation mode of the power converter;

[0043] Figure 5 A power converter in the form of a modified boost converter is shown, in which the energy storage (inductor) is formed as two parallel inductors;

[0044] Figure 6 Shows Figure 5 Normal continuous operation mode of the power converter;

[0045] Figure 7 Shows Figure 5 The effect of reducing the inductance of the power converter;

[0046] Figure 8 shows the removal of the output capacitor for Figure 6 The influence of waveform;

[0047] Fig. 9 A power converter based on a buck converter is shown;

[0048] Fig.10A timing diagram is shown to explain the Fig. 9 functionality of the circuit during normal operation in discontinuous mode; and

[0049] Fig.11 A method of controlling a system is shown. DETAILED DESCRIPTION

[0050] The present invention will be described with reference to the accompanying drawings.

[0051] It should be understood that the detailed description and specific examples, although indicating exemplary embodiments of the device, system and method, are intended only for illustrative purposes and are not intended to limit the scope of the invention. These and other features, aspects and advantages of the device, system and method of the present invention will be better understood based on the following description, the appended claims and the accompanying drawings. It should be understood that the drawings are only schematic and are not drawn to scale. It should also be understood that the same reference numerals are used throughout the drawings to represent the same or similar parts.

[0052] The present invention provides a system including a battery, a load and a power converter for converting battery power from the battery into load power to be applied to the load. The system controller is capable of selecting between a continuous discharge mode and a discontinuous discharge mode of the battery. In this way, the battery can be operated in different ways according to the most suitable battery discharge characteristics.

[0053] The concept of pulse discharge of a battery is based on continuous changes in current rate and / or direction, rather than using a constant discharge current. Basically, the current can be interrupted, a shorter rest period introduced, or replaced by a short charging pulse. These short rest periods are believed to increase the rate of relaxation, and short current reversals can achieve faster relaxation and reverse the direction of the electrochemical process in the battery.

[0054] Some literature suggests that pulsed current or voltage can affect the transport mechanism of lithium-ion batteries. Depolarization pulses can greatly improve the charging and discharging performance of batteries. For example, CF Chiasserini and RR Rao, "Pulsed Battery Discharge in Communication Devices", Proceedings of the 5th International Conference on Mobile Computing and Networking, Seattle, Washington, 1999, pp. 88-95, show the use of pulse discharge profiles with carefully selected amplitudes and rest periods to increase both the specific energy and specific power of battery cells. In addition, other studies have shown that pulsing the potential of the electrode can control the thickness of the equivalent diffusion layer, thereby limiting the concentration of the overpotential. Therefore, pulse discharge (and optionally charging) can be considered an interesting candidate for battery performance optimization.

[0055] The interfacial resistance is highest at low temperatures and at the end of discharge. For a given state of charge, the interfacial resistance increases exponentially with decreasing temperature. Therefore, the need for efficient battery use is particularly important at low temperatures.

[0056] Figure 1 A system 10 is shown that includes a battery 12 , a load 14 , a power converter 16 for converting battery power from the battery into load power for application to the load, and a system controller 18 .

[0057] The system controller 18 is adapted to be able to select between a continuous discharge mode and a discontinuous discharge mode of the battery. The discontinuous discharge mode of the battery is used to improve battery utilization, in particular the utilization of available energy storage, cycle life, specific energy and specific power. Discontinuity leads to faster relaxation, which helps optimize battery performance.

[0058] Figure 1 Another external power source 20 is shown. In one example, this is a mains power source. In this case, the battery can be used to deliver power at peak times (and therefore peak charges) for mains power, and the mains power can be used to recharge the battery. In another example, the power source 20 is a solar power source. In this case, the battery is used to supply power when direct solar power is not available.

[0059] Figure 2 One example of a possible power converter 16 is shown comprising a switch mode power converter. The switch mode power converter comprises an energy store 22 and switching means 24, 26 for controlling the switching of battery power to the energy store and from the energy store to a load.

[0060] The example is a boost converter, in which the switching device comprises a high-frequency main switching transistor 24 and a diode 26, and an inductor 22 as energy storage. A smoothing capacitor 28 is arranged in parallel with the output load 14.

[0061] In a first example, the controller 18 is adapted to control the switching frequency of the main transistor 24. By controlling the switching frequency, the time available for the charging and discharging cycles is varied. As a result, the current waveform can be switched between a continuous mode and a discontinuous mode.

[0062] Figure 3 The waveform for the continuous operation mode is shown.

[0063] The top graph shows the current drawn from the battery, I B , and the bottom graph shows a pulse width modulation (PWM) signal applied to the gate of the main transistor by the controller 18. The energy conversion rate is controlled based on the duty cycle of the PWM signal. The rising current ramp corresponds to the inductor charging, while the falling current ramp corresponds to the inductor discharging to the load.

[0064] This continuous operation may be achieved by operating at a first frequency, such as 50 kHz.

[0065] Figure 4 The waveform for the discontinuous operation mode is shown.

[0066] Likewise, the top graph shows the current drawn from the battery, I B , and the bottom diagram shows a pulse width modulated (PWM) signal applied to the transistor gate by the controller 18. This discontinuous operation can be achieved by operating at a lower second frequency, such as 30kHz. The effect is that the decreasing current drawn from the battery drops to zero as the energy storage is completely depleted.

[0067] The discontinuous mode has, for example, a switching frequency between 10% and 70% of the switching frequency of the continuous mode, for example a switching frequency which is half of the switching frequency of the continuous mode.

[0068] For example, this type of boost converter is often used in LED lamps with integrated batteries, because in most cases the battery voltage is lower than the required LED string voltage. Figure 3 When operating at high frequencies as shown, for example in the range of 50kHz to 100kHz, the battery current is continuous because the converter will operate in continuous conduction mode (CCM), resulting in a continuous DC battery current. By keeping the maximum inductor current within the saturation limit, the operating frequency can be reduced so that the battery current becomes discontinuous or in boundary conduction mode.

[0069] A consequence of this frequency reduction method of implementing discontinuous conduction mode is that the output current, i.e. the LED current, may need to be reduced because the power delivery from the battery to the LED string will be limited by the inductor saturation limit. Frequency reduction may be used, for example, during dimming operation to improve battery performance.

[0070] Figure 5 An improved boost converter is shown in which the energy storage (inductor) is formed as two parallel inductors 22a, 22b and with a switch 22c for controlling the connection of the second inductor 22b. Thus, the inductance can be switched between two values. Different inductance values ​​change the time characteristics of the circuit. The switching is controlled by the system controller 18.

[0071] A switchable inductor is used to overcome the inductor saturation limit and maintain the same LED current despite the reduced frequency.

[0072] Figure 6A normal continuous operating mode is shown with only the first inductor 22a.During this normal high frequency (50-100kHz) operating mode, the battery current is continuous and DC in nature, and the inductor 22a carries the battery current.

[0073] Figure 7 The effect of reducing the inductance by connecting two inductors in parallel and reducing the switching frequency is shown. The slope of the current profile increases due to the reduced inductance, while the reduced frequency results in a discontinuous mode.

[0074] The benefit of this combined approach is that the rms current is kept at a constant level. The increase in current compensates for the reduced time that current is flowing. By switching two inductors in parallel, the current delivery limit is improved by a factor of two, even at low frequency operation.

[0075] In both approaches, the LED current will be continuous due to the output capacitor filter. By controlling the impedance, the charge and discharge time constants are altered, which has the effect of changing the slope of the battery current waveform. The two inductors in parallel enable the peak current to be carried to be doubled, so the pause time between peaks can be longer.

[0076] The controller 18 may alternatively or additionally be adapted to control the coupling of the smoothing output capacitor 28 to the power converter. By controlling the output smoothing, it is again possible to select between a continuous mode and a discontinuous mode.

[0077] Figure 8 The output capacitor pair is shown to be removed, for example, by opening the switch 30 connected in series with the output capacitor. Figure 6 As a result, the LED experiences a pulsed current I LED , the pulse current I LED Has the same frequency as the battery pulse current.

[0078] During the inductor charging period (main switch 24 is closed), current flows from the battery to the inductor, and during the inductor discharging period (main switch 24 is open), the current I LED flows to the LED load.

[0079] Fig. 9 An embodiment based on a buck converter is shown. This is suitable for applications where the battery voltage is higher than the LED string voltage. Components that are the same as in the boost converter have the same reference numerals. There is an input filter circuit across the battery, including a filter capacitor 90 and a series switch 92 connected in parallel with the battery 12. When the main switch 24 is closed, current flows from the capacitor 90 through the switch 24 to the inductor 90, and when the switch 24 is open, the depleted charge in the capacitor is replenished by the battery.

[0080] A series resistor (or inductor) may be provided between the battery 12 and the capacitor 90. The current path from the capacitor to the down converter is through a low ohmic connection. When the series switch 92 is open, recharging by the battery is interrupted and the current ripple of the down converter is fully supported by the battery.

[0081] Fig.10 A timing diagram is shown to explain the circuit function during normal operation in discontinuous mode.

[0082] The battery experiences a pulse current I B In order to maintain the same LED current at different frequencies and duty cycles, inductor switching is employed as described above.

[0083] As shown in the figure, the LED current I LED is continuous.

[0084] like Figure 1 As shown, the system has an input for connection to an external power source 20. This can be used to ensure continuous power is provided to the load during the discontinuous discharge mode by using the external power source during the discontinuous period. The external power source is, for example, a mains supply. In this way, the battery current and the mains supply (or charger) current can be interleaved. The load current pulses are then made independent of the battery current pulses. Also during battery charging, the LED can be used to intermittently load the battery, thereby causing current ripple.

[0085] Alternatively, the system may include multiple batteries, and the controller 18 is then adapted to select at least two batteries to deliver output power alternately to the load. This achieves a discontinuous discharge mode of at least two batteries while maintaining a continuous supply to the load. Two or more batteries may be used to transfer current between them to provide another way to shape the current pulses.

[0086] Fig.11 A method of controlling a system is shown, the system including a battery, a load and a power converter for converting battery power from the battery into load power for application to the load. The method includes selecting between a continuous discharge mode and a discontinuous discharge mode of the battery in step 110. In step 112, the load is powered according to the selected operation mode.

[0087] As mentioned above, mode selection can be based on one or more of the following:

[0088] controlling the impedance of an energy storage of a switch mode power converter;

[0089] controlling a switching frequency of a switching device of a switch mode power converter;

[0090] Controls the coupling of a smoothing output capacitor to a switch mode power converter.

[0091] The present invention provides a system that can switch between continuous and discontinuous conduction modes. There is a trade-off between the two modes. Discontinuous mode can improve battery performance but reduces power delivery capability, so it can be operated during dimming mode operation and not at the rated power operation of the LED string. However, adding an inductor switch allows the rated current output to be maintained.

[0092] Pulsing battery discharge may not always improve battery performance. This may depend on various operating conditions, such as temperature (DCM works well at low temperature operation) and battery chemistry. Therefore, by providing the driver with the ability to switch between continuous mode and discontinuous mode, the driver is forward-looking and can be used for battery performance improvement for any battery chemistry under any external operating conditions. This also enables a single driver design to be compatible with multiple applications, such as low temperature areas and high temperature areas.

[0093] It can be seen that the decision to switch between modes may be based on various sources of information, such as:

[0094] temperature;

[0095] Dimming levels of LED lamps;

[0096] Type of connected battery;

[0097] The battery's state of charge;

[0098] Battery health status.

[0099] The battery impedance can be measured during the current ripple. This can be used for state of charge and state of health assessment. Since the current ripple is intentionally generated, the impedance can be directly monitored by measuring the battery terminal voltage waveform generated by the current pulse.

[0100] Battery impedance measurement gives interesting information about the state of charge and the state of health of many battery systems. However, the ohmic resistance of the battery is usually very small and therefore the phase shift between voltage and current can be small or dominated by the cable. In battery-integrated lamps, the battery cable is usually short and therefore the impedance influence can usually be monitored.

[0101] Examples of a buck converter and a boost converter are given above. However, the converter may be a buck-boost converter, a push-pull converter, a forward converter or a half-bridge or full-bridge inverter.

[0102] For example a purely resistive circuit can be used to generate the LED current, with the result that the battery current is the same as the LED current. At high frequencies, assuming a flicker rate above about 400 Hz, the resulting ripple is acceptable, as the naked eye will not see it. In some applications (such as street lighting), slower flickering may also be acceptable. Local energy such as capacitors and / or inductors may not be used as part of the switch mode power supply, again giving the freedom to generate a battery current discharge ripple that is independent of the LED flux ripple.

[0103] The invention is of interest for outdoor solar luminaires, where in winter and at night, low temperature discharge is expected so that pulse discharge will be effective. The battery can alternatively be used for time shifting, i.e. demand response lighting. During high demand periods, the luminaire operates normally in dimming mode and the battery performance can be improved again by DCM discharge.

[0104] However, the invention may be more generally applied to battery operated loads.

[0105] Other variations of the disclosed embodiments may be understood and effected by those skilled in the art in practicing the claimed invention by studying the drawings, the disclosure and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. A power conversion system (10), comprising: Battery (12); load(14); a power converter (16) for converting battery power from a battery into load power for application to a load; and a system controller (18), wherein the system controller is adapted to be able to select between a continuous discharge mode and a discontinuous discharge mode of the battery, wherein the controller (18) is adapted to achieve equal root mean square output current when switching between the continuous discharge mode and the discontinuous discharge mode, wherein the power converter (16) comprises a switch mode power converter, the switch mode power converter comprising an energy storage (22) and a switching device (24) for controlling the switching of battery power to the energy storage and from the energy storage to the load, Therein, the controller (18) is adapted to control the impedance of the energy storage.

2. The system according to claim 1, wherein: The controller (18) is adapted to control a switching frequency of the switching device.

3. The system according to claim 1, wherein: The energy storage device includes: a first inductor and a second inductor (22a, 22b) connected in parallel; and a switch (22c) for selectively connecting or disconnecting one of the inductors.

4. A system according to any preceding claim, wherein: The controller (18) is adapted to control the coupling of a smoothing output capacitor (28) to the power converter.

5. A system according to any one of claims 1 to 3, comprising an input for connection to an external power source (20), and wherein: The controller is adapted to provide continuous power to the load during the discontinuous discharge mode by using the external power source during discontinuous periods.

6. The system according to any one of claims 1 to 3, comprising a plurality of batteries, and wherein: The controller is adapted to select at least two batteries to alternately deliver output power to a load, thereby achieving a discontinuous discharge mode of the at least two batteries.

7. The system according to any one of claims 1 to 3, wherein: The load (14) is a lighting device, and the system includes an LED lamp.

8. A method of controlling a system, the system comprising a battery (12), a load (14) and a power converter (16), the power converter being used to convert battery power from the battery into load power for application to the load, wherein: The method comprises: (110) selecting between a continuous discharge mode and a discontinuous discharge mode of the battery, wherein equal root mean square output current is achieved when switching between the continuous discharge mode and the discontinuous discharge mode, wherein the power converter (16) comprises a switch mode power converter, the switch mode power converter comprising an energy storage device and a switch device for controlling the switching of battery power to the energy storage device and from the energy storage device to the load, Therein, a controller (18) is adapted to control the impedance of the energy storage.

9. The method of claim 8, wherein the method comprises achieving the selection by: controlling a switching frequency of a switching device of the switch mode power converter; and / or Controls the coupling of a smoothing output capacitor to a switch mode power converter.

10. A method as claimed in claim 8 or claim 9, comprising providing continuous power to the load during the discontinuous discharge mode by using an external power source during the discontinuous periods.

11. The method according to claim 8 or 9, comprising selecting at least two batteries from a plurality of batteries to deliver output power alternately, thereby achieving a discontinuous discharge mode of the at least two batteries.

12. A method according to claim 8 or 9, comprising controlling an LED lamp.

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