Battery charging controller

[0002] This invention relates to batteries and in particular to battery charging controllers for such batteries. This application is a CIP of Ser. No. 09/152,049 filed Sep. 11, 1998, now U.S. Pat. No. 6,111,391.

[0003] Most electricity used in the United States comes from power grids fed by large power stations. However, for many reasons alternate energy systems are becoming economically attractive in special situations. These alternate energy systems include solar, wind, hydroelectric and thermoelectric generators. Solar electric generators (SG's) have been commercially available in the United States for about 25 years. These units generate electric power from the energy of sunlight, which is free. Attempts have been made to produce electric power from sunlight to supply utility electric grids but these efforts have been largely unsuccessful because the total cost per kilowatt-hour from the solar generators substantially exceed the cost per kilowatt hour for electric power generated at central generating stations powered by burning coal, oil, gas or by nuclear power plants.

[0004] However, when it is not feasible to hook up to a power grid fed by a central generating station, the solar electric generator is often the power source of choice. Competitive power sources include gasoline powered motor generating units and thermoelectric devices. A very lucrative market for solar generators is to provide electric power for recreation vehicles (RV's) when the engine of the vehicle is not being utilized for travel. In this situation, the solar unit provides electric power (considering all applicable cost including depreciation, maintenance, etc.) at a small fraction of the cost of operation the vehicle gasoline engine to charge the battery or batteries of the RV. The typical RV has one or two batteries. When there are two batteries, one is for the engine and one is for the “house” portion of the RV. A controller is needed to control the supply of electricity to the batteries.

[0005] Prior art controllers have typically been rather simple devices and not much effort has gone into utilizing controllers to maximize the efficiency of solar power generators. Perhaps, the thinking has been “why worry about efficiency when the energy (from the sun) is free?”

[0006] The typical prior art solar generating unit sold for RV units is designed to produce power at about 17 volts for charging 12-volt batteries. The typical control unit comprises control switches (either relay control switches or solid state control switches) for connecting the output of the solar generator to the battery and a control unit which monitors the battery voltage and opens the switch when the battery voltage reaches a high target voltage, such as 14 volts and closes the switch when the respective battery voltage drops to a low target voltage such as 13 volts. The prior art control units are also typically constructed with a series diode to assure that current does not flow in reverse through the solar generator discharging the battery at night.

[0007] Most solar generating units are designed to operate in what is called constant current mode. This means that for a given level of solar radiation such as 1000 W/m², a substantially constant current is produced for any battery voltage within the design range of the solar generating unit. For example, FIG. 1 shows current vs. voltage for a typical solar unit, which is the BP275 Module available from BP Solar with offices in Fairfield, Calif. This graph shows that in the sunshine of 1000 W/m²at a solar generator temperature of about 25° C., the current produced by this unit is about 4.7 amps for battery voltages between 0 and 14 volts. The current drops off slightly to about 4.5 amps at 17 volts and drops to substantially zero at 21.4 volts. This is referred to as the open circuit voltage. Power is the product of current and voltage. Thus, if the battery being charged is at a low voltage level the rate of power delivery, and hence charging, can be substantially reduced.

[0008] Battery lifetime can be adversely affected if it is not maintained in accordance with instructions of the manufacturers. These instructions include recommendations on techniques for charging and maintaining the charge of the batteries.

[0009] What is needed is a better controller permitting the solar generating unit to function safely at or near its maximum power capacity and at same time to provide charging to maintain long battery life.

[0010] The present invention provides a controller for a solar electric generator that permits the generator to produce power substantially at its maximum capacity while also providing efficient charging at three charging stages; i.e., bulk charging, acceptance charging and float charging. Power is transferred from the generator to a temporary electric storage device that is periodically partially drained of power to maintain the temporary electric storage device at a voltage corresponding to the voltage needed by the generator to provide maximum generator power. The electric power drained from the temporary storage device is used to charge conventional batteries. In a preferred embodiment, the temporary storage device is a capacitor that is part of a buck regulator operating at 50 kHz with duty factor control between 0% and 100%. This buck topology switching type regulator provides the periodic draining. In the preferred embodiment control of the duty factor of the buck regulator is utilized to limit current, to prevent battery over charging, to test for the voltage corresponding to maximum power, and to operate the solar generator at is maximum power voltage. When operated at its maximum power operating point, the output to the battery is constant power, providing greater battery charge current than prior art controllers. Additional controls are provided to adjust battery charge voltage to permit maximum current flow during bulk charging, and at a first pre-selected charge voltage during acceptance charging and at a second pre-selected charge voltage during float charge. In a preferred embodiment provision is made for periodic equalization overcharging to improve battery performance and lifetime.

[0011] FIG. 1 shows an estimated Voltage—Current curve for BP275 Modules at 25° C.

[0012] FIG. 1A shows an estimated Voltage—Power curve for BP275 Modules.

[0013] FIG. 2 shows a simplified functional drawing of a preferred embodiment of the present invention.

[0014] FIG. 3 shows an estimated Voltage—Current curve demonstrating array efficiency as a function of temperature for BP275 Modules at 1000 W/m².

[0015] FIG. 4 is a modified version of FIG. 2 to permit three-stage battery charging.

[0016] FIG. 5 is a chart showing the three distinct battery stages in a preferred embodiment.

[0017] FIG. 6 is a chart showing charge voltage vs. temperature for acceptance and float charging stages.

[0018] A solar generator controller is described in FIG. 2. This unit is designed to extract the maximum energy from a solar generating unit (such as the BP275 solar generator) which can be used for providing solar power for RV vehicles.

[0019] The data displayed in FIG. 1 was used to plot the curves in FIG. 1A. FIG. 1A reveals that (at 1,000 W/m²and 25° C.) the unit provides the maximum power at about 17 volts. At 17 volts, 1,000 W/m²and 25° C., the power (which is the product of current and voltage) is about 75 watts. (In terms of energy production this would be 75 watt-hours/hour). However, at 10 volts the power production is only 40 watts and at 20 volts the power production is also only 40 watts. FIG. 1A also shows the power vs. voltage curve for 500 and 250 W/m².

[0020] The present invention recognizes the importance of operating the solar generating unit at its maximum power voltage (V_MP) which in this case (at 1000 W/m²and 25° C.) is about 17 volts. V_MPdoes not vary very much with solar radiation levels, but varies significantly and predictably with array temperature.

[0021] As shown in FIG. 3 the open circuit voltage changes substantially with array temperature. However, the difference between SG open circuit voltage V_OCand V_MPis essentially constant regardless of array temperature. The actual operating point V_MPis determined in this system by periodically sampling V_OC, which changes with SG temperature, then subtracting the difference between a particular SG panel's data sheet values of V_OCand V_MPfrom the sampled V_OC. The delta between V_OCand V_MPfor the BP275 SG panel is approximately 4.4 volts. Applicant has determined that for the BP275 unit and similar units V_MPis about 4.4 volts below the open circuit voltage at each radiation level over a wide range of levels from 1000 W/m²down to about 50 W/m².

[0022] In many installations, several units like the BP275 are operated in parallel so that sufficient power can be generated under minimum radiation conditions. This means that when the sun is very bright, in summer at mid-day and with no clouds, the current generated may exceed the current carrying capacitance of the charging circuits. Applicant's controller deals with this issue.

[0023] FIG. 2 is a simplified functional drawing of the solar generator controller system. A solar generator 2 comprising five parallel BP Solar Modules generates electric power for charging battery 3 from solar radiation 4 at voltages ranging from 0 to about 21.4 volts. Referring to FIG. 2, controller 1 includes a buck type switching voltage regulator 6 consisting primarily of a 43 μH inductor L1, a bucking capacitor C1, a field effect transistor Q1, a circulating diode CR1, a gate driver 8, a pulse width modulation controller 10 and a relay control switch 18. Within the basic buck regulator there are two current sensing resistors, R1 and R2, which measure solar generation (SG) current (input current to the buck regulator), and battery current (output current from the buck regulator) by means of differential amplifiers 12 and 14. The differential amplifiers produce voltages proportional to current through their respective resistor, which feed other circuit elements. One of the circuit elements fed by the differential amplifiers is a three and one-half digit voltmeter 16. This meter also reads battery voltage. Battery voltage is displayed to 10-millivolt resolution, whereas SG current and battery current are displayed to 100 milliamps resolution.

[0024] Whenever photons of sunshine illuminate the solar panels of solar electric generator array 2, each of the five panels of the generator will produce a quantity of electric current as indicated by FIG. 1. The total current is the sum of the current produced by each of the panels. The current produced is primarily dependent on the radiation level and the voltage on bucking regulator C1 and to a lesser degree, the temperature of the solar array.

[0025] In early morning when the sun begins to illuminate array 2, the array begins to charge bucking capacitor C1. Comparator 20 closes relay switch 18 when the I_SGcurrent reaches 110 milliamps and the voltage on capacitor C1 has reached 14 volts. Current source 97 comprises a saturable inductor and has a saturation voltage of approximately 14 volts. Therefore, current will not flow until available voltage is approximately 14 volts. The voltage on bucking capacitor C1 determines the current flowing in circuit 22, in accordance with FIG. 2. As indicated above, a principal element of this invention is to assure maximum power transfer from a solar electric generator array 2 to bucking capacitor C1. This is in general accomplished by having Q1 operate at 50 kHz and at a duty cycle such that the voltage on C1 is maintained at a target voltage chosen to assure maximum power transfer from solar array 2 to bucking capacitor C1. Current is allowed by transistor Q1 to flow to battery 3 at a rate as necessary to assure that C1 remains at the proper target voltage. Inductor L1 limits the current flow at the beginning of each cycle of the duty cycle of transistor Q1 and serves as an energy storage unit in the buck regulator.

[0026] Once the charging system turns on, it remains enabled as long as SG current is greater than approximately 80 milliamps. This hysteresis of approximately 30 milliamps (i.e., 110 milliamps minus 80 milliamps) in turn on/off threshold assures that operation will be stable near the turn on/off transition range. If current through R1 drops below 80 milliamps, comparator 20 shuts the generator down.

[0027] The required SG current should be available at this relatively high voltage of 14 volts to assure that charge current will flow to the battery. If the on/off decision was based on short circuit current, partial shading of SG array 2 would produce sufficient current for the system to turn on under SG short circuit conditions, but current would not flow to the battery since partial shading would prevent the SG array from developing a sufficiently high voltage to overcome battery voltage, causing the charge control system to turn off. Under these conditions the charge on/off control system would be unstable.

[0028] At very low radiation levels, relay switch 18 is open, duty cycle is clamped to 0% preventing current flow to the battery. However, the small quantity of SG current generated is allowed to flow through an on/off controllable sinking current source 97. Current source 97 has a soft saturation voltage of approximately 14 volts and a current limit of approximately 140 milliamps. It is enabled whenever the pulse width modulator (PWM) duty cycle is less than approximately 20 percent and is disabled whenever PWM duty cycle is greater than approximately 20 percent. (The PMW is described below.) When the charge control system is off, the PWM duty cycle is clamped to 0%. At this point, current source 97 is on and it, in combination with R1, differential amplifier 12, and SG_ONcomparator 20, essentially search for sufficient SG voltage and current. If it is available, controller 1 turns on. Current source 97 also provides the function of maintaining a minimum SG current for controller 1 to remain on if duty cycle goes to 0% due to unusually high battery voltage, i.e. greater than setpoint. This assures that controller 1 will remain on whenever sufficient SG current and voltage are available regardless of PWM duty cycle. This also assures that current source 97 is turned off when the controller is delivering charge current to the battery and duty cycle is in the normal operating range of 50-100%.

[0029] The PWM control system of the switching regulator uses a PWM device that attempts to deliver 100 percent duty cycle at all times. It is configured in such a way that duty cycle can be limited by five separate controlling inputs. The analog OR'ing function is such that whichever of the five inputs is attempting to decrease PWM duty cycle, will override other inputs requesting greater duty cycle. The inputs that can reduce duty cycle are: 1) SG open circuit voltage sample pulse, 2) peak power SG voltage control, 3) SG_ONcomparator output low, 4) battery voltage control, and 5) output current limit.

[0030] As shown in FIG. 2, an approximation of the open circuit voltage of array 2 is measured every eight seconds by sample and hold circuit 22 based on a 15 ms signal from oscillator 24. PWM controller 10 reduces the duty cycle on Q1 transistor to zero for the 15 ms sample period to obtain the open circuit voltage approximation. During this 15 ms period the charge on C1 increases to approximately open circuit voltage and the voltage reading is stored by sample and hold circuit 22. After the 15 ms period, PWM controller 10 returns to normal operation.

[0031] When SG voltage is sufficiently high, relative to battery voltage plus system voltage drops, such that 100% PWM duty cycle would produce an SG voltage below the maximum power voltage (V_MP), SG setpoint block 98 and SG servo block 99 reduce duty cycle such that SG voltage increases to V_MP, and is servo controlled at this value.

[0032] The proper V_MPsetpoint is determined by SG setpoint block 98. SG setpoint block 98 has three inputs which are used to determine the V_MPsetpoint for the SG peak power voltage control SG servo 99. These inputs are; the sampled value of V_OCas described above, a voltage proportional to SG current derived from resistor R1 and differential amplifier 12, and a user programmable voltage ΔV. ΔV is the difference between SG datasheet values of V_OCand V_MPand is substantially constant for the full expected SG temperature range as shown in FIG. 3. The user programs this value into the controller at the time of installation, which is 4.4 V for the BP275 SG. The output of SG setpoint block 98 is equal to; ((sampled V_OC)-ΔV-(0.07 V/amp of SG current)). The 0.07 V/amp of SG current correction factor decreases SG servo setpoint voltage to compensate for voltage drop in cabling between the controller and the SG. Due to cost, manageable wire size, etc., a typical installation will produce approximately a 0.7 volt drop at 10 amps between the SG and the controller terminals. Since the controller servos V_MPat the controller terminals, actual SG voltage will typically be 0.7 volts higher than the desired SG voltage at the SG array terminals, at an SG current of 10 amps. This is also key to the invention as the correction factor eliminates the need for remote sensing of actual SG voltage.

[0033] The SG voltage setpoint feeds SG servo block 99, which controls the PWM duty cycle to maintain SG voltage at V_MP. Note that the SG servo operates in a reverse polarity to a typical servo since lower SG voltage requires a decrease in duty cycle to raise SG voltage to the desired setpoint value.

[0034] Since under conditions of constant radiation and SG temperature the SG servo forces constant SG voltage at V_MPregardless of battery voltage and current, the output operates as constant power due to the well understood characteristics of the traditional buck topology switching regulator. As battery voltage changes with constant SG input power, PWM duty cycle changes to maintain constant SG power. Since output power is essentially constant, a decrease in battery voltage produces an increase in charge current going to the battery. This application of buck topology power conversion technology is key to the invention.

[0035] But, whenever SG voltage is not sufficiently high, relative to battery voltage plus system voltage drops, such that a 100% PWM duty cycle produces a SG voltage above the maximum power voltage (V_MP), the SG servo saturates at 100% PWM duty cycle, and the system reverts to straight through direct connection to the battery the same as prior art. If the voltage becomes high enough, battery voltage servo limits and controls the voltage.

[0036] A key to proper sampling at low SG currents is the need to minimize the size of C1 so that zero SG current is flowing at the end of the sample pulse. In this application the United-Chemicon URZA series capacitor is used due to its very high ripple current capability at relatively low capacitance values. This unique capacitor allows proper V_OCsampling, and therefore proper boost operation, at SG currents as low as 0.8 amps, while having a suitably high ripple current rating for long life in a 20 amp buck converter. Another key requirement to keeping the minimum SG current required for boost operation low is a large enough value of L1 relative of switching frequency to keep the buck converter in a continuous conduction operating mode. The combination of a 50 KHz operating frequency and 43 μH L1 inductor maintains continuous conduction under normal operating conditions down to an output current of approximately 0.9 amps. Therefore boost reliably operates down to an output current of just under 1.0 amp.

[0037] SG comparator 20, in addition to providing a signal to operate relay switch 18, provides a low current signal at 80 mA to initiate a zero duty cycle of buck regulator 6. This means that the controllable current source 97 should be on all the time whenever controller 1 is off.

[0038] In this preferred embodiment the duty factor is also subject to reduction based on battery high voltage. This high voltage setting is preferably set based on data provided by the battery manufacturer. A battery temperature signal from temperature sensor 26 is used by battery servo 28 to establish the high voltage limit which is used to direct PWM controller to reduce the duty factor as the limit is approached. In the preferred embodiment, an analog circuit is used to provide the temperature adjustment but a digital processor could also be utilized. For example, the voltage limit of typical lead acid battery decreases by about 5 millivolt per cell for each ° C. rise in the battery temperature.

[0039] This preferred embodiment provides a current limit servo 30 to provide a signal to PWM controller 10 to limit duty factor to limit the current in the charging circuit. In this embodiment the current limit is set at 21 amps. In the event this limit is reached current limit servo 30 will provide a signal to PWM controller 10 to limit the current to 21 amps.

[0040] The above sections of this specification and FIG. 2 describe a solar generator controller designed to permit the solar generator to operate at or approximately at its maximum efficiency by controlling the drain from an interim storage device to assure that the interim storage device is at the proper voltage to permit efficient solar generator operation. FIG. 4 is a modified version of FIG. 2 which shows additional features which together with the equipment described in FIG. 2 provide a preferred embodiment of the present invention. This embodiment in addition to permitting the solar generator unit to operate at close to maximum efficiency also provides for three-stage battery charging which permits fast charging of the battery when it is low, rapid charging when approaching full charge and at a slightly lowered voltage at full charge to increase battery lifetime.

[0041] As shown in FIG. 5 the first stage is referred to as the “bulk charge” stage. During this stage the battery is at a low (e.g., less than 70 percent of full charge) state of charge. Bulk charge is initiated when (1) the charge current during acceptance or float stages increases above a pre-selected transition current or (2) when insufficient power is available from the solar generator 2 for the voltage control servo to regulate battery voltage. During bulk charging the maximum current available is allowed to flow up to a current limit preferably set to prevent circuit overload.

[0042] Following bulk charge when the state of the charge is preferably about 70 percent, the system changes to a voltage control mode where the acceptance voltage is applied to the battery. In this embodiment the acceptance voltage is determined by the battery temperature. FIG. 6 shows acceptance voltage as a function of battery temperature for a commercial grade battery having a recommended factory set point at 14.3/28.6 volts at 80 degrees F. When the charge current during acceptance decreases to a pre-selected float transition current, the battery is considered “fully charged”. Preferably, the float transition current is set at about 1.0 amps per 100 amp-hours of battery capacity.

[0043] Once the battery is fully charged the generator controller 1 switches the system to float control where the battery is maintained at a voltage level slightly below the acceptance voltage level. In this preferred embodiment that voltage is about 13.3/26.6 volts. This keeps the battery fully charged without excessive water loss. It provides a very small current to offset self-discharge. During float a healthy battery will draw about 0.1 to 0.2 amps per 100 amp-hours of battery capacity. If a battery in float charge attempts to draw more than the float transition current (typically because of an increase in power drainage from the battery) control will switch to acceptance.

[0044] A proper determination of when the battery is fully charged is when the net charge current drops to a pre-selected value based on the amp-hour capacity of the battery. In this embodiment the charge current is used as the determining factor to switch between acceptance and float. Current for this determination could be output current of the charger but preferably it is the net charge current measured via an external shunt as shown at 103 in FIG. 4. The advantage of the external shunt can be illustrated as follows: Suppose a 350 amp-hour battery is at a fairly high state of charge in the float mode and is drawing 3 amps which is being provided by SG2. If a 10-amp load is then placed on the battery controller 1 automatically increases the current to the battery to hold it at the acceptance voltage. SG 2 is now delivering 13 amps. Using the internal shunt R2 would make it appear that the battery is consuming 13 amps that would call for a switch to the acceptance mode. However, if external shunt 103 is used for the current signal for mode determination, a signal of 3 amps is recognized and the battery control remains at float mode with the current for the 10 amp load being provided by SG unit 2.

[0045] FIG. 4 shows the additional circuitry for three stage charging with external current shunt. Current shunt 103 is a 0.001 ohm precision resistor with Kelvin sense terminals. It is wired into the system so that all battery charge or discharge current must flow through it so it measures net battery current. Current delivered by SG 2 directly to loads 104 do not flow through shunt 103. Precision amplifier 101 conditions and amplifies the signal from the shunt. Switch 102 selects the signal from external shunt 103 or internal shunt R2. Comparator 106 in combination with battery servo set point generator 105 determines if the acceptance voltage set point (preferably 14.3 Volts) or the float voltage set point (preferably 13.3 Volts) will be sent to battery voltage control servo 28. If the measured current is greater than the float transition current set point (preferably 3.5 amps for this 350 amp-hour battery) the acceptance voltage is applied to the battery and if the current is less than the set point the float voltage is applied to the battery. Since the battery consumes less current with lower applied voltage, a natural hysterises is created which helps maintain stable operation.

[0046] In this preferred embodiment, periodic equalization is provided for and recommended. Equalization is essentially a controlled over charge and should be performed periodically on vented liquid electrolyte lead acid batteries. Since each cell of the battery is not identical, repeated charge/discharge cycles can lead to an imbalance in the specific gravity of the individual battery cells. Stratification of the electrolyte can also occur. Equalization brings all battery cells up to the same specific gravity and eliminates stratification by heavily gassing the battery. This preferred embodiment features a manually operated equalization function although the function could be automated. Manual is preferred since an operator may be needed to ensure the equipment connected to the battery can tolerate the higher equalization voltage and that preferred time periods are not exceeded. Preferably the equalization voltage is the bulk voltage plus about 1 or 2 Volts for 12 or 24 Volt systems respectively. Note that with temperature compensation, the equalization voltage can be quite high at cold temperatures. In this preferred embodiment a push button is provided to enable equalization. An LED is provided which blinks rapidly when Equalization is selected. Preferably equalization is performed about once per month and the equalization period is about 2 hours. Preferably it is performed when the battery is fully charged. After equalization the battery preferably should be topped off with distilled water.

[0047] While the present invention has been described in relation to a particular embodiment, persons skilled in the art will recognize that many potential variations are possible. For example, smaller or larger solar generating systems will require appropriate changes. Other generators such as wind powered, hydroelectric or thermoelectric generators could be substituted for the solar unit. A small rechargeable battery could be used in place of the C1 capacitor. The maximum power voltage could be determined periodically by forcing a voltage swing on C1 and measuring the current across R1 and then using recorded voltage and current values to calculate the maximum power voltage.

[0048] The present invention has many obvious applications other than RV's. All that is needed is a little sunshine and a location some distance from a utility power grid. For these reasons the scope of this invention is to be determined by the appended claims and their legal equivalents.