Battery Equalization Circuit for a Dual Voltage Charging System

The present invention relates to a dual voltage charging system and more particularly to a battery equalization circuit for such a system.

Dual voltage charging systems are known for maintaining two series connected batteries at equal voltages for supplying power to a pair of variable loads. Such dual voltage charging systems have included a battery equalization circuit with two independent sensing networks each requiring an individual reference voltage. This type of equalization circuit does not provide automatic tracking of the battery voltages. Further, because of the duplicate sensing networks and the need for dual calibration, this type of circuit is costly and inefficient. This type of circuit is also susceptible to temperature drift.

Other battery equalization circuits for dual voltage charging systems have included a voltage divider circuit coupled to the batteries. The voltage divider circuit provides a reference voltage that is equal to half the total voltage across the batteries. A regulated D.C. to D.C. converter has its output connected across one of the batteries for charging that battery to a level indicated by the reference voltage. The voltage divider of this battery equalization circuit must either be calibrated or include expensive precision resistors in order to set a precise reference voltage for maintaining a balance between the voltages provided by the batteries. Further, the resistors in the voltage divider must have matched temperature coefficients to maintain the balance between the battery voltages over a range of temperatures.

In accordance with the present invention, the disadvantages of prior battery equalization circuits as discussed above have been overcome. The battery equalization circuit of the present invention samples the voltages across each of two series connected batteries to provide a representation thereof; and compares the representations to provide an error signal that is used to regulate the charge on at least one of the batteries to balance the voltage provided by the batteries.

More particularly, the battery equalization circuit of the present invention includes a capacitor coupled through a first switching means across the battery that is not referenced to ground to sample the voltage across the battery and provide a representation thereof when the first switching means is closed. The capacitor is also coupled to an error amplifier through a second switching means to compare the representation of the voltage across the battery not referenced to ground, as stored by the capacitor, to the voltage across the other battery that is referenced to ground. The error amplifier produces an error signal indicative of the difference between the voltages across the batteries, the error signal being applied to a charging and regulation circuit that is responsive to the error signal to minimize the difference between the voltages provided by the batteries.

The battery equalization circuit further includes a hold network coupled between the second switching means and the error amplifier to stabilize the charging current.

The battery equalization circuit of the present invention compensates for battery imbalance independent of the method employed to charge the batteries and the value to which the batteries are charged. Further, the effects of temperature drift are minimized and calibration is unnecessary since the circuit is not dependant on absolute component values.

Other features and advantages of the invention will be apparent from the following description taken in connection with the accompanying drawing wherein:

FIG. 1 is a block diagram of the battery equalization circuit of the present invention.

The battery equalization circuit of the present invention for a dual voltage charging system is illustrated in FIG. 1. The dual voltage charging system includes a pair of +12 volt batteries 12 and 14 connected in series across a 24 volt load 16. The +12 volt battery 14 is additionally connected across a 12 volt load 18. Because different loads are coupled across the batteries 12 and 14, the batteries can discharge at unequal rates. The battery equalization circuit of the present invention insures a balance is maintained between the voltages across the batteries 12 and 14.

The battery equalization circuit includes a sampling circuit 20 for sampling the voltage across the battery 12 to provide a signal representative thereof to an error amplifier. Although the voltage across the battery 14 may be similarly sampled as discussed below, because this voltage is referenced with respect to ground, it may be applied as a direct input to the error amplifier 22. The error amplifier 22 compares the signal representing the sampled voltage across the battery 12 to the signal representing the voltage across the battery 14 to producing an error signal indicative of the difference between the voltages across the batteries 12 and 14. A charging/regulating for circuit 24 is responsive to the error signal output from the error amplifier 22 to minimize the difference between the voltages across the first and second batteries 12 and 14.

The sampling circuit 20 includes a capacitor 26 coupled across the battery 12 through a first pair of ganged switches 28 and 30. Similarly, the capacitor 26 is coupled to a hold network 32 through a second pair of ganged switches 34 and 36. The first pair of switches 28 and 30 and the second pair of switches 34 and 36 are alternately opened and closed by a toggle control 38 to charge and discharge the capacitor 26. More particularly, the toggle control 38 includes a free-running oscillator providing an output having a fifty percent duty cycle that opens and closes the switches 28, 30, 34 and 36 for equal amounts of time, the switches 28 and 30 being closed when the switches 34 and 36 are opened and visa versa.

The operation of the sampling circuit 20 of the present invention involves closing the first pair of switches 28, 30 while the switches 34, 36 are open to periodically sample the voltage across the battery 12. The voltage sampled is stored in the form of charge on the capacitor 26 until coupled to a hold network 32 by the simultaneous opening and closing of respective switch pairs 28, 30 and 34, 36. More particularly, when the first pair of switches 28 and 30 is closed in response to the output of the toggle control 38, current is conducted along a path 40 to charge the capacitor 26 to a voltage representative of the voltage across the battery 12. Following the charging of the capacitor 26, the toggle control 38 opens the first pair of switches 28 and 30 and simultaneously closes the second pair of switches 34 and 36 to provide a current path 42 for discharging the capacitor to ground via the hold network 32 which temporarily stores the voltage representative of the voltage across the battery 12. The hold network 32 may be formed of a capacitor or other conventional sample and hold circuits may be used. Due to the periodic switching of the circuitry mentioned above, the hold network 32 is needed to produce a stable charging current for the battery 14 if the voltages across the respective batteries 12 and 14 are not equal.

The error amplifier 22 which may be formed of a differential amplifier is coupled to the hold network 32 as well as to the battery 14 for comparing the sampled voltage across the battery 12 to the voltage (or a representation thereof) across the battery 14. In response to the comparison, the error amplifier 22 produces an error signal representing the difference between the voltages across the batteries 12 and 14. The error signal output from the error amplifier 22 is coupled to the charging/regulating circuit 24 for minimizing the difference between the voltages across the batteries 12 and 14.

The charging/regulating circuit 24 includes a conventional 24 volt alternator 50 coupled to a 24 volt regulator 52 for providing +24 volts which is coupled across the series combination of the batteries 12 and 14 for independently charging the battery 12. The charging and regulation circuit 54 is responsive to a nonzero error signal for charging the battery 14 until the error signal is reduced to zero.

The battery equalization circuit of the present invention compensates for battery imbalance independent of the method employed to charge the batteries and the value to which the batteries are charged. Further, the effects of temperature drift are minimized since the circuit is not dependent on absolute component values.

Although the battery equalization circuit described consists of two batteries, any number of batteries could be equally charged using this scheme. Likewise, even though demonstrated in FIG. 1 as having an alternator as a power source, it is equally effective using alternative power sources.

Many modifications and variations of the present invention are possible in light of the above teachings. Thus, it is to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as described above.