Abstract:
A method for charging batteries using a solid state battery charger is disclosed which includes the steps of supplying a constant current to the batteries being charged until the battery voltage reaches a first threshold voltage value, supplying a constant voltage to the batteries being charged until current being drawn by the batteries reaches a threshold current value and supplying a constant current to the batteries being charged until a voltage rise per unit time threshold is met.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/134,463, filed May 17, 1999, which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to charging batteries, and more specifically to methods and systems for charging batteries and extending battery life. 
     Known methods of charging rechargeable batteries, such as lead-acid batteries found in automobiles and other electric vehicles, typically are constant voltage source chargers. Since the proper voltage for such a battery depends on a variety of factors including battery age, temperature, and the amount of charge in the battery, known constant voltage chargers tend to overcharge older or lower voltage or hotter batteries and tend to undercharge newer, cooler higher voltage batteries. In addition batteries tend to discharge over time due to an internal current draw or self discharge leakage between the cells of the battery. Depending on storage temperatures, known batteries discharge to a 50% charge anywhere from three to nine months. 
     It would be desirable to develop a battery charger and method of battery charging which can take into account the battery factors listed above and develop a more efficient charge for a battery to be charged. In addition it would be desirable to develop a method and system for retaining full charges on under utilized batteries, for example, batteries in an electric vehicle in storage for a winter. 
     BRIEF SUMMARY OF THE INVENTION 
     In an exemplary embodiment, a solid state battery charger algorithm implements an algorithm for charging batteries comprising the steps of supplying a constant current to the batteries being charged until the battery voltage reaches a first threshold voltage value, supplying a constant voltage to the batteries being charged until current being drawn by the batteries reaches a threshold current value, and supplying a constant current to the batteries being charged until a voltage rise per unit time threshold is met. The above described method overcomes overcharging and undercharging problems known to exist in known constant voltage supply battery chargers. 
     In addition the battery charging algorithm constantly monitors battery performance and can reinitiate a charging cycle when monitored battery parameters so dictate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a solid state battery charger according to one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a block diagram  10  of a solid state battery charger according to one embodiment of the present invention. Included in block diagram  10  are a microprocessor  12  and memory  14  which contain a battery charging algorithm to be executed by microprocessor  12 . As used herein, microprocessor  12  refers to controllers and processors, including microcontrollers, programmable logic controllers, input/output controllers, reduced instruction set circuits, application specific integrated circuits, logic circuits, and any other circuit, processor or microcomputer capable of processing the embodiments described herein. Memory  14  can be either internal or external to microprocessor  12 . 
     A buck converter  16  and phase shift controller  18  form a switching power supply from which the electronics in the charger are powered. Switching power supply include filtering circuitry for removing the switching frequency in a differential mode. Electromagnetic interference is reduced through use of ceramic FET insulators to lower common mode emissions at harmonic frequencies. Phase shift controller  18  uses a phase modulator so that outputs are always square wave regardless of a pulse width modulated controller used in buck converter  16 . As phase is shifted, DC output is zero. A current transformer is included in controller  18  so that control circuitry may be run off the battery being charged. A current signal at the switching frequency introduces noise at the switching frequency and a filter is within controller to filter out the switching noise. 
     A receiver  20  and transmitter  22  are included as options to the charger to provide external communication with an outside device, for example, a computer, such as a laptop or other handheld unit. Receiver  20  and transmitter  22  communication can be any of several known formats including a hard wired interface, such as an RS-232 serial interface, or alternatively can be infrared (optical), for example using the television remote standard of a 40 kilohertz carrier or other standard. Alternatively a radio frequency interface can be used. Information to be communicated across receiver  20  and transmitter  22  interface could include, but is not limited to battery charging voltage, battery charging current, battery charger temperature, total ampere-hours used, a number of times a battery voltage has dropped below a particular voltage threshold, initial battery charge state, error diagnostic messages, battery charge time, and other user specified messages. 
     A light emitting diode (LED) panel  24  is an option to the charger which includes a plurality of indications as to the state of the battery charger and of the battery itself. Although described in terms of an LED indicator, panel  24  can be in a number of formats, including incandescent indicators, meters, etc. Indicators monitor charger and battery operation as part of an algorithm stored in memory  14 . A partial list of functions monitored include an indication of a charger over temperature condition, battery disconnect, a percentage of battery charge threshold, for example 80%, a charge cycle complete indication, an active charge cycle indication, and an indication of low battery charge, for example 20%. 
     A voltage measurement circuit  26  and a current measurement circuit  28  monitor the state of the battery during charge cycles and use. Output circuits  30  generate the voltage and currents necessary for charging of the battery or batteries. 
     One exemplary algorithm for the charging of batteries includes a three stage charging cycle including a constant current cycle followed by a constant voltage cycle followed by a constant current cycle described in more details as follows: a constant current is applied to the battery being charged until the battery voltage reaches a specified voltage threshold stored as part of the charging algorithm stored in memory  14 . At that point the battery charger becomes a constant voltage source charger until the current being absorbed by the battery decreases to a current threshold value, also stored as part of the charging algorithm. Then a constant current charge, at a different current level then the first current charging cycle, is applied to the battery or batteries until the change in voltage of the battery over a predetermined time approaches a predetermined threshold (dv/dt), at which time the charger shuts off. In one embodiment, the constant current charge of the third stage runs until the change in battery voltage (dv/dt) is less than 0.1 volt per cell per hour, at which time the charger shuts off. The above described charging algorithm compensates for older batteries or the temperatures at which charging is taking place. 
     In another embodiment, the charger is configured with a maximum time charge cycle, for safety purposes. One example is a twelve hour maximum charge cycle. 
     The battery charger is configured to monitor battery voltage when the battery is not in a charge cycle. When the charger detects that battery voltage has dropped below a preprogrammed threshold, the charging cycle will automatically be restarted. Automatic recharging extends battery life as batteries should not be stored in a partially discharged state. Other embodiments of the battery charger include inhibition of use during a charge cycle, maintaining a running total of ampere-hours used, and a running total of a number of times a battery voltage has dipped below a preprogrammed voltage. Inhibition of use during charging cycles is accomplished by feeding the signal signifying that an active charging cycle is taking place back into the microprocessor. 
     Using a battery charger as described above adds life, sometimes up to a year, to known lead-acid batteries. The life is extended due to the final part of the charging algorithm which supplies a constant current to the battery until the change in battery voltage over time is reduced to a predetermined value, as described above. Battery life is extended because the complete charging cycle of the algorithm causes an equalizing charge each cycle. Plates located in the battery cells are scrubbed and complete material replacement for the deposits on the plates is achieved. The algorithm thus compensates for older or hot batteries that typically charge to lower voltages and newer or cold batteries that typically charge to higher voltages using other known battery chargers. Therefore known battery chargers tend to overcharge older batteries and undercharge newer batteries. 
     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.