This present non-provisional patent application hereby incorporates by references U.S. patent application Ser. No. ______ (20679-103), filed Mar. 29, 2011, entitled BATTERY CHARGING SYSTEM AND METHOD; U.S. patent application Ser. No. ______ (20679-104), filed Mar. 29, 2011, entitled INTELLIGENT BATTERY MANAGEMENT SYSTEM AND METHOD; and U.S. patent application Ser. No. 12/650,401 filed Dec. 30, 2009, entitled SYSTEMS AND METHODS FOR MANAGING CHARGE AND DISCHARGE FOR BATTERIES.
The present invention relates in general to an individual cell charger and a method of using it. It more particularly relates to an individual cell charger and method for charging multi-cell series connected batteries.
This section describes the background art of the disclosed embodiment of the present invention. There is no intention, either express or implied, that the background art discussed in this section legally constitutes prior art.
There is a need for an individual cell battery charger which is primarily to insure that all cells become fully charged and balanced in a fast expeditious manner. In this regard, battery cell balance is an issue during charging operations for multi-cell series connected batteries. This may be particularly true for Lithium ion cells. If one cell in a series string charges significantly differently from the others, it can cause performance to be degraded.
A conventional method of individual cell charging is to provide a charger for each cell comprised of an individual switcher with its associated transformer (or transformer winding), rectifiers, filters and control circuitry. For charging a multi cell battery pack this approach may be expensive to manufacture, may be bulky and may have limited reliability because it involves many parts.
In order to better understand the invention and to see how the same may be carried out in practice, non-limiting preferred embodiments of the invention will now be described with reference to the accompanying drawings, in which:
Certain embodiments of the present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all, embodiments of the invention are shown. Indeed, these embodiments of the invention may be in many different forms and thus the invention should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided as illustrative examples only so that this disclosure will satisfy applicable legal requirements. Like numbers refer to like elements throughout.
It will be readily understood that the components of the embodiments as generally described and illustrated in the drawings herein, could be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of the certain ones of the embodiments of the system, components and method of the present invention, as represented in the drawings, is not intended to limit the scope of the invention, as claimed, but is merely representative of the embodiment of the invention.
An individual cell charger and method of using it are disclosed. The voltage of each individual battery cell of a series connected cell configuration is measured. A controlled power source charges the cells individually at an initial voltage. The voltage on each individual one of the charged cells is subsequently measured by a battery management system. The voltage of the controlled power source is incrementally increased to charge the cells individually at an incrementally higher voltage. The measuring and charging of each individual cell is repeated until the voltage on at least one of the cells reaches a predetermined voltage. Once the voltage on at least one of the cells reaches a predetermined voltage, the measuring and charging each individual cell continues at substantially the same last incremental voltage.
According to an embodiment of the present invention, an individual cell charger is provided which is greatly simplified as compared with a conventional approach, partly due to its integration with a Battery Management System, or BMS, resulting in a small, inexpensive component count.
A conventional BMS usually has the capability to measure the voltage of each individual cell in a multi-cell battery. Its usual function is to act on the monitored data to avoid over charging or over discharging any of the cells. It is this voltage measuring capability that makes it useful when integrated with the present embodiment of the individual cell charger of the present invention.
An embodiment of the invention relates to an individual cell charger, which measures voltage increases on all of the cells and then charges alternating ones of a first set of the cells only. The charger then determines that voltage increases on all cells are within predetermined limits. The charger then switches to alternating ones of a second set of the cells and charges them with a higher charging current. This cycle continues until at least one of these cells has reached the predetermined limit. The one or more cells having reached the predetermined limit are no longer charged, and the remaining cells are continued to be charged at the last charging current. This charging cycle is continued until all of the cells reach the pre-determined limit.
By utilizing the disclosed embodiments of the invention, the individual cell charger ensures that each cell becomes fully charged during each charge cycle. According to the prior art, where the series connected cells were charged across the entire series and not individually, individual cells could lag behind and never become fully charged. This is particularly true for lithium ion batteries. In this manner, the battery could be determined to be discharged prematurely due to the lagging cell not being fully charged, even though the remaining cells were not fully discharged and still capable of functioning. Also, such a lagging cell which does not become fully charged during each charge cycle, may require early replacement.
Considering now the drawings with reference to
A direct current power source 261 supplies current through an H bridge switcher 263 under the control of a switcher control 264 to the primary winding 265 of a transformer 267. The switcher control 264 causes the duty cycle of the switcher 263 to create alternating current flowing through the primary 265 of the transformer 267. A secondary winding 269 of the transformer 267 is connected across a double rail pair of conductors 270 and 271 to provide alternating current to a set of capacitor coupled bridge rectifiers 272, 274, 276 and 278 for supplying rectified current individually to the cells 252, 254, 256 and 258, respectively.
A battery management system (BMS) 279 measures the voltage on each cell during a charging operation and can selectively connect charging current to the cells by selectively coupling the charging current to individual ones of the cells or disconnecting or decoupling the charging current from cells reaching the predetermined voltages. A control signal lead 266 from the BMS 279 to the switcher control 264 causes the switcher 263 to increase or decrease its duty cycle incrementally. The BMS 279 may be similar to the battery management system disclosed in U.S. patent application Ser. No. 12/650,401, filed Dec. 30, 2009, which is incorporated herein by reference.
Each one of the bridge rectifiers such as the bridge rectifier 272 is coupled between the double rail conductors 270 and 271 by capacitors such as a pair of capacitors 281 and 283 for the rectifier 272. The capacitors 281 and 283 are each preferably low effective series resistance capacitors, such as functional polymer electrolytic capacitors. Each one of the bridge rectifiers such as the bridge rectifier 272 includes four diodes such as the diodes 285 through 288 for the bridge rectifier 272. Each bridge rectifier supplies energy to its cell such as the cell 252 (cell A) by a switch such as a switch 289 which are controlled by the battery management system 297 by ON/OFF CONTROL leads such as the lead 303. A pair of small current sense resistors 290 and 291 connect the bridge rectifier 272 to the cell 252.
A set of four low pass filters are provided for the four bridge rectifiers. The four low pass filters are similar to one another, and include a low pass filter 292 comprising a capacitor 293, resistor 294 and the resistor 305, for the bridge rectifier 272. The filtered output of bridge rectifier 272 is fed to the BMS 279.
The switcher 263 includes a set of four switches 295 through 298 connected in an H bridge configuration. A set of four diodes 299 through 302 are connected individually and are suitably poled across each one of the switches 295 through 298, respectively. The switcher control 264 selectively controls the switcher switches to alternately reverse current flow through the primary 265 of the transformer 267. For example, when the switches 295 and 297 are activated and the other two switches 296 and 298 are deactivated, current flows from the DC power source 261 through the switch 295, through the primary 265 and the switch 297, and back to the power source 261. Thereafter, the switches 295 and 297 are switched off for a time, and then later switches 296 and 298 are turned on, to provide the alternating current flow through the winding 265. There will be inductive current in the winding 265 when the switches turn off, and it flows through the diode 302 to the winding 265 and back through the diode 300 to the power source 261 just after switches 295 and 297 turn off. Likewise, current flows through diode 301, winding 265 and diode 299 after switches 296 and 298 turn off.
As shown in
The output of each bridge rectifier is fed through an off/on switch such as switch 289, and small value current sense resistors such as RA and RB (resistors 290 and 291, respectively), to its corresponding cell, such as cell A. It is also fed through the low pass filter such as the circuit including resistors RFA, RFB, and capacitor CFA to the cell voltage monitoring terminals of a BMS.
When charging is not taking place, no current flows through the bridge rectifiers and the filtered cell voltages are fed through to the BMS so it can perform its normal cell monitoring function. When charging is taking place, however, the switcher feeds an AC signal into the primary winding of the transformer T1, causing an AC output on its secondary. The secondary voltages are AC coupled to the bridge rectifiers such as the rectifier 272 including diodes DA, DB, DC, and DD, causing rectified current such as I1 to flow through them and through the cells such as cell A, providing it is switched on, thus charging the cell.
A concern, however, is that since all of the discharged cells may not be at the same voltage prior to charging, the lowest voltage cell limits the amplitude of the AC voltage from the transformer, and may draw current that is excessive, especially for the coupling capacitors and/or the bridge rectifiers. The disclosed embodiments of the present invention are designed to avoid, or at least greatly reduce this situation.
According to the method of the disclosed embodiments of this invention, prior to charging, the switcher 263 is off, and the BMS 279 monitors the voltage of all cells to establish a baseline. It then switches on alternate cells, such as cell A and C and then initiates the switcher 263 at its lowest or initial duty cycle by means of the switch control 264 in response to the control signal from the BMS 279 while monitoring the cells.
The BMS 279 then determines which, if any of the voltages have increased in voltage. An increase would occur if a current such as I1 flowed from a cell's rectifiers through the small current sense resistors 290 and 291. The measured voltage, V1M, would be the sum of the cell voltage V1 plus the voltage drop across the current sense resistors caused by the charge current neglecting the internal cell resistance, which in this case is small by comparison. If the voltage increase is within a preset limit, then therefore the amount of charge current is satisfactory and the process is then repeated. This time an alternate set of cells, such as cells B and D are switched on, and the cells A and C are switched off. It is then determined whether or not the voltage has increased within the preset limit, and the charge current is satisfactory. If in either case the voltage increase exceeds the limit, the BMS 279 concludes that a cell is faulty such as being shorted, and the BMS then stops the charge cycle, and provides a status indication.
The voltage limit is set by calculating a voltage increase based on a target value for cell charge current, such as I1, where I1=(V1M−V1)/(RA+RB). It is assumed that no current flows into the BMS 279 through resistors RFA and RFB, and they therefore have no steady state influence on the measured voltage V1M.
If the voltage increase is within the limit for both sets of cells, the BMS 279 increases the switcher duty cycle by one increment to increase the overall current flow, and repeats the above process. It continues to do so until the voltage increase of at least one cell has reached the predetermined limit and therefore its current is at its predetermined limit. Thereafter, as charging continues at that current level, the voltage on that cell continues to increase until it is close to a voltage on another. At that point, some of the charge current will begin to flow into other cells. Thus, the current in the first cell will start to decline. Subsequent measurements will detect that, and the charge current increases until at least one cell is being charged at its maximum current. This means that the maximum acceptable current is flowing into at least one cell, and no other cells are drawing excessive current.
The BMS 279 then turns on all cells, and charges at the last duty cycle setting for a predetermined period of time, preferably 5 seconds. After this charging has occurred, the cell voltages may have increased, so the charging is stopped and the cell voltages are again read to establish a new baseline.
Next, the switcher 263 is restarted at the same duty cycle that it left off and voltage increases are again measured in two groups, as before. If the increases are less than the maximum acceptable value, the duty cycle is incremented up again and measurements taken until at least one more cell reaches the maximum. If the increases are above the maximum value the duty cycle is decremented until they are all within the limit.
As the cycle is repeated, and the lowest voltage cell or cells become partially charged, the transformer voltage will be at successively higher values, and current begins to flow into other cells that had higher initial voltages, as well as continuing to flow into the cell or cells that initially had the lowest voltage. This process continues until the baseline voltages of all the cells are at the full charge value whereupon the charging algorithm is complete and charging stops. At this point the cells are also balanced.
In accordance with a presently preferred embodiment of the invention, the voltage increase measurements may be taken with pairs of alternate cells only, on at one time to avoid current flowing into an adjacent cell and causing an error. For example, in
Although the invention has been described with reference to the above examples, it will be understood that many modifications and variations are contemplated within the true spirit and scope of the embodiments of the invention as disclosed herein. Many modifications and other embodiments of the invention set forth herein will come to mind to one skilled in the art to which the invention pertains having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention shall not be limited to the specific embodiments disclosed and that modifications and other embodiments are intended and contemplated to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.