Method and apparatus for battery charging

Abstract

The present invention concerns a method and apparatus for battery charging. The method comprises the steps of connecting a battery to a battery charging apparatus and supplying a voltage and current to the battery, wherein a constant voltage is supplied for an extended period of time. At the beginning of the constant voltage being supplied, the initial charging current being supplied to the battery is noted (Amp). During the period in which a constant voltage is supplied, the rate of change of the charging current supplied to the battery (dl/dt) is monitored. The method includes calculating the ratio: K=Amp/(dl/dt) during the period in which a constant voltage is supplied, and when K equals a preselected value, maintaining the charging current at the instant value for an extended period of time.

Claims

1. A method of charging a battery, the method comprising the steps of: connecting a battery to a battery charging apparatus; supplying a voltage and current to the battery, wherein a constant voltage is supplied for an extended period of time; noting, at the beginning of the constant voltage being supplied, the initial charging current being supplied to the battery (I=Amp); during the period in which a constant voltage is supplied, monitoring the rate of change of the charging current supplied to the battery (dl/dt); calculating the ratio:
K=Amp/(dl/dt) during the period in which a constant voltage is supplied; when K equals a preselected value, maintaining the charging current at the instant value for an extended period of time.

2. A method as claimed in claim 1, the method including the step of applying a constant current charge prior to applying the constant voltage charge.

3. A method as claimed in claim 2, the method including the step of applying a constant current charge until the cell voltage in the battery reaches a set regulation voltage.

4. A method as claimed in claim 3, wherein the regulation voltage is selected from a range of values between 2.30V pc and 2.50V pc.

5. A method as claimed in claim 2, where the value of the initial charge is chosen to be between 20% and 70% of the capacity of the battery.

6. A method as claimed in claim 5, wherein the constant current is selected to be 20% of the capacity of the battery.

7. A method as claimed in claim 5, wherein the constant current is selected to be 70% of the capacity of the battery.

8. A method as claimed in claim 5, including the step of choosing one of a range of constant current values available.

9. A method of calibrating a battery charging method, the battery charging method as described according to claim 1, the method comprising the steps of: performing a series of battery discharge and charge cycles, the battery being connected to a test bench; in each charge cycle, charging the battery such that a charge factor of one is reached, monitoring the value of K=Amp/(dl/dt), and recording the K value when the charge factor equals one, and storing the K value when the charge factor equals one for use in the battery charging method as claimed in claim 1.

10. A battery charging system, the battery charging system comprising a battery charging apparatus configured to supply a battery charging profile in accordance with the method as described according to claim 1.

Description

DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which:

(2) FIG. 1 shows a graph of a battery charging profile according to a first embodiment of the invention;

(3) FIG. 2 shows a battery charging system arranged to supply a battery charging profile as shown in FIG. 1; and

(4) FIG. 3 shows a flow chart of battery charging steps according to a first embodiment of the invention; and

(5) FIG. 4 shows a table with test results relating to a calibration process according to a second embodiment of the invention.

DETAILED DESCRIPTION

(6) FIG. 1 shows an example charging profile according to the invention. FIG. 2 shows a battery charging system which is arranged to provide a charging profile as shown in FIG. 1.

(7) FIG. 2 shows a battery charging system comprising a battery charging apparatus 100 connected to a battery 102 via battery charging leads 104. A shunt 106 is provided on the battery charging leads and is arranged to measure the current and voltage being supplied to the battery 102. The battery charging apparatus 100 comprises a control unit 108 which controls the current and voltage being supplied by the battery charging apparatus 100 to the battery 102. The control unit 108 also includes a central processing unit (CPU) 110 which is arranged to set the appropriate charging profile for the charging process. The CPU 110 is also arranged to monitor the K value during the charging process, and adjust the charging process accordingly.

(8) FIG. 1 shows an IUI charging profile (constant current, constant voltage, constant current) for charging a 12 volt forklift truck battery. The charging current, generally indicated by the reference 10, moves between three phases, as indicated by the letters A, B, and C. Firstly, in phase A, the charging current has a constant phase 12 where the current is approximately constant with time, followed by a decreasing phase 14 (indicated as phase B), where the current value decreases with time, and a final constant phase 16 (indicated as phase C), where the current is approximately constant with time. The voltage, generally indicated by the reference 18, of the charging profile also varies across the three phases, with an increasing phase 20 (phase A), where the voltage increases with time, a constant phase 22 (phase B), where the voltage is approximately constant with time, and a final increasing phase 24 (phase C), where the voltage increases with time. The voltage indicated on the scale on the right hand axis of the graph is the voltage for a single cell, so the values on this scale have to be multiplied by 6 for a 12V forklift truck battery. As can be seen, the initial constant current phase 12 corresponds with the increasing voltage phase 20, the decreasing current phase 14 corresponds with the constant voltage phase 22, and the final constant current phase 16 corresponds with the final increasing voltage phase 24.

(9) The value of the initial charging current 12 is determined based on the speed of charging requirements. The battery charging apparatus may include more than one battery charging profile, for example, a normal charge profile, and a fast charge profile. The value of the initial charge may be chosen to be between 20% and 70% of the capacity of the battery. Typically, for a normal charge, the value of the initial charging current is set to be approximately 20% of the capacity of the battery being charged. If a faster charging time is required, the initial charging current 12 may be set to be approximately 70% of the battery capacity. The voltage supplied by the battery charging apparatus increases in order to maintain the constant charging current 12, as can be seen in FIG. 1. When the voltage across the battery cells reaches a regulation voltage U1, the battery charging process moves into the constant voltage phase, indicated in FIG. 1 by the reference B. The regulation voltage is set by the manufacturer of a battery for use during the charging of the battery, and in the case shown in FIG. 1, U1 is approximately 2.32V pc (or 13.92V for a 12V battery).

(10) The current and voltage being supplied by the battery charging apparatus 100 is monitored by a shunt 106 located on the battery charging connection 104. Alternatively, the shunt 106 may be located on the battery charging apparatus 100. The value of the current 12 being supplied by the battery charging apparatus when the battery charging profile moves into the constant voltage phase B, is noted as being I=Amp. During the constant voltage phase, the current 14 supplied by the battery charging apparatus is constantly decreasing. The rate of decrease, dl/dt, is monitored by the CPU 108 associated with the shunt 106 located on the battery charging connection 104. The CPU 108 calculates a ratio K, where K=Amp/(dl/dt) during the constant voltage phase B. When the K value matches a predetermined value, the charge factor of the battery is approximately 1 (plus or minus 1.5 percent). In the example shown in FIG. 1, the value of K when the charge factor is approximately 1, is 35. At this point, the battery charger maintains the current at the instant value (approximately 5 A in the present example), without voltage limitation, to complete the charge. This is shown as phase C in FIG. 1. The length of time the current 16 is maintained at the constant value, with the increasing voltage 24 is calculated based on the battery reaching the optimum charge factor that has been determined for that battery type. For example, it may be a charge factor of 1.03.

(11) An advantage provided by the invention is giving a simple method by which a battery being charged may be charged to a state in which the charge factor is 1, without needing to know the depth of discharge of the battery prior to the initiation of the charging process. Once the battery has reached the charge factor equals 1 position, it provides a fixed point from which the optimum final charge factor may be reached.

(12) The applicant has found that the method of monitoring the constant voltage phase of the charging process allows the charge factor to be determined independently of the depth of discharge of the battery, the age of the battery, and the battery temperature during the charging process. The method also provides a method of monitoring and controlling the battery charging process without requiring there to be any communication between the battery 102 being charged and the battery charging apparatus 100. Instead, the battery charging process may be monitored and controlled based on the readings taken by a shunt 106 located on the battery charging connector 104.

(13) The value of K that indicates the charge factor of a battery has reached 1 is determined by running tests on that battery type. The battery is connected to a discharge bench, where the battery is discharged, and the depth of discharge is monitored. The battery is then charged according to the charging profile shown in FIG. 1. As the depth of discharge is known, it is straightforward to determine when the charge factor is 1. Therefore, by testing a battery under various different conditions, varying the depth of discharge and the temperature of the battery, the value of K can be determined, as shown in the table in FIG. 4, which represents such a test charging and discharging process. The K value may then be applied to a normal charging process, where the depth of discharge of the battery being charged is not known.

(14) FIG. 3 shows a flowchart including various steps of the invention, according to the first embodiment. The first step 200 comprises connecting a battery to be charged to the battery charging apparatus 100. Then a constant current is applied to the battery in step 201. Once the regulation voltage U1 has been reached across the battery cells, the battery charging apparatus 100 starts supplying a constant voltage to the battery in step 202. The initial charging current applied when the constant voltage is applied is noted as I=Amp in step 204. The rate of change of the current (dl/dt) as the constant voltage is applied is monitored in step 206. A ratio, K=Amp/(dl/dt) is calculated during the constant voltage phase in step 208. If K is not equal to a predetermined value, x, then the constant voltage continues to be applied to the battery as shown in the flowchart. If K=x, then the current being applied to the battery is kept constant at the same value as when K=x, for a set period of time, as shown in step 210. Once the constant current has been applied for a set time period, the charging process finishes 212.

(15) FIG. 4 shows a series of test results obtained during a calibration process according to a second embodiment of the invention. A battery is connected to a test bench, and discharged and charged a number of times. As can be seen from the table, there are six cycles, the first five with a depth of discharge of 60 percent and the final with a depth of discharge of 100 percent. The charge rates and voltage regulation are also shown, together with the temperature of the battery at the start of the charge, when the battery starts a constant voltage phase, the battery temperature at the end of the battery charge, and the maximum battery temperature during the charge. It can be seen that when the ratio K is equal to minus 35, the charge factor of the battery ranges from 1.012 to 99.4. Therefore, regardless of the temperature of the battery, or the depth of discharge, a K value of minus 35 returns a battery with a charge factor of approximately 1. Therefore, the K value of minus 35 may be applied when charging a similar battery according to the method of the first embodiment of the invention such that the battery is charged to a charge factor of approximately 1, using only a shunt monitoring the current and voltage supplied to the battery via the battery charging apparatus.

(16) Whilst the present invention has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein. By way of example only, certain possible variations will now be described.

(17) The use of the K value may be used in any charging profile where there is a constant voltage stage. For example, the method may be applicable to a constant power, constant voltage, constant current (WUI) charging profile, or a constant current, constant voltage, constant current (IUIo) charging profile, or a constant current, constant voltage, constant current, constant voltage (IUIUo) charging profile.

(18) Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims. Moreover, it is to be understood that such optional integers or features, whilst of possible benefit in some embodiments of the invention, may not be desirable, and may therefore be absent, in other embodiments.