Fast charging method and apparatus for secondary cells

A fast charging method and apparatus for secondary cells, especially for lithium ion cells. The charging process has two charging phases. In the first charging phase, a constant current is supplied to the secondary cell from a charging apparatus whilst monitoring the output voltage of the charging apparatus. The first phase terminates when the output voltage of the charging apparatus reaches a predetermined maximum voltage (e.g., 4.2 volt for a lithium ion cell). Then, in the second charging phase, the output voltage of the charging apparatus being monitored and constant current pulses of fixed duration (e.g., 10 seconds) are supplied to the secondary cell in a manner such that the duration of intervals between such constant current pulses is controlled to maintain an average output voltage of the charging apparatus at a predetermined average voltage (e.g., 4.2 volt for a lithium ion cell). The charging is terminated when the duty cycle of the pulses falls below a predetermined value.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a method and apparatus for the fast charging of 
secondary cells such as lithium ion cells. 
2. Description of the Invention 
Secondary cells, in particular lithium ion cells are sensitive to 
over-voltage during charging. Any significant rise of the charging voltage 
above a prescribed maximum charging voltage is likely to reduce the cycle 
life of the cell. 
A charger is used for charging a secondary cell which is installed in an 
electric appliance. With a remote charger, such as is conventionally used 
with a portable appliance such as a mobile telephone, it is difficult to 
measure the cell voltage accurately, because of voltage drops in 
components connecting the charger to the cell. 
Accordingly, it has previously been proposed to operate a remote charger 
during an initial charging phase at a high level of charging current, 
whilst monitoring the output voltage of the charger, and to reduce the 
charging current when the output voltage reaches a predetermined level 
representing a 70% fully charged condition of the cell. At the reduced 
charging current, the voltage drops referred to are smaller and a more 
accurate estimate of the cell voltage is thus available based on the 
charger output voltage. With this arrangement, charging is made more 
protracted because of the reduction in the charging current. 
SUMMARY OF THE INVENTION 
It is the object of the present invention to provide a fast charging method 
and apparatus for charging secondary cells such as lithium ion cells in 
which this disadvantage is substantially overcome. 
In accordance with one aspect of the invention, there is provided a method 
of fast charging a secondary cell which comprises a first charging phase 
of supplying a constant current to the secondary cell from a charging 
apparatus whilst monitoring an output voltage of the charging apparatus, 
and terminating the first charging phase when the output voltage of the 
charging apparatus reaches a predetermined maximum voltage; and a second 
charging phase in which constant current pulses of fixed duration are 
supplied to the secondary cell, the output voltage of the charging 
apparatus being monitored and duration of intervals between such constant 
current pulses being controlled to maintain an average output voltage of 
the charging apparatus at a predetermined average voltage. 
Charging may be terminated when the duty ratio of the constant current 
pulses falls below a predetermined level. 
In accordance with another aspect of the invention there is provided a 
charging apparatus for use with a portable appliance having a secondary 
cell, the charging apparatus comprising a constant current source, a 
switching device connecting the constant current source to an output 
connection, means for sensing a voltage at the output connection, and a 
control device connected to the sensing means and controlling opening and 
closing of the switching device such that, in a first charging phase the 
switching device is maintained conductive until the voltage at the output 
connection reaches a predetermined maximum voltage, and in a second 
charging phase the switching device is rendered conductive in charging 
pulses of fixed duration, with an interval between the charging pulses 
being such that a average voltage at the output connection is maintained 
at a predetermined average voltage. 
In the present invention, the secondary cell is typically a lithium ion 
cell. 
The above and other objects, features and advantages of the present 
invention will be apparent from the following description referring to the 
accompanying drawings which illustrate an example of a preferred 
embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring firstly to FIG. 1, the charging apparatus 10 includes a PSU 
(power supplying unit) 11 which can be connected to a main supply 18 and 
provides a reduced voltage to a controller in the form of a CPU (central 
processing unit) 12 and to a constant current source 13. A switching 
device 14, preferably in the form of a semiconductor switch such as an FET 
(field effect transistor), connects the out of the constant current source 
12 to an output terminal 15 of the charging apparatus 10. 
The switching device 14 is controlled by the CPU 12, which receives an 
input from an analog-to-digital (A/D) converter 16. The input of A/D 
converter 16 is connected to a midpoint on a voltage divider resistor 
chain which has resistors 17a, 17b and is connected to the output terminal 
15. 
In use, the charging apparatus 10 in connected to a cell powered appliance 
20 such as a mobile telephone. As shown in FIG. 1, the appliance 20 
includes a lithium ion cell 21 which is shown as having virtual resistors 
22a, 22b connected in series with the cell 21. These resistors 22a, 22b 
represent resistance which occurs in the cell itself and in its 
connections to the circuitry rather than actual added resistors. The 
appliance 20 includes a load circuit 23 to which power is supplied from 
the cell 21. 
The operation of the charging apparatus is illustrated by the flow chart 
shown in FIG. 2. The flow chart indicates a software routine executed by 
the CPU 12. 
As shown, when the charging operation is started, the switching device 14 
is switched on at step 51 so that a constant charging current of a 
predetermined value, e.g. about xxx milliampere, starts to flow into the 
cell 21, and the voltage at the output terminal 15 is monitored by the CPU 
12 by periodically inputting the digitized output voltage value V into the 
CPU 12 from the A/D converter 16 at step 52 and checking it against a 
maximum voltage value Vmax of about 4.2 volt at step 53. If the output 
voltage V has not reached this level Vmax, another voltage sample is 
converted and fed to the CPU 12 by returning to step 52. This loop 
continues until the output voltage V reaches the maximum value Vmax at 
step 53. Then, the first phase of charging terminates and a second phase 
of charging starts. 
In the second phase of charging, the switching device 14 is closed and 
opened repeatedly. The duration of the switch closed interval is fixed at 
about 10 seconds (although longer or shorter fixed duration pulses may be 
utilized) so that a constant current pulse of 10 seconds is applied to the 
cell at step 55. Each time the switching device 14 is closed to in, a 
count n of the voltage sample periods is restarted at zero and a variable 
Vav is reset to zero at step 55. Then, the output voltage V is sampled at 
step 56. After each voltage sample is processed, the CPU 12 calculates, at 
step 57, the new average voltage Vav at the output terminal 15 since the 
current pulse cycle commenced by multiplying the existing valve of Vav by 
n addling the latest sample value V and dividing the result by n+1. The 
CPU 12 increments the value of n at step 58 and tests whether the value of 
n has reached a predetermined maximum value Nmax at step 59. If it has 
not, the CPU 12 compares, at step 60, the value of Vav with a 
predetermined desired average value Vs, typically also 4.2 volt. If the 
calculated value Vav is greater than the desired value Vs the routine 
loops back to the sampling step of step 58. If the value of Vav is less 
than Vs at step 60 and the pulse duration has expired, the routine loops 
back to the point, i.e. step 55, at which a new 10 second pulse is 
initiated. 
In the early stages of the second phase of charging, the value of Vav may 
fail to reach the desired value Vs by the time the pulse ends, in which 
case, a new 10 second pulse is started immediately. As the cell voltage 
rises as it nears its fully charged condition, the time taken for the 
average voltage Vav to fall to Vs will increase, until the cycle length 
becomes so long that the count n reaches its maximum value Nmax indicating 
that the duty cycle of the pulses has fallen to such a low level that the 
cell 21 can be regarded as fully charged. When this occurs and is detected 
at step 59, the charging operation is terminated. 
It is found that the process described above adequately protects the cell 
from substantial over-voltage damage, whilst permitting full charging to 
be obtained quickly as compared with the prior art. It is not necessary to 
include any additional components in the appliance to protect the cell. 
It should be noted that during the second phase of charging, proper control 
is maintained of the charging process even if a load current is drawn by 
the load circuit 23. 
It will be understood that although the example described a single cell is 
employed in the appliance, the invention is also applicable to the 
charging of multiple cells connected in series. Further, it is obvious 
that the fast charging method and apparatus of the present invention can 
be applied to secondary cells other than lithium ion cells. The voltage 
values with which the output voltage is compared during the first phase 
and with which the average output voltage is compared during the second 
phase would be changed according to the type and the number of cells to be 
charged. 
It is to be understood that variations and modifications of the fast 
charging method and apparatus disclosed herein will be evident to those 
skilled in the art. It is intended that all such modifications and 
variations be included with in the scope of the appended claims.