Charging method for storage batteries

The invention relates to a charging method for storage batteries, in particular for NiCd and NiMH cells. According to the invention, the rise in the charging voltage during each of a number of consecutive charging cycles of a specified duration is measured in the form of a unit corresponding to a certain voltage. The number of these units is transmitted in the form of a counting pulse to two up-down counters during the period of the charging cycle, whereby one counter functions in the down mode and is set at the beginning of a charging cycle to a number corresponding to the number of units established in the preceding charging cycle. In the following charging cycle, the roles of the two counters are reversed, so that the up-counter is now set to the down mode and the other counter to the up mode, whereby the latter is set to a Certain starting counting status. At the end of each charging cycle, the final status of the down counter is compared with the specified starting status, from which a switch-off criterion is derived for the charging process. According to the invention, this process enables both positive and negative changes in the dU/dt value of the charging curve to be detected with a high voltage resolution and, at the same time, minimal cost.

BACKGROUND OF THE INVENTION 
The invention relates to a charging method for batteries, in particular for 
NiCd (nickel-cadmium) and NiMH (nickel hydride) cells, in which the 
batteries are supplied from a charging current source. 
A number of such charging methods are known to date (cf. "Intelligent 
charging of NiCd batteries" from Design und Elektronik 22, 1992, pages 
106-107), all of which are based on the objective of providing a low-cost, 
optimum battery charging concept. The intention is to ensure that the 
battery is charged as nearly as possible to its rated capacity, while at 
the same time avoiding overcharging, which could shorten its service life. 
The charging time also constitutes a criterion for evaluating the process, 
in that prolonged charging periods are no longer accepted. On the one 
hand, rapid charging with high-strength current considerably reduces the 
charging time. On the other hand, however, the charging period must then 
be precisely adhered to, otherwise the battery may be damaged beyond use. 
This means that a battery charger is required with a control unit which 
constantly monitors the charming current and the state of charge of the 
battery. Thus, various processes are known which monitor the dU/dt pattern 
of the charging curve, from which a switch-off criterion is derived. 
Typically, the charging process can be terminated when the battery voltage 
falls, i.e. with a negative du/dt value, or even with a pronounced 
increase in the battery voltage, i.e. with a positive dU/dt value. Up to 
now, the monitoring of the voltage gradient in a charging curve of this 
type has only been carried out with the aid of considerable circuitry 
resources, which in turn are reflected in high-cost batteries. 
SUMMARY OF THE INVENTION 
The object of the invention is to provide a charging method of the type 
initially described for the detection of the dU/dt pattern of the charging 
curve relating to the battery to be charged, which can be achieved by 
means of modest circuit technology. 
According to the invention, the rise in charging voltage in the battery 
being charged is established as a unit of measurement corresponding to a 
certain voltage during consecutive charging cycles of a specified period 
of time. The number of these measured units is transmitted in the form of 
counting pulses, during the period of a charging cycle, to two up-down 
counters, of which one counter operates in the down mode and is set to a 
numerical value at the beginning of a charging cycle corresponding to the 
sum obtained from a pre-defined starting counter status and the number of 
units measured in the preceding charging cycle. In the following charging 
cycle, the roles of the two counters are reversed, so that the up-counter 
is now set to the down mode and the other to the up mode, the latter being 
set to the specific starting counter status mentioned above. After each 
charging cycle, the final counter status of the down counter is compared 
with the pre-defined starting status by the generation of a differential, 
from which a switch-off criterion is derived for the charging process. 
Since the down counter always starts at an initial counter status, which 
contains a measurement of the rise in charging voltage in the previous 
charging cycle, i.e. representing the dU/dt value, in which dt represents 
the length of the charging cycle, the differential between the two 
designated counter statuses represents the value of the change in the 
dU/dt value over two consecutive charging cycles. In mathematical terms, 
this differential therefore represents a measurement of the second 
derivation of the charging curve over the two consecutive charging cycles. 
In this way, a limit can be established which determines whether the 
charging process is continued or should be regarded as completed. This 
limit, which represents a positive natural number, thus lays down the 
change in the dU/dt value over two consecutive charging cycles at which 
the charging process is terminated. 
According to the invention, this process is particularly suitable for those 
types of batteries whose charging curve exhibits two turning points and a 
maximum, e.g., NiCd or NiMH batteries. With the rapid charging of 
batteries of this type, the charging voltage rises continuously until it 
is fully charged at the maximum of the charging curve, when approximately 
110% of the rated capacity is reached. If charging continues beyond this 
maximum, the electrical energy delivered is only converted into heat. With 
an NiCd battery, the voltage maximum is considerably more pronounced than 
with an NiMH battery; in particular, the negative gradient of the voltage 
curve at the conclusion of charging does not always reliably occur. As a 
result, with the process according to the invention, the transition from a 
shallow gradient to a pronounced gradient in the area of the second 
turning point of the charging curve can be reliably detected by an 
appropriate choice of limit. 
In an advantageous development of the process according to the invention, 
equidistant points in time are defined for the measurement of the voltage 
rise during each charging cycle. At each of these time stages, a 
comparison is made between the charge voltage and a reference voltage, 
whereby the reference voltage is increased in accordance with a 
predetermined voltage rise by the voltage value specified as the unit of 
measurement, This new reference value serves as the reference value in the 
subsequent time stage. In this way, each increase of a unit of measurement 
is transmitted to the two counters as a counting pulse. It is preferable, 
with this development of the process according to the invention, to begin 
with a value for the reference voltage which corresponds to the charging 
voltage immediately at the start of the charging cycle. 
Since, with the charging of batteries having a charging curve complying 
with the aforementioned pattern, it is essentially a question of detecting 
the rise in the charging curve to the maximum point, i.e., the area about 
the second turning point of the curve, it is possible with another 
embodiment of the process according to the invention to start the charging 
process with a pre-charging process, whereby a comparison of the charging 
voltage with a reference voltage is carried out after the beginning of the 
charging at equidistant time stages. If the charging voltage at a specific 
time stage coincides with the reference voltage, the two up-down counters 
are activated, i.e., the actual evaluation of the change in the dU/dt 
value: over two consecutive charging cycles only now begins, whereby the 
two counters, which function in the reversed mode in the process described 
above, receive counting pulses corresponding to the units of measurement 
representing the value of the increase in charging voltage, so as to 
enable the counter statuses at the end of a charging cycle to be 
evaluated. 
The charging process is now continued with the same strength of charging 
current. If, by contrast, the charging voltage at this particular time 
stage is greater than the reference voltage, the reference voltage is 
increased by a specific voltage and thus forms the reference value for the 
comparative measurement at the following time stage. Ideally, the voltage 
provided for the increase in the reference voltage can be of the same 
value as the voltage specified for the unit of measurement. In this way, 
the area of the charging curve which is of importance relative to the 
termination of the charging process is most quickly reached. 
In order to prevent an erroneous value for the charging voltages from being 
used for the comparative measurement processes, the charging process is 
interrupted at the measurement time stages. In this way, the measured 
charging voltage corresponds only to the electrochemical cell potential 
and contains no resistance components based on the conductor resistance, 
electrode resistance or electrolyte resistance. 
In order to ensure that the battery is also effectively fully charged, the 
charging process is concluded with a so-called top-off charge, which takes 
place with a reduced charging current. This post-charging process takes 
place for a specified period of time and only after it is concluded is the 
entire charging process terminated.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIG. 1, a process control unit 3 is shown which, when connected to a 
battery to be charged 1 initially tests marginal conditions such as the 
temperature or the presence of a short circuit, and only after a 
successful test result is the charging current released for a certain 
period of charging time, typically 20 s. At the end of this first charging 
phase, the charging current is switched off for some 100 ms. During this 
charging current interval, the actual value Uact for the battery voltage 
is compared with a reference value Uref with the aid of a comparator 2. 
This reference voltage Uref is generated by a D/A converter 7, which 
converts a digital value supplied by a counter 6 into a corresponding 
analog value, i.e., the reference value Uref. 
If, at the first charging current interval, the actual value Uact is 
greater than the reference value Uref, a further charging phase of the 
same duration follows, but the reference value Uref is adjusted by a 
certain amount. For this purpose, a converter adjustment logic (4), which 
is connected in series to the comparator 2, triggers a circuit 5 to start 
the counter 6 at a certain number of steps which corresponds to a minimum 
voltage interval dUmin of 5 mV. In this way, after every charging phase, 
if the actual value Uact is greater than the reference value Uref, the 
reference value Uref is increased on each occasion by one voltage interval 
dUmin. When the actual value Uact and reference Uref coincide, the 
precharging phase switches to the main charging phase, in that the process 
control unit 3 triggers the two up-down counters 9 and 10. In this way, 
the charging process is continued with the same strength of charging 
current. 
Thus, when the comparator 2 indicates to the converter adjustment logic 4 
that the actual value Uact coincides with the reference value Uref, the 
converter adjustment logic 4 gives clearance to an up-down counter control 
unit 8 to start the two up-down counters 9 and 10. These two four-bit 
counters 9 and 10 serve to identify an increase in the gradient of the 
charging voltage and deliver their respective counter status to an up-down 
discriminator 11, which carries out an evaluation of the counter statuses. 
The main charging phase which now follows takes place on a cyclical basis 
and is interrupted after 160 s, during which charging intervals the 
evaluation of the counter status is carried out by one of the two counters 
9 or 10. These intervals are shown in FIG. 2 as t1 to t4. Each charging 
cycle, lasting 160 s, is itself cyclically interrupted again, with the 
result that 8 charging phases, each of approximately 20 s duration, are 
created. In each of the seven charging intervals thus arising, the actual 
value Uact and the reference value Uref are compared in turn by the 
comparator 2. 
At the beginning of the main charging phase, i.e. at the beginning of time 
stage t1, as per FIG. 2, both the first counter 9 and the second counter 
10 are set to a predefined starting condition, in this case at numerical 
value 8. In this connection, the first counter 9 is initially set to the 
up mode and the second counter 10 to the down mode. If, in a charging 
interval, the actual value Uact is greater than the preset value Uref, the 
D/A converter 7 is adjusted. At the same time, the up-down counter control 
unit 8 receives an adjustment pulse from circuit 5, which is transmitted 
by circuit 8 to the two counters 9 and 10 as a counting pulse, whereby the 
first counter counts one step up and the second counter one step down. 
This process is repeated seven times, until the first counter typically 
shows counter status 11 and the second counter shows counter status 5. At 
time stage t2, the first counter 9 is set to the down mode and the second 
counter to the up mode, while simultaneously being set to its starting 
counting mode, i.e., numerical value 8. The charging current is now 
interrupted again after every 20 s, a comparison of the actual value is 
made with the reference value, and if necessary, the reference value Uref 
readjusted. After every adjustment stage, this leads to a step-wise 
reduction in the counter status of counter No. 1 and a step wise increase 
of the status of counter No. 2. At the end of the second charging cycle, 
i.e. at time stage t3, counter No. 1 will typically indicate counter 
status 7 and counter No. 2 will indicate counter status 12. 
The up-down discriminator 11 now carries out its first evaluation, in that 
initially the differential between the final counter status e of counter 
No. 1 and the starting counter status a is formed. In the present case, 
this differential is as follows: 
EQU m=/a-e/=1. 
This differential m shows the relative change in the gradient of the 
charging curve by comparison with the preceding 160 s charging cycle. In 
the present case, this signifies that the gradient during the second 
charging cycle has increased by the relative value 1 by comparison with 
the first charging cycle. 
In this way, it is possible to determine a specific value for differential 
m, at which the rapid charging process should be terminated. A practical 
value for the differential has proved to be m=3. Thus the switch-off 
criterion becomes: 
EQU /a-e/.gtoreq.m. 
During the third charging cycle between time stages t3 and t4 as per FIG. 
2, this switch-off criterion is reached at time stage t4. At time stage 
t3, the first counter is first returned to the up mode and simultaneously 
set to a starting value of a=8, while the second counter is switched to 
the down mode. As shown in FIG. 2, readjustment stages were required 
during this third charging cycle 7, with the result that the final counter 
status of the first counter is 15, while the final counter status e of the 
second counter is 5. Thus the differential amounts to m=3, fulfilling the 
aforesaid condition, with the consequence that the rapid charging process 
is terminated. If, by contrast, the switch-off condition is not met, a 160 
s charging cycle is repeated until a switch-off criterion is obtained. 
If the gradient of the charging curve increases constantly, the 
differential a-e is also positive. If the charging voltage gradient 
reduces instead of increasing, the final counter status e is Greater than 
the starting counter status a, i.e. differential a-e is negative. Thus, 
with the aforementioned switch-off condition, the rapid charging process 
is also terminated here. This switch-off criterion is reached when the 
charging voltage has exceeded its maximum. 
FIG. 3 shows a voltage curve during the charging of NiCd and NiMH 
batteries. With the charging process according to the invention, factor m 
which determines the switch-off criterion is adjusted in such a way that 
the rapid charging process is switched off before the maximum point of the 
curve is reached. This occurs when a pronounced increase gradient is 
present between charging cycles or, in mathematical terms, when the second 
derivative of the charging curve exhibits a high value. The second 
switch-off criterion, at which the maximum has already been exceeded, i.e. 
differential a-e is negative, does not normally come into effect, since 
the first criterion, i.e., when the differential a-e is positive and value 
m is exceeded, occurs beforehand. For this reason, this second switch-off 
criterion is only co-evaluated for reasons of safety. 
In this way, both the switch-off criteria can also be evaluated with 
different factors, in contrast to the above method, i.e. 
EQU a-e.gtoreq.m.sup.1 and 
EQU e-a.gtoreq.n, 
whereby m' and n are positive whole numbers. 
In order to mask out sudden voltage changes in the battery, a shift 
register (a so-called 3-bit plausibility shift register) can be connected 
downstream of the comparator 2. Only when the same converter adjustment 
signal is output by the comparator three times in succession does the 
converter adjustment logic 4 receive a clock signal, as do also the two 
counters 9 and 10. If the shift register previously indicates the status 
"000", only 5 out of the possible number of adjustment clock signals 
within a 160 s charging cycle can effect a change in the two counters 9 
and 10 and the D/A converter 7. In the counter diagram shown in FIG. 2, no 
allowance is made for a 3-bit plausibility shift register of this type, 
since this is only intended to explain the charging process according to 
the invention. 
On the completion of the rapid charging process, the residual capacity of 
the battery is carefully charged at a reduced charging current (Top-off 
charging), the period of this post-charging process being capable of being 
limited to approx. 20 minutes. On the completion of the post-charging 
process, there follows a so-called holding charging process, during which 
the strength of the charging current is again reduced. The holding 
charging process is continued until the battery is removed from the 
battery charger.