Method of determining and displaying battery charge status

A method of determining and displaying the charge status of a storage battery of a battery-operated appliance, such as a razor. A time-based charge status is calculated, based at least in part on the cumulative operating time of the appliance since recharging. The battery terminal voltage is also monitored. The time-based charge status is displayed until the battery voltage indicates a low voltage condition, at which point a low-charge alert indicator is displayed only if the time-based charge status is below a predetermined level (i.e., if the cumulative operating time is above a predetermined level). Methods of detecting battery aging, and for accounting for periods of excessive current flow, are included.

BACKGROUND OF THE INVENTION 
This invention relates to a method of determining the charge status of a 
storage battery, and to a method of determining the aging of a storage 
battery. 
More accurate determinations are needed of when the low-charge status of a 
storage battery is actually reached. This low-charge status equates to a 
terminal voltage corresponding to a residual capacity of, for example, 10 
to 20 percent of the rated capacity of the battery. With this residual 
charge it is still possible to operate the power-consuming device 
reliably. The battery voltage drops as its capacity (i.e., its charge) 
drops. 
If the power-consuming device places a higher than normal load on the 
storage battery (i.e., if a greater amount of current flows through the 
storage battery), the storage battery's terminal voltage will drop at an 
accordingly higher rate that with a current of normal magnitude. Inside 
the storage battery itself a voltage drop occurs that is equal to the 
product of the internal resistance of the storage battery and the current. 
From this it results that a drop in the storage battery's terminal voltage 
equivalent to a drop in capacity of the storage battery can also be 
observed when the current increases. If, therefore, the terminal voltage 
is used as the sole criterion for determining the storage battery's 
capacity, an accordingly far advanced discharge status of the storage 
battery will be assumed to exist when lowering of the voltage is caused by 
a comparatively large current through the power-consuming device, even 
though its capacity is comparatively well above the low-charge point. In 
this case the storage battery still has a comparatively large capacity. 
A prior art method for determining and displaying charge status is 
disclosed in DE-OS 41 31 981 A1 (U.S. Pat. No. 5,623,193), which describes 
how the operating time of an electrical appliance, particularly a shaving 
apparatus, is monitored with a counter. This prior art method features the 
activation of segments of a display in accordance with the operating time, 
indicating to the user the current charge status of the storage battery or 
batteries. If a voltage discriminator detects that the measured terminal 
voltage of the storage batteries has reached a predetermined lower 
threshold value, then the readout of the display is set to a condition 
indicative of a charge status that corresponds to the lower threshold 
value and a counter presetting a specified residual operating time of the 
electrical appliance is started. After this residual operating time has 
elapsed, the electrical appliance is switched off by appropriate 
activation of an electronic switching device. This switching off can be 
effected by instant switch-off or by a slow reduction of power, thus 
resulting in the case of a shaving apparatus in a slow reduction of speed. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to improve the determination and 
display of the charge status of storage batteries in battery-operated 
appliances. 
According to one aspect of the invention, a method is provided for 
determining and displaying charge status of a storage battery connected to 
a power-consuming device through which the storage battery is discharged 
to a predetermined residual charge. The method includes the steps of 
calculating a time-based charge status corresponding to cumulative 
operating time of the power-consuming device, comparing terminal voltage 
of the storage battery to a predetermined lower voltage threshold value, 
and displaying a predetermined low charge alert indicator on a display 
only when both the time-based charge status is at or below a predetermined 
lower limit and the terminal voltage is below the lower voltage threshold 
value. 
In some instances, the method includes comparing the frequency at which the 
lower threshold value is lowered to a predetermined frequency, and, when 
the frequency at which the lower threshold value is lowered exceeds the 
predetermined frequency, displaying a message signaling that the 
power-consuming device requires service. This can alert the operator when 
an adaptation of the variation of the characteristic curve is no longer 
suitable to reliably maintain the function of the appliance, and that the 
storage battery has to be replaced. 
In some embodiments, the method also includes lowering the predetermined 
lower voltage threshold value by a specified amount when the terminal 
voltage falls below the lower voltage threshold value before the 
time-based charge status reaches its predetermined lower limit. In this 
manner, the method can advantageously adapt the detection of the 
low-charge point to the variation of the characteristic curve of the 
storage battery's terminal voltage with advancing discharge as a function 
of time. 
In some embodiments the method also includes comparing the frequency at 
which the lower threshold value is lowered to a predetermined frequency, 
and, when the frequency at which the lower threshold value is lowered 
exceeds the predetermined frequency, setting, in the power-consuming 
device, a fault code readable during servicing. Thus, the servicing 
operation can be simplified because the corresponding fault condition is 
then quickly identified in the course of servicing, without having to 
subject the appliance to a complete function test. 
Some embodiments include the steps of, once the low charge alert indicator 
is displayed, waiting a preset length of time, and then displaying a 
"zero" charge status and disconnecting the power-consuming device from the 
storage battery unit by means of a controllable switching device. By 
automatically disconnecting the power-consuming device from the storage 
battery, the function of the appliance is suspended demonstrably to the 
user on account of the draining of the storage battery at a defined 
moment, and the storage battery is subjected to less strain when it is not 
deep discharged to complete exhaustion. This residual charge of the 
storage battery enables components such as microprocessors or ASICs, which 
require a minimum voltage to perform certain basic functions, to run for 
some further time. This time can amount to a magnitude of several weeks. 
Preferably, the power-consuming device is switched off by decreasing power 
consumption of the power-consuming device to zero over an extended period 
of time. This is advantageous with an electric shaver or hair cutter, for 
example, because the risk of hairs being caught by a sudden switching off 
of the appliance can be eliminated. The reduction of speed can extend over 
several seconds. 
In some preferred embodiments, the decrease in power consumption of the 
power-consuming device generally follows near-discharge voltage discharge 
characteristics of the storage battery. This can enable the user to 
identify the noise of the appliance's suspension of function when the 
storage battery is discharged and to identify (even without observing the 
display at that particular moment) why the function of the appliance has 
been suspended. 
In some cases the method includes, once the low charge alert indicator is 
displayed, indicating on the display a time remaining until the preset 
length of time has elapsed. This can provide a simplified means of 
indicating to the user the residual running time of the appliance as 
accurately as possible when the storage battery is nearing the end of its 
discharge. 
In some embodiments, the low charge alert indicator is displayed only when 
both the time-based charge status is at or below a predetermined lower 
limit and the terminal voltage is below the lower voltage threshold value, 
and when electrical current flowing from the storage battery is determined 
to be below a predetermined current limit, such as a current limit 
corresponding to an expected maximum current flow under normal operating 
conditions. 
According to another aspect of the invention, a method of determining and 
displaying charge status of a storage battery connected to a 
power-consuming device includes the steps of: 
(1) calculating a time-based charge status based at least in part on 
cumulative operating time of the power-consuming device since a latest 
recharge; 
(2) comparing terminal voltage of the storage battery to a predetermined 
lower voltage threshold value; 
(3) displaying via a charge status indicator the calculated time-based 
charge status until the time-based charge status is at or below a 
predetermined lower limit and the terminal voltage is below the lower 
voltage threshold value; and, 
(4) only when both the time-based charge status is at or below a 
predetermined lower limit and the terminal voltage is below the lower 
voltage threshold value, displaying a low-charge status indicator. 
According to another aspect of the invention, a method of determining and 
displaying charge status of a storage battery connected to a 
power-consuming device includes the steps of monitoring cumulative 
operating time of the power-consuming device since a latest recharge; 
monitoring terminal voltage of the storage battery; and, only when 
cumulative operating time is above a predetermined upper limit and 
terminal voltage is below a predetermined lower limit, displaying a 
low-charge status indicator. 
According to another aspect of the invention, an improved battery-operated 
appliance is provided, employing the methods disclosed herein for 
determining and displaying the charge status of a battery of the 
appliance. In some embodiments, the appliance comprises an electric shaver 
or hair cutter. 
The invention features the advantageously combined evaluation of charge 
status as determined from terminal voltage (voltage-based charge status) 
and the charge status established from the operating time of the 
power-consuming device (time-based charge status), resulting in an 
improved indication of the storage battery's low charge status. 
If, with a calculated time-based charge status above its predetermined 
lower limit, the storage battery's terminal voltage drops below the 
predetermined lower threshold value, this is taken to mean that the 
storage battery is being loaded momentarily with a higher current, and 
that the storage battery's charge status has not yet reached the 
corresponding low-charge point. The falling of the terminal voltage to a 
value below the predetermined lower threshold value is ignored, therefore, 
in this case. The charge status indicated on the display thus continues to 
be the charge status as determined on the basis of the cumulative 
operating time of the storage battery. 
If, on the other hand, the storage battery's terminal voltage falls below 
its predetermined lower threshold value when the determined time-based 
charge status has already reached a predetermined lower limit, it is 
possible to conclude with relative certainty that this drop in terminal 
voltage is attributable to a drop in capacity of the storage battery. In 
this case the readout on the display is reset to indicate a charge status 
of the storage battery in accordance with the lower voltage threshold 
value. 
In some cases it is advantageously possible to conclude, by evaluating the 
current, whether the drop in the storage battery's terminal voltage is 
directly attributable to a drop in the storage battery's capacity, or 
whether the drop in terminal voltage is owed to a higher current. In the 
former case it is deduced that the storage battery's charge status has 
reached the value corresponding to the detected low terminal voltage, and 
the readout of the display is reset to indicate this low voltage-based 
charge status. In the latter case the time-based charge status will 
continue to be indicated on the display. 
However, one advantage of some embodiments of the invention is that it can 
provide improved determination of charge status without measuring and 
evaluating current. 
The invention can also provide a means for detecting battery aging. This 
may be accomplished by recording the actual effect of the internal 
resistance of the storage battery through evaluation of the current, 
thereby enabling the actual reaching of the low-charge point to be 
reliably detected by the simultaneous recording of the terminal voltage. 
Aging of the storage battery exists when this actual low-charge point is 
reached while the time-based charge status indicates a more or less large 
residual capacity. It is considered that the corresponding charge status 
at the recorded low-charge point deviates from the time-based charge 
status and that this deviation only occurs after a period of time when 
this is to be expected as the result of normal aging of the storage 
battery. It is considered that this deviation occurs with a certain 
frequency and regularity. It is also considered whether there is any 
reason for a high load current that could explaining the deviation. 
An embodiment of the invention is presented in greater detail in the 
accompanying drawings. From the context it will be understood that the 
term "storage battery" means a storage battery unit which may be comprised 
of one or more cells.

DETAILED DESCRIPTION OF EMBODIMENTS 
Referring first to FIG. 1 of the drawings, a load 2 is concerned to a 
battery (accumulator) B by means of an On/Off switch S and an electronic 
switching device 21. The battery B can be recharged by means of a charging 
circuits not shown. The electronic switching device 21 is driven by a 
pulse-duration modulator 4 via a driver circuit 3. 
A counter 5 records the time period during which the load L, which has an 
essentially constant power consumption, is connected to the battery B via 
the switch S, the count starting from a fully charged condition of the 
battery. 
The rated capacity of the battery B being also known, a display 9 
indicating the charge status of the battery is controlled by the counter 5 
on a time basis. After a predetermined connection time has elapsed which 
is determined on the basis of the known power consumption and the known 
rated capacity of the battery B, such that the battery then contains only 
a low residual charge, counter 5 will deliver an output signal. At this 
point, a charge indicator on display 9 will indicate the charge status 
"zero." This signal will cause the electronic switching device 21 to be 
driven. Driving may be accomplished by a pulse-duration modulator 4 which 
gradually reduces the pulse duty factor for the control voltage down to 
zero, thus also gradually reducing the current flowing through the load 2. 
An oscillator 6 provides the clock pulses for the counter 5 and the 
pulse-duration modulator 4. 
Referring to FIG. 2, load 2 is a direct current motor M of an electric 
shaving apparatus, for example. The series arrangement comprising the 
motor M and the switching transistor 21 is applied to the battery B via 
the On/Off switch S when the device is set in operation. A diode 22 
connected in parallel with the motor M prevents inductive voltage peaks. 
The base of the switching transistor 21 is driven by the output of the 
pulse-duration modulator 4 via a driver circuit 3 comprised of transistors 
31, 36 and resistors 32, 33, 34, 35, 37, 38. 
Connected directly in parallel with the battery B (i.e., bypassing the 
On/Off switch S) are a voltage discriminator 8 and counter 5. The voltage 
discriminator 8 determines when the battery terminal voltage reaches a 
predetermined terminal voltage U corresponding to a residual battery 
capacity of 10 to 20 percent of its rated (full) capacity. This 
predetermined terminal voltage threshold is called the "low charge" 
voltage level. Until this low charge level is reached, the display 9 will 
indicate the charge status calculated on the basis of the preceding time 
period during which the motor M has been connected to the battery B (the 
"time-based" charge status). 
In the prior art method, as soon as the voltage discriminator 8 has 
detected the low charge level of the voltage U across battery B, counter 5 
and a second indicator on display 9 are started. When the detected 
low-charge level is attained, the motor has, for example, only eight 
minutes of operation left until battery B is completely depleted. The time 
period predetermined for counter 5 is then six minutes, for example, in 
order to preclude complete discharging of the battery. This period of six 
minutes may be subdivided into six fixed time intervals of equal length so 
that each minute one segment of the second indicator of display 9 is 
driven. These segments may also indicate the minutes remaining until 
expiration of the predetermined time period. After the predetermined time 
(six minutes) has elapsed (i.e., when counter 5 has reached zero), 
pulse-duration modulator 4 will be driven. 
Pulse-duration modulator 4 then reduces the pulse duty factor of the 
control voltage supplied to the driver circuit 3, such as in the manner 
shown in FIG. 3. The individual pulse duty factors of FIG. 3 are assigned 
numerals from 1 (full control voltage) through 9 (zero control voltage). 
FIG. 4 illustrates the rotational frequency (as a percent of the rated 
speed) plotted against these pulse duty factors, with the pulse duty 
factors 1 to 9 corresponding to those shown in FIG. 3. The curve 
corresponds essentially to the voltage curve of a battery with the load 
switched on until it is completely discharged. 
In this manner, deep discharging of the battery is avoided. Because an 
abrupt stopping of the motor is prevented, the risk of hair being 
painfully snagged in a shaver or hair clipper by accident is reduced. It 
will be appreciated that this effect can be achieved with an analog 
circuit as well as with the digital circuit described above. 
As will be explained more fully below with respect to FIGS. 5-7, the method 
of the invention helps to assure that the storage battery's charge status 
actually corresponds to this low-charge voltage U detected by voltage 
discriminator 8 before counter 5 and the second indicator of display 9 are 
started. 
In the flow sequence of FIG. 5 it is checked whether the low-charge point 
of the storage battery is actually reached at the instant of time when the 
storage battery's terminal voltage falls below the predetermined lower 
threshold value. In step 501 it is checked, therefore, whether the storage 
battery's terminal voltage has reached or fallen below the predetermined 
lower threshold value. If the answer is no, the routine branches to step 
505 in which the readout of the display is controlled in accordance with 
the cumulative operating time of the power-consuming device recorded by 
the counter (i.e., in accordance with the calculated "time-based" charge 
status). 
If the terminal voltage of the storage battery has fallen below the 
predetermined lower threshold value, the routine proceeds to step 502 in 
which it is checked whether the calculated time-based charge status has 
reached or fallen below a predetermined lower limit. If the low-charge 
point of the storage battery is set at 10 or 20 percent of the rated 
capacity of the storage battery, for example, the value of this lower 
limit of the time-based charge status can correspond to a 30 to 40 percent 
residual capacity of the storage battery, for example. 
If the check in step 502 turns out negative, it can be concluded that an 
abnormal operating situation is present. It is possible to trace this to 
certain operating situations that can be indicated likewise to the user. 
This is shown in more detail in FIG. 7 dealing with step 504. 
At step 505 the storage battery charge status continues to be indicated on 
the display in accordance with the calculated time-based charge status. 
If the check in step 502 turns out positive, the routine proceeds to step 
503 in which the readout of the display is reset to show a charge status 
of the storage battery that corresponds to the storage battery terminal 
voltage in accordance with the lower threshold value of the storage 
battery terminal voltage at normal load (the "voltage-based" low charge 
status). 
The sequence outlined in FIG. 6 is an alternative to the sequence of FIG. 
5. In step 601, as in step 501 of FIG. 5, the storage battery terminal 
voltage is compared to its predetermined lower threshold value. 
If the battery voltage is still above its lower threshold value, the 
routine branches to step 607 in which the readout of the display (the 
charge indicator) is controlled in accordance with the calculated 
time-based charge status. 
If battery voltage is at or below the predetermined lower threshold value, 
the routine proceeds to step 602 in which the current flowing with the 
switch "ON" is compared to a predetermined current limit value 
corresponding to the current consumption of the power-consuming device 
under normal operating conditions. 
If the measured current is lower than the predetermined limit value, in 
step 603 the time-based charge status is compared to a predetermined lower 
limit. If the low-charge point of the storage battery is set at 10 to 20 
percent of rated capacity, for example, the value of this lower limit can 
correspond to 30 to 40 percent of the rated battery capacity. 
If the comparison made in step 603 reveals that the time-based charge 
status has not yet reached its predetermined lower limit, this is 
interpreted as an indication that the storage battery has aged, that the 
characteristic curve of battery voltage as a function of time has shifted 
to lower voltage values with progressive discharge. The routine of FIG. 7 
is then initiated in step 604. The flowing of a high load current is 
considered in the routine of FIG. 7 as a possible explanation for this 
operating condition. This explanation of FIG. 7 only applies, however, to 
the extent that the routine of FIG. 7 is called from the routine of FIG. 
5. As called from FIG. 6, the high current possibility is already excluded 
by the query in step 602. Thus with the routine of FIG. 7 included in step 
604, the comparison of step 701 could actually be omitted. However, in 
order to rule out a faulty indication regarding the aging of the storage 
battery due to a non-recurring deviation of the calculated charge status 
from the actual charge status of the storage battery, we recommend 
including the comparison of step 701 even when called from FIG. 6. 
Subsequent to step 604 and also in the event of the answer "no" upon the 
comparison in step 603, the routine proceeds to step 605 in which the 
readout of the display is reset to indicate the voltage-based charge 
status corresponding to the lower threshold value of the battery voltage 
under normal load. 
It is also conceivable, however, to omit steps 603 and 604, proceeding 
directly to step 605 in the event of a positive comparison in step 602. 
A comparatively large load current is flowing if the check in step 602 
turns out negative. With an electric shaver this may be attributable, for 
example, to the user having a relatively strong beard or alternatively, to 
a relatively advanced state of soiling of the shaver. In this case a 
signal can be issued in accordance with step 606 to indicate to the user 
that the shaver is soiled. Since this operating condition and its cause 
are not unequivocally repeatable, however, it is also conceivable to 
combine this signal with signals generated elsewhere in order to obtain 
more certain evidence and to subject the signal to appropriate evaluation 
in accordance with step 606, for example by driving a readout or by 
suitably adapting the motor speed. 
At step 607 the time-based charge status continues to be indicated on the 
display. 
In one preferred embodiment, fixed time counter 5, oscillator 6 and pulse 
duration modulator 4 are components of a single microprocessor, resulting 
in a simple implementation of the methods of FIGS. 5 and 6. It is then 
also possible to readily calculate time-based charge status by 
accumulating the operating time in the microprocessor. Advantageously, the 
display indicator is divided into several segments, corresponding to a 
certain percentage charge of the storage battery. The storage battery's 
charge can then be indicated by means of the individual segments in steps 
of, for example, 20% of rated charge. As the progressive discharge of the 
storage battery is monitored, the corresponding segments of the display 
are used for the indication. Provision is also made for additional display 
indicators which can be employed when the low-charge status is reached. In 
this connection it is also possible to indicate how many shaves' worth of 
charge remain instead of showing how many minutes of operating time are 
left before the shaving apparatus is switched off. For this purpose it is 
possible to employ the microprocessor to evaluate how much time the user 
needs for an average shave. On the basis of this average shaving time it 
is then possible to convert a residual operating time of the shaving 
apparatus into a remaining number of shaves. 
FIG. 7 shows a detailed expansion of steps 504 and 604 of FIGS. 5 and 6, 
respectively. The situation at this point of the routine in FIG. 5 is that 
battery voltage has reached or fallen below its predetermined lower 
threshold value, but the time-based charge status has not yet reached or 
fallen below its predetermined lower limit. 
The flowing of a comparatively large load current is a possible reason for 
this situation. With an electric shaver this may be attributable to the 
user's relatively strong beard, for example, or also to a relatively 
advanced state of soiling of the shaver, as discussed above. 
Another explanation is that the storage battery has aged, causing the 
characteristic curve of the storage battery's voltage as a function of 
time to shift to lower voltage values when the storage battery discharges. 
This means terminal voltage, which in a new storage battery reaches a 
predetermined lower threshold value only at an advanced state of 
discharge, will in an aged storage battery reach the same threshold at a 
higher state of residual charge. 
A sign of aging of the storage battery is, for example, when the routine in 
FIG. 5 branches to step 504 for the first time after several years of use 
of the battery-powered appliance. It is also conceivable to deduce aging 
of the storage battery if the routine branches to step 504 several times 
during a single charge/discharge cycle, it being possible for this 
branching to occur repetitively on successive executions of the routine. 
It is also conceivable to combine this criterion of repeat occurrence with 
the criterion of first-time occurrence after the appliance has been in use 
for several years. If additionally a signal is present indicating a 
certain degree of soiling of the appliance, detected independently of the 
method herein described, it is conceivable to interpret a branching to 
step 504 as a detection of storage battery aging only when the degree of 
soiling of the appliance does not exceed a predetermined threshold. 
It is also possible to take account of the aging of the storage battery by 
lowering the predetermined lower threshold value by a specified amount. 
This amount is preferably in the range of from 40 to 50 mV when the 
predetermined lower threshold value for a new storage battery is 2.38 
volts. Better coordination between time-based and voltage-based charge 
status is thus possible for further operation. Since the battery capacity 
available to a connected power-consuming device decreases as the aging of 
the storage battery increases, the time intervals of the counter as well 
as the value of the fixed time counter can be correspondingly adapted to 
improve the time-based charge status. 
If the predetermined lower threshold value was lowered at or above a preset 
frequency, it is taken to indicate that the aging of the storage battery 
is so far advanced as to necessitate battery replacement. It is possible 
for this preset frequency to be set so low that the storage battery is 
determined to need replacement after the first branch to step 504; it is 
also possible, however, for the present frequency to be a triple lowering, 
for example, of the predetermined lower threshold value. It is then 
possible for the aging of the storage battery to be indicated at least 
indirectly by driving an indicating element signaling to the user that the 
shaving apparatus requires servicing. During this servicing the 
possibility may exist to read a memory inside the shaver in which a code 
indicative of a defective storage battery has been entered. By virtue of 
this fault code it is then possible without any great effort to identify 
the storage battery as a source of trouble during troubleshooting and to 
replace it before the shaving apparatus becomes unusable. If the shaving 
apparatus contains appropriate devices to identify other sources of 
trouble, other fault codes indicative of the source of trouble can also be 
entered in the memory. When one of the these fault codes is entered, the 
indicating element which signals to the user that the appliance requires 
servicing is then activated likewise. Hence for the user it is only 
evident that the appliance needs to be serviced. During servicing the 
respective fault can then be found quickly by evaluation of the individual 
fault code, without the appliance having to undergo a complete 
examination. 
If the check reveals that branching to step 504 is attributable to the 
flowing of a relatively high load current, it is possible to produce a 
signal indicating to the user a soiled condition of the shaver, for 
example. However, since this condition may have other causes, such as a 
particularly strong beard, it may be preferable to combine this signal 
with signals generated elsewhere in order to obtain more certain evidence 
and to subject the signal to appropriate evaluation. This evaluation can 
consist, for example, of a readout being driven or the motor speed being 
suitably adapted, signalling to the user the need for shaver maintenance 
(e.g., that it needs a cleaning). 
Referring to FIG. 7, step 701 consists of an evaluation of whether or not 
there is sufficient indication that the battery has excessively aged, 
based on one or more criteria. The evaluation criteria can include, for 
instance, whether the branching to step 504 occurs for the first time 
after several years of using the appliance, whether the branching to step 
504 occurs repeatedly during discharge and each intermediate recharge of 
the storage battery, and/or whether the soiling of the appliance does not 
exceed a predetermined threshold. 
If it is determined that the battery has not excessively aged, the routine 
branches to step 705 in which, preferably in combination with a further 
signal 706, the user is notified that cleaning is necessary, or 
alternatively, the motor speed of the appliance is suitably adapted. 
If the check in step 701 turns out positive, the predetermined lower 
threshold value is lowered in accordance with step 702 in order to take 
account of the aging of the storage battery. 
In step 703 it is then checked whether a preset frequency has been reached 
in the lowering of the predetermined lower threshold value. This check can 
be made in particular by increasing a counter for this lowering in step 
702 for each instance of lowering, this counter being then checked in step 
703. If the count is at a preset maximum value, which in particular may 
lie in the range from 2 to 5, the routine process to step 704. 
A fault code, which is indicative of the progressive aging of the storage 
battery and can be read during servicing of the appliance, is set in step 
704. At the same time a readout is activated, signaling to the user the 
need to service the appliance. 
If the check in step 703 reveals that the preset frequency has not yet been 
reached, the routine of FIG. 7 is ended.