Battery charging system with battery type and polarity discrimination

A battery charging system detects if multiple battery cells to be recharged include rechargeable, secondary batteries or non-rechargeable, primary batteries. If the system detects that any one of the cells is a primary battery, then the charging system halts the charging operation. If all of the cells are secondary batteries, but the polarity of any battery cell is incorrect, then the charging system halts the charging process.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to rechargeable batteries and, more particularly, to 
systems for charging of rechargeable batteries and preventing charging of 
non-rechargeable batteries. 
2. Description of the Related Art 
Most conventional battery-operated consumer products utilize 
non-rechargeable, or primary, batteries that are available in standard 
battery cell sizes. Primary batteries are batteries such as carbon-zinc or 
alkaline batteries that must be discarded when fully discharged. 
Rechargeable, or secondary, batteries are batteries such as nickel-cadmium 
batteries that can be repeatedly charged and discharged for a significant 
number of cycles before they must be discarded. Most primary batteries, if 
subjected to the charging current provided by a charging circuit, will 
become damaged and will possibly damage the product or device in which the 
battery is placed. 
Traditionally, secondary batteries have been constructed with terminals and 
case configurations and sizes that are incompatible with the primary 
batteries that can be obtained by retail customers. More recently, 
secondary batteries have become available with terminals and in 
configurations and sizes identical to those of standard primary batteries. 
That is, primary and secondary batteries are no longer physically 
distinguishable and are now interchangeable. This interchangeability makes 
it easy for a consumer to inadvertently place a primary battery into 
either a product with a battery charging circuit or a dedicated battery 
charger device with a charging circuit and then inadvertently attempt to 
recharge the primary battery. 
Efforts have been underway to prevent such inadvertent charging of primary 
batteries. For example, U.S. Pat. No. 4,577,144 to Hodgman et al., 
describes a battery charger that electrically determines if a battery cell 
being recharged is a primary battery or a secondary battery. Charging of a 
primary battery cell is inhibited and charging of a secondary battery cell 
is permitted. 
Battery chargers that electrically discriminate between a primary battery 
cell and a secondary battery cell incorporate a desirable safeguard, but 
generally are configured for recharging only a single battery cell at a 
time. It would be advantageous if chargers that electrically discriminate 
between primary and secondary batteries could recharge more than one cell 
at a time. The ability to charge multiple battery cells simultaneously, 
however, introduces the possibility of installing a mixture of primary and 
secondary batteries into a charging circuit. If the user attempted to 
charge the mixture, damage to all of the batteries could occur. Present 
chargers are not configured electrically to detect a mixture of primary 
and secondary battery cells. 
It also would be advantageous to electrically determine if a battery cell 
were installed in a charging circuit with the incorrect polarity. 
Incorrect polarity also can damage battery cells. The ability to charge 
multiple battery cells simultaneously also introduces the possibility of 
installing secondary batteries with mixed polarity. Thus, it would be 
advantageous to charge multiple battery cells simultaneously without 
risking damage to the batteries or the associated product or charger due 
to mixing types of batteries or installing with incorrect polarities. 
From the description above, it should be apparent that there is a need to 
provide a battery charging system that recharges multiple secondary 
battery cells simultaneously and prevents charging primary battery cells 
mixed in with secondary battery cells and prevents charging battery cells 
installed with incorrect polarities. The present invention satisfies this 
need. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a battery charging system, 
adapted to receive one or more battery cells having first and second 
electrical contacts, includes a battery charging circuit having first and 
second terminals coupled to the contacts of the battery cell(s) and 
operating in a charging mode and a non-charging mode, a battery 
discriminating circuit that detects the voltage across the battery cell(s) 
during the charging mode to place the charging circuit in the non-charging 
mode if the detected voltage indicates a primary battery cell is present, 
and includes a battery polarity discriminating circuit, coupled to the 
charging circuit, that electrically detects when the first and second 
contacts of the battery cell(s) are connected with the incorrect polarity 
to the first and second terminals of the battery charging circuit and 
operates the battery charging circuit in the non-charging mode. 
The battery charging system can be advantageously included within a 
charging cradle that mates with a product containing a plurality of 
rechargeable battery cells. The charging system detects if the user is 
attempting to charge primary batteries or secondary batteries and detects 
if the battery cells are installed with the correct polarity. If primary 
batteries are indicated, then charging is halted. If secondary batteries 
are indicated, but polarity is incorrect, then charging is halted and an 
indicating signal is produced. 
Other features and advantages of the present invention should be apparent 
from the following description of the preferred embodiment, which 
illustrates, by way of example, the principles of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
A charging system in accordance with the present invention can be 
incorporated into a dedicated battery charger or can be part of a 
product/charger combination including a product that accepts secondary, or 
rechargeable, battery cells and a specially designed charging cradle that 
mates with the product. FIG. 1 and FIG. 2 illustrate a charging system 
embodied in a camera 10/charging cradle 22 combination constructed in 
accordance with the present invention. FIG. 1 shows a camera 10 that 
receives battery cells 12 in a battery receiving well located on the 
underside of the camera, shown covered by an access door 14. The camera 
produces photographs through an objective lens 16 and includes, for 
example, an electronic flash unit 18 that receives power from the battery 
cells 12. The camera 10 includes a pair of electrical contacts 20 through 
which a charging current can be delivered to the battery cells. The camera 
10 mates with a charging cradle 22, which includes a pair of electrical 
terminals 24 that are coupled to the camera electrical contacts 20 when 
the camera is mated with the cradle 22. The operating status of the 
charging system is indicated by an indicator light 26 on the charging 
cradle. When the camera 10 is mated with the charging cradle 22, the 
charging circuitry contained in the cradle determines if the battery cells 
12 are primary or secondary batteries. If any one of the battery cells is 
a primary battery, then the system does not proceed with charging and the 
indicator light 26 flashes on and off. If all of the battery cells are 
secondary batteries, but any one of the cells is installed with incorrect 
polarity, then the system does not proceed with charging and the indicator 
light 26 is not illuminated. If the battery cells are secondary batteries 
and are properly installed with the correct polarity, then the system 
proceeds with charging and the indicator light 26 is steadily illuminated. 
In this way, the charging system 10, 22 detects primary and secondary 
batteries, detects correct polarity, and permits charging to proceed only 
if all battery cells are secondary batteries and are installed with the 
correct polarity. 
FIG. 3 is a schematic diagram of a charging system 30 constructed in 
accordance with the present invention. The numerical values shown adjacent 
the resistors are the resistance values, in ohms, of the preferred 
embodiment and therefore are exemplary only. With respect to the exemplary 
camera 10 and charging cradle 22 combination illustrated in FIG. 1 and 
FIG. 2, the battery cells 12 to be charged are electrically connected to 
the camera contacts 20, which are then engaged with the charging cradle 
terminals 24. The electrical connection to the flash unit 18 also is 
represented in FIG. 3 by an additional terminal 25. Electrical power is 
produced for the charging system 30 by typical household voltage of 120 
volts AC (120 VAC). It is to be understood that other supply voltages also 
can be used with appropriate circuit changes known to those skilled in the 
art. The electrical power is provided to a transformer circuit 32 
including a transformer T1 that changes the household voltage to 5.8 VAC. 
This voltage is rectified into a direct current via two diodes, CR1 and 
CR2. The voltage from the first diode, diode CR1, is filtered by a 
capacitor C1 and supplies electrical power V.sub.f at a level of 
approximately 8.2 volts to the remainder of the charging system, which 
comprises a charging circuit 34. A resistor R1 and a zener diode VR1 
provide a reference voltage V.sub.z of 5.1 volts used for control purposes 
as described further below. The rectified voltage from the second diode, 
diode CR2, is current-limited by a resistor R2 and supplies electrical 
charging power to the battery cells 12 through a transistor Q2. The 
transistor Q2 is, for example, a 2N4401 transistor obtainable from Texas 
Instruments. The voltage across the battery cells when they are connected 
to the charging circuitry is represented by V.sub.batt. 
The charging circuitry 34 includes four operational amplifiers OA1, OA2, 
OA3, and OA4 that act as switching devices. The first operational 
amplifier OA1 is used as a master reset switch. The output of OA1 goes to 
a high state (8.2 volts) at any time that the battery voltage, V.sub.batt, 
is higher than the zener reference voltage V.sub.z (5.1 volts). A high 
output state for OA1 resets the third operational amplifier, OA3, by 
driving the positive input terminal of OA3 high. The charging circuitry 30 
also includes four voltage comparators, CMP5, CMP6, CMP7, and CMP8. CMP5 
and CMP6 also are reset by a high output state from OA1 by having their 
negative input terminals driven high. A resistive network 36 having four 
resistors R4, R5, R6, and R7 is supplied with the zener diode reference 
voltage V.sub.z and produces a set of secondary reference voltages that 
are determined in accordance with battery cells 12 that are assumed to 
have a fully-charged voltage of 1.5 volts each. 
The second operational amplifier OA2 is used as a reverse polarity 
detector. If the battery voltage V.sub.batt falls below an R5-R6 secondary 
reference voltage obtained from between resistors R5 and R6 of the 
resistive network 36 as shown, then OA2 will switch from a low output 
state to a high output state. The third operational amplifier OA3 is used 
as a battery type detector. The positive input terminal of OA3 is coupled 
to the battery voltage V.sub.batt through a capacitor C2. The negative 
input terminal of OA3 is coupled to an R6-R7 reference voltage obtained 
from between resistors R6 and R7 of the resistive network 36 as shown in 
FIG. 3. If the R6-R7 reference voltage from the resistive network is 
greater than the battery voltage, then OA3 will switch from a low output 
state to a high output state. 
The voltage comparator CMP8 is used to monitor the output signals from OA2 
and OA3. If either OA2 or OA3 switches from a low output state to a high 
output state, then CMP8 will detect a voltage that is higher than an R4-R5 
reference voltage obtained from between resistors R4 and R5 of the 
resistive network 36 as shown in FIG. 3 and the output of CMP8 will be 
shorted to ground. If OA2 and OA3 are both in a low output state, then 
CMP8 will detect a voltage lower than the fourth reference voltage from 
the resistive network, and the output from CMP8 will be left 
open-circuited. When the output of CMP8 is open-circuited, the base of the 
transistor Q2 will be at a positive voltage potential, as determined by 
resistors R14 and R15. This positive potential permits the transistor Q2 
to conduct current from the second resistor R2 to the battery cells 12, 
which are thereby charged. If the output of CMP8 is shorted to ground, 
then the base of the transistor Q2 is held in a low state and Q2 therefore 
is an off condition, halting any charging. 
The voltage comparator CMP5 is used to reset an RC timing network 38 
comprising a resistor R11 and a capacitor C3. If the output signal of OA1 
is in a high state, then the zener diode reference voltage V.sub.z 
maintains CMP5 in a grounded output condition, which permits the capacitor 
C3 to be charged. When the output of OA1 goes to a low output state, then 
the output of CMP5 is open-circuited. This permits the capacitor C3 to 
discharge through the resistor R11. The voltage comparator CMP7 monitors 
the voltage at the resistor R11 and capacitor C3. If the voltage at R11 
and C3 falls below the zener diode reference voltage, then CMP7 will be in 
an open-circuited condition. If the monitored voltage of R11 and C3 goes 
above the zener diode reference voltage V.sub.z, then the output of the 
voltage comparator CMP7 is shorted to ground. With the outputs of both 
voltage comparators CMP7 and CMP8 open-circuited, the base of the 
transistor Q2 reaches its most positive value through a resistor R15. With 
the output of CMP7 shorted to ground and the output of CMP8 
open-circuited, the base of the transistor Q2 is at less than its maximum 
positive value, as determined by the two resistors R14 and R15. The output 
of the voltage comparator CMP6 is shorted to ground if the output of OA1 
is in a high output state. The output of CMP6 is permitted to float to a 
predetermined value, set by resistors R12 and R13, if the output of OA1 is 
in a low state. Finally, OA4 is used to drive an indicating light, such as 
LED1, and is controlled by the voltage comparator CMP6 and the combined 
outputs of OA2 and OA3. 
If any one of the battery cells 12 is a non-rechargeable primary battery, 
then the operational amplifier OA3 will detect a higher AC-coupled battery 
voltage V.sub.batt than the R6-R7 reference voltage from the resistive 
network 36. That is, the values of the resistors R4-R7 are selected so the 
R6-R7 reference voltage, as shown in FIG. 3, discriminates between all of 
the battery cells 12 being secondary batteries and any one of the cells 
being a primary battery. The resistance values shown in FIG. 3 provide 
this discrimination. The high V.sub.batt causes OA3 to switch to a high 
output state for the positive portion of the AC cycle. A resistor R8 
provides positive feedback to the positive input terminal of OA3, which 
causes OA3 to remain locked in that high output state and ignore AC cycle 
fluctuations. The only way to clear this locked situation is to remove the 
battery cells 12 from the charging circuit, thus resetting OA1 to a low 
output state. As OA3 switches to a high output state, the second 
operational amplifier OA2 has switched to a low output state. This in turn 
causes the fourth operational amplifier OA4 to oscillate output states due 
to the charging and discharging of the capacitor C4. Therefore, the signal 
indicator LED1 will blink on and off at a rate determined by C4, R12, and 
R13. In addition, the voltage comparator CMP8 will switch to a grounded 
output condition, thereby shutting off the transistor Q2 by pulling the 
base of the transistor to ground and shutting off any charging. 
If one or more of the battery cells 12 is reversed from its correct 
polarity in the charging circuit, then the second operational amplifier 
OA2 will detect a lower battery voltage V.sub.batt than the R5-R6 
reference voltage supplied from the resistive network 36. That is, the 
values of the resistors R4-R7 are selected as shown in FIG. 3 to 
discriminate between all of the battery cells being installed with the 
correct polarity and any one of the cells being installed with the 
incorrect polarity. The resistance values shown in FIG. 3 achieve this 
discrimination. As a result of V.sub.batt being lower than the R5-R6 
reference voltage, OA2 will switch to a high output state. If OA2 is in a 
high output state, then CMP8 will switch to a grounded output state and 
will shut off the transistor Q2 by pulling the base of the transistor to 
ground. Charging will be halted. At the same time, OA3 will switch to a 
high output state, causing OA4 to switch to a low output state. Therefore, 
LED1 will not be illuminated. 
If the battery cells 12 are rechargeable secondary batteries and polarity 
is correct, then OA1 permits normal operation of the charging circuitry. 
If the battery voltage V.sub.batt rises above the R5-R6 reference voltage 
from the resistive network 36, then the OA2 output signal stays in a low 
output state. If V.sub.batt also does not rise above the R6-R7 reference 
voltage from the resistive network, then the OA3 output signal will stay 
in a low output state. The combination of outputs from OA2 and OA3 will 
switch OA4 to a high output state and therefore the indicating light LED1 
will be steadily illuminated through a current-limiting resistor R17. 
In another aspect of the present invention, a quick charge cycle is 
followed by a trickle charge cycle, which most efficiently charges 
secondary batteries. When the charging cycle is begun, the capacitor C3 is 
charged and begins to discharge through the resistor R11. The output 
signal of the voltage comparator CMP7 is held in an open condition while 
the capacitor C3 is discharging. This permits the maximum positive 
potential to be applied to the base of the transistor Q2 through the 
resistor R15, thereby providing a maximum current to the battery cells 12. 
When the capacitor C3 has discharged to a voltage level that is higher 
than the zener diode reference voltage level, then the output of CMP7 
switches to a grounded condition. This in turn reduces the positive 
potential applied to the base of the transistor Q2, thereby permitting a 
trickle charge of less than the maximum charge current to be applied to 
the battery cells. 
In summary, if non-rechargeable primary batteries are installed into the 
camera 10 when the camera is placed onto the charging cradle 22, then the 
third operational amplifier OA3 switches to a high output state as 
described above and the output of the voltage comparator CMP8 goes to a 
grounded condition, turning off the transistor Q2. This halts the charging 
of the battery cells 12 and flashes the indicating light LED1 on and off. 
If rechargeable secondary batteries are installed in the camera 10 when 
the camera is placed onto the charging cradle, but one or more battery 
cells is installed with the incorrect polarity, then the light LED1 is not 
illuminated. If all cells are installed with the correct polarity, then 
the third operational amplifier OA3 stays in a low output state and the 
output of CMP8 stays in an open-circuited condition, permitting the 
transistor Q2 to pass current determined by the resistors R14 and R15. 
This permits charging of the battery cells 12 and steadily illuminates the 
indicating light LED1. In addition, CMP7 permits a one-hour quick charging 
cycle and then reduces the current being passed through the transistor Q2 
for a trickle charge, as described above. 
It is to be understood that the charging circuitry described and 
illustrated in FIG. 3 can be embodied in a dedicated battery charger 
rather than a camera/charging cradle combination. Other alternatives, as 
well as alternative details of configuration, will occur to those skilled 
in the art. For example, the operational amplifiers OA1 through OA4 can be 
obtained in a single chip package LM 324 obtainable from National 
Semiconductor, Inc. and the comparators CMP5 through CMP8 can be obtained 
in a single chip package LM 339, also obtainable from National 
Semiconductor, Inc., but alternative packages and arrangements will occur 
to those skilled in the art. Similarly, the transistors Q1 and Q2 can be 
obtained from Texas Instruments, Inc. 
Thus, the present invention provides relatively simple charging circuitry 
that electrically detects whether multiple battery cells connected to the 
charging circuitry include one or more non-rechargeable primary batteries, 
in which case charging is halted. The charging circuitry also detects if 
one or more of the battery cells are installed with the incorrect 
polarity. Again, charging is halted. In this way, charging circuitry in 
accordance with the invention prevents damage to batteries, the product in 
which the batteries are installed, and the charging circuitry. 
The present invention has been described above in terms of a presently 
preferred embodiment so that an understanding of the present invention can 
be conveyed. There are, however, many configurations for charging systems 
not specifically described herein, but with which the present invention is 
applicable. The present invention therefore should not be seen as limited 
to the particular embodiment described herein, but rather, it should be 
understood that the present invention has wide applicability with respect 
to charging systems generally. All modifications, variations, or 
equivalent arrangements that are within the scope of the attached claims 
should therefore be considered to be within the scope of the invention. 
The following elements and their corresponding reference numerals are used 
in the drawings: 
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camera 10 
battery cells 12 
access door 14 
objective lens 16 
electronic flash unit 18 
electrical contacts 20 
charging cradle 22 
electrical terminals 24 
additional terminal 25 
indicator light 26 
charging system 30 
transformer circuit 32 
charging circuit 34 
resistive network 36 
RC timing network 38 
capacitor C1 
capacitor C2 
capacitor C3 
capacitor C4 
diode CR1 
diode CR2 
indicating light LED1 
operational amplifier OA1 
operational amplifier OA2 
operational amplifier OA3 
operational amplifier OA4 
resistor R1 
resistor R2 
resistor R4 
resistor R5 
resistor R6 
resistor R7 
resistor R8 
resistor R11 
resistor R12 
resistor R13 
resistor R14 
resistor R15 
resistor R17 
transformer T1 
transistor Q1 
transistor Q2 
voltage comparator CMP5 
voltage comparator CMP6 
voltage comparator CMP7 
voltage comparator CMP8 
zener diode VR1 
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