The present invention relates to a control device to control charging
and discharging of rechargeable cells for a battery operated device which consumes
power from the cells.
Rechargeable batteries are commonly used in electrical appliances
such as mobile phones, electric shavers and portable music systems. It is well
known that to maximise battery performance and to prolong the life of the battery,
as measured by the number of charge/discharge cycles, that the battery must be
fully discharged before being fully charged. However, for convenience, it is preferable
to keep the battery in a state of constant readiness, that is, always fully charged.
Therefore, it is common practice to charge a partially drained battery. This causes
the battery performance to deteriorate, and the battery is said to exhibit a "memory
The present invention seeks to prevent the deterioration of rechargeable
batteries due to improper charging.
According to the present invention there is provided a control device
to control charging and discharging of rechargeable cells for a battery operated
device which consumes power from the cells, comprising charging control means to
select a discharged cell and permit charging of said cell to a maximum charging
level and discharging control means to select a partially charged cell and permit
discharging to a discharged level.
The device may include measuring means for measuring the level of
charging of a cell, which can be a volmeter, and a comparing means for comparing
levels of charging.
The device may also include a switching means to switch between the
A battery operated device, for example a mobile phone, or a battery
pack may incorporate such a control device. The battery operated device may comprise
a battery pack incorporating the control device.
According to the present invention there is provided a method of controlling
charging and discharging battery cells comprising selecting a discharged cell and
permitting charging of said cell to a maximum charging level, and selecting a partially
charged cell and permitting discharging to a discharge level, prior to recharging.
Thus, only full charging and discharging of the battery cells are
performed which maximises their lifetime.
An embodiment of the present invention will now be described, by way
of example, with reference to the accompanying drawings, in which:
- Figure 1 illustrates a conventional mobile phone handset and a power supply
- Figure 2 illustrates the drop in the voltage across a typical rechargeable battery
as it is discharged;
- Figure 3 shows a mobile phone handset including a partitioned battery and a
control device in accordance with the present invention;
- Figures 4 shows two rechargeable batteries sharing the same battery housing;
- Figure 5 shows the control device circuits in accordance with the present invention;
- Figure 6 is a process flow diagram for selecting a battery;
- Figure 7 is a process flow diagram for monitoring charge left in the selected
- Figure 8 is a process flow diagram for determining each battery's suitability
to be charged.
Referring to Figure 1, a known mobile phone handset 1 comprises a
Nickel-Metal Hydride (NiMH) rechargeable battery 2 with a 1000 mAh capacity. The
battery is charged with power supplied from a power supply unit 3, which also serves
as a cradle to hold the handset steady during charging. Power is delivered from
male connecting members 4 of the power supply 3 to the battery 2 through female
connecting members 5.
Referring to Figure 2, the battery 2 can be safely charged to a maximum
charging voltage, Vmax., which is typically about 1.4 volts. When in
use, the handset 1 draws power from the battery 2 and the voltage, V, appearing
across the battery terminals falls. When most of the charge is drained, there is
a rapid drop in voltage. The battery 2 is effectively drained of charge and can
no long power the handset 1, once the voltage drop below a fully discharged voltage,
Vmin, which is typically 1.1 volts. The handset 1 then needs to be placed
in the power supply unit 3 to recharge the battery 2.
Draining a fully charged battery 2 such that the battery voltage,
V, drops from the maximum charging voltage, Vmax, down below the fully
discharged voltage, Vmin, is called a full discharge. Charging the battery
2 from below the fully discharged voltage, Vmin, to the maximum charging
voltage, Vmax, is called a full charge. A full discharge followed by
a full charge is called a full charging cycle. It is well known to those skilled
in the art that a full charging cycle maximises the lifetime of the battery. Typically,
a NiMH rechargeable battery has a lifetime of about 500 to 1000 cycles. After that,
the battery capacity falls and is incapable of holding a useful amount of charge
for any significant period of time.
The lifetime of the battery 2 is considerably shortened if the battery
is not charged using full charging cycles. Incomplete charge cycles can include:
- fully charging a partially discharged battery,
- partially charging a fully discharged battery and
- partially charging (i.e. not to full charge) a partially discharged battery
Repeated incomplete charging cycles leads to the battery 2 exhibiting
a "memory effect", in other words, showing signs of accelerated aging. The battery
2 becomes quickly discharged, so quickly in fact, that the battery 2 is rendered
Unfortunately, full charging cycles are not always practical for mobile
phone users and incomplete charging cycles are commonly performed. For example,
prior to a journey, outing or day at work, when the user expects to be make or
receive a calls and will not have access to a charger, the user will "top-up" a
partially discharged battery with further charge.
Referring to Figures 3 and 4, a first embodiment of the present invention
is a mobile phone handset 1 comprising a partitioned NiMH battery 6, which yields
first and second batteries 6a, 6b, each having approximately half the capacity
of the prior art battery. The partitioned battery 6 further comprises a battery
charging and discharging control device 7 built into the body of the battery casing.
The partitioned battery 6 and the prior art battery 2 are much the same in look,
weight and total charge capacity. Furthermore the partitioned battery 6 is characterised
by the same maximum charging voltage, Vmax, and the same fully discharged
voltage, Vmin. However, the control device 7 ensures that only full
charge cycles of each battery 6a, 6b are performed, thereby maximising the lifetime
of the batteries 6a, 6b. The batteries 6a, 6b are charged using the same power
unit 3, from male connecting members 4, through female connecting members 5. However,
rather than passing directly to the batteries 6a, 6b, the power is switched through
the control device 7. The control device 7 also controls the power supply from
the batteries 6a, 6b, to the handset 1.
Referring to Figure 5, the control device 7 serves as an interface
between the partitioned battery 6 and a stabilised power supply 8 contained in
the power supply unit 3, and the partitioned battery 6 and telecommunications circuitry
9 contained in the mobile handset 1. The control device 7 switches between charging
and discharging circuits, measures the degree to which each battery 6a, 6b is charged
and switches between them accordingly. Thus, when the handset 1 draws power, the
control device 7 can choose the most suitable of the two batteries 6a, 6b. Also,
when the batteries 6a, 6b are being charged, the control device 7 can choose which
of the two batteries, if any, is ready for a full charge.
The criteria by which first and second batteries 6a, 6b are charged
and discharged are as follows:
If both first and second batteries 6a, 6b of the partitioned battery
6 are fully discharged, with battery voltages less than the fully discharged voltage,
Vmin, then control device 7 permits both batteries to be charged. The
first battery 6a is fully charged to the maximum charge voltage, Vmax.
Then the second battery 6b is charged in a similar fashion.
If either one of the batteries 6a, 6b is partially charged, with a
battery voltage greater than the fully discharged voltage, Vmin, then
said battery is not charged. The first battery 6a is tested and, if it is fully
discharged, it is charged. If it is already partially charged, charging is skipped.
The second battery 6b is tested. Again, if it is already charged, no further charging
is performed. Charging is finished once both batteries 6a, 6b have been tested
and, if appropriate, charged.
If both first and second batteries 6a, 6b of the partitioned battery
6 are fully charged with battery voltages greater than the fully discharged voltage,
Vmin, then control device 7 selects the second battery to be drained.
This choice is arbitrary.
If both the first and second batteries 6a, 6b of the partitioned battery
6 are partially charged to the same degree, with battery voltages greater than
the fully discharged voltage, Vmin, then control device 7 still selects
the second battery.
If both the first and second batteries 6a, 6b of the partitioned battery
6 are partially charged with battery voltages greater than the fully discharged
voltage, Vmin, but one of the batteries is less charged than the other,
then control device 7 selects the less charged battery to be drained first.
Once a battery has been selected it is exclusively used until it is
fully discharged. Then the control device 7 tests both batteries again and, if
one is available, selects a partially or fully charged battery for use.
To prevent a break in power supply when the control device switches
from one battery to the other, a smoothing capacitor can be employed parallel to
the telecommunications circuit. The required capacitance will be dependent on the
switching time and current drawn.
Detailed description of control device
Referring to Figure 5, the control device 7 comprises a toggling unit
10 which switches the control device between two modes: a) control of discharging
and b) control of charging. The toggling unit 10 can be a electrical relay that
uses power from the power supply unit 3 to switch the control device 7 to charging
mode when the mobile handset 1 is inserted into the cradle of the power supply
unit 3. The control device 7 includes a volmeter unit 11, to measure voltages across
the first and second batteries 6a, 6b. This information is sent by information
lines 12 to a control unit 13, which compares the voltages against each other and
against pre-programmed levels. In response to these comparisons, the control unit
commands a switching unit 14 to switch between the first and second batteries 6a,
Referring to Figures 6 and 7, a description of how the control device
7 controls discharging of the batteries will now be described.
When the mobile handset 1 is not sitting in the power supply unit
3, the toggling unit 10, switches the battery into a discharge mode. i.e. power
from the batteries is diverted to the telecommunications circuitry 9 of the handset
1. When the handset 1 is switched on, the control device 7 selects either the first
or second battery 6a, 6b for use. This selection is illustrated in Figure 6. Initially,
the control unit 13, commands the volmeter unit 11, to measure the voltages of
the first battery 6a (step S1) and the second battery 6b (step S2), which returns
values of Va and Vb respectively. These voltages are compared
(step S3) in the control unit 13, to determine whether the first battery 6a is
more fully charged than the second battery 6b.
If Va ≥ Vb is true, then the first battery
6a is more fully charged. The second battery 6b is tested to determine whether
it is partially charged (step S4). If it is charged, it is selected to be used
(step S5). If on the other hand it is discharged, then the first battery 6a is
tested to determine whether it is charged (step S6). If the first battery 6a is
charged, it is selected (step S7). If on the other hand it too is discharged, then
neither battery is sufficiently charged to power the mobile handset 1.
If the test in step S3 indicates that the first battery 6a is less
charged than the second battery 6b, the first battery is tested to determine whether
it is partially charged (step S8). If partially charged, the first battery 6a is
selected (step S9). Otherwise, the same test is performed on the second battery
6b (step S10) and is selected (step S11) if partially charged.
Referring to Figure 7, once a battery is selected for use, it is used
exclusively until it is discharged. While the selected battery is in use, its voltage
is measured (step S12) and tested to determine whether it still is sufficiently
charged (step S13). Once it becomes discharged, with V falling below the fully
discharged voltage, Vmin, the control device 7 returns to the battery
selection routine as shown in Figure 6.
Referring to Figure 8, a description of how the charging circuit operates
will now be described in detail.
During charging, the voltage of the first battery 6a is measured (step
S15). The control unit 13, tests the first battery 6a to determine whether it is
discharged (step S16). If the first battery 6a is discharged, the routine allows
it to be charged (step S17). The voltage of the first battery 6a is measured (step
S18) and compared with the maximum charging voltage, Vmax (step S19).
If the battery voltage, V, has not reached maximum charging voltage, Vmax,
then the first battery 6a has not been fully charged and further charging is allowed
(step S17). This cycle continues until the first battery 6a is fully charged. The
routine moves on to test the second battery 6b. The routine also moves on if it
is determined that the first battery 6a is already partially charged (step S16).
The testing/charging routine for the second battery 6b is identical.
The second battery 6b is measured (step S20) and tested to determine whether it
is discharged (step S21). If it is discharged, then charging starts (step S22)
otherwise the routine ends. The battery voltage is measured (step S23) and checked
whether it has reached the maximum charging voltage, Vmax (step S24).
If it has not, charging continues. Once charging is complete (step 24), the routine
When the battery is placed in the power supply unit 3 for charging,
it is very likely that the control device 7a will find the batteries 6a, 6b in
a partially charged state. There are two contributing reasons. Firstly, all rechargeable
batteries self-discharge, even when disconnected from a circuit. The rates of self-discharge
can be high, especially for a fully charged battery. Secondly, charging may be
interrupted. The control device 7, having detected that a battery is fully discharged,
permits charging of a battery. If charging process is interrupted and the battery
is only partially charged then the control device 7a prevents the battery from
being charged further.
If the user wishes, the charging process can be overridden to always
provide fully charged batteries. One method is to indicate the battery level, so
allowing the user to manually discharge the battery through a load resistor, before
continuing charging. Alternatively, the control device 7 can be configured to fully
discharge any partially charged batteries that it finds and then fully charge them.
In summary, the control device 7 maximises the lifetime of the batteries.
It does this in two ways. Firstly, the control device 7 only allows fully discharged
batteries to be charged. Secondly, the control device 7 allows only the least charged
battery to provide power to the handset 1 and it is compelled to do so until it
is fully discharged. In doing so, the control device 7 ensures that batteries are
always discharged fully. In this way, batteries are always subjected to full charging
It will be appreciated that many modifications can be made to the
For example, the rechargeable batteries may be of the Nickel Cadmium
(NiCd) type or any battery type that exhibits memory effect.
The partitioned battery may be partitioned into three, four, five
or even more batteries. This has the advantage of ensuring that the most charge
is stored at any one time and for quick charging times. However, this is at the
expense of a more complex partitioning of battery and more complicated measuring,
comparison and switching circuits.
There may be occasions when one battery is close to being fully discharged
but still has a battery voltage greater than Vmin. According to the
embodiment of the invention, such a battery will not be charged. The control device
can be modified to detect whether a battery is close to being fully discharged
and if it is to fully discharge the battery a load resistor. Charging of the battery
is then permitted.
The control device may be located in the handset circuitry rather
than in the battery housing. Neither does the control device necessarily have to
be a unitary circuit, but can be distributed throughout the handset, the battery
and/or the power supply unit.
The control device can be modified to detect whether one battery is
being charged and discharged more often. To rectify this imbalance, the control
device can bias towards using the other battery more often.
A single battery need not be partitioned to obtain a plurality of
batteries. The batteries may be discrete entities.
Charging of the batteries can be done in parallel, rather than sequentially,
to save time.
The rechargeable batteries can be used in other battery operated devices,
such as electric shavers, portable music systems, computers and toys.