The invention relates to an apparatus for recharging a battery, comprising
a compartment into which the battery can be placed so that its electrical terminals
are in contact with a pair of electrodes, said electrodes being connectable to
corresponding poles of a controllable source of electrical energy.
The invention also relates to a method of recharging a battery, in
which changes in a given physical parameter are electronically monitored and exploited
as an indicator of the accumulated quantity of electrical charge in the battery.
The term "battery" is here intended to refer to either single cells
or battery packs, and refers specifically to secondary (i.e. rechargeable)
An apparatus and method as specified hereabove are known
from laid-open Japanese Patent Applications JP 63-268445 (filed
24.04.87) and JP 5-326027 (filed 22.05.92). These documents describe how the physical
dimensions of a battery can change as a result of increased internal pressure and
temperature during charging. Such increase is attributable to the occurrence of
certain chemical reactions within the battery, the rate and type of which are determined
by the battery's charge state at any given time. In particular, at the end of a
charging procedure, additional electrical energy supplied to the battery will,
in general, no longer cause an increase in its internal charge, but will instead
provoke a sharp rise in the battery's internal pressure and temperature, causing
the battery to expand (these effects being particularly prominent in NiCd and NiMH
batteries). Using a mechanical strain gauge in contact with the battery casing,
such mechanical deformation of the battery can be measured. The detection of (the
onset of) sharply increasing deformation can then be used as an indication that
efficient charging has been completed, and that the electrical energy source should
be switched off.
With the aid of calibration experiments, it is possible to monitor
the mechanical strain in a battery as a function of charging time, whilst simultaneously
measuring the quantity of charge supplied to the battery. When recharging the battery
on subsequent occasions, the measured strain at any given time then allows the
corresponding charge value to be deduced. In this way, the electrical voltage and/or
current supplied to the battery during charging can, if so desired, be automatically
tailored to particular requirements. For example, it is possible to automatically
interrupt charging when the battery is 90% charged, or to slow down the charging
procedure as charge saturation is approached.
It is an object of the invention to provide an alternative method
of monitoring the mechanical deformation of a battery during recharging, without
reliance on a mechanical strain gauge. In particular, it is an object of the invention
that such a method should be accurate and sensitive.
These and other objects are achieved in an apparatus as specified
in the opening paragraph, characterised in that the compartment comprises a ferromagnetic
body which is positioned so as to be subjected to mechanical stress in response
to mechanical deformation of the battery, and that the apparatus comprises electrical
means for monitoring the magnetic permeability of the ferromagnetic body.
The inventive apparatus exploits the insight that, when subjected
to mechanical stress, a ferromagnetic body demonstrates a change in magnetic permeability
(µ) as a result of magnetostriction effects. This phenomenon is elucidated in more
detail by S. Chikazumi in Physics of Magnetism, John Wiley & Sons (1964);
see chapter 8, and particularly section 8.4. If, therefore, the ferromagnetic body
is positioned so as to experience a mechanical force as a result of expansion of
the battery during charging, then such expansion will lead to a change in the value
of µ, which, in turn, can be monitored electrically.
In a preferential embodiment of the inventive apparatus, the electrical
means for monitoring µ comprise a coil of conductive material which surrounds at
least a portion of the ferromagnetic body and which is connected to an electrical
circuit for measuring the self-inductance (L) of the coil, which circuit comprises
an electrical oscillator. In principle, such a "coil" need only comprise a single
loop, but a multiple-loop coil is preferable, since it has a larger inductance
(assuming other parameters, such as the loop area, to be equal).
In a variant of the embodiment elucidated in the previous paragraph,
the coil is connected across an AC voltage source, and L is then measured by further
incorporating the coil in one leg of a Wheatstone bridge. Alternatively, by employing
a second coil which is part of an electrical oscillator circuit, and by placing
this second coil in the vicinity of the coil already referred to hereabove, the
value of µ can be indirectly monitored by measuring the inductance of the second
coil (mutual inductance).
In all such embodiments, it is advantageous if the ferromagnetic body
is a closed magnetic circuit, so as to conserve magnetic flux and optimise the
sensitivity of induction measurements. In particular, the body may simply take
the form of a ring.
Whatever way the electrical means for monitoring µ are embodied, a
further embodiment of the inventive apparatus can be characterised in that these
means are embodied to generate an output signal which is dependent on µ, and that
this signal serves as an input signal to regulate the controllable source of electrical
A preferential embodiment of the apparatus according to the invention
is characterised in that the ferromagnetic body comprises ferrite material,
e.g. MnZn ferrite. Such materials have a relatively high value of µ (at
room temperature), and are commercially available in various forms, including rings.
Other materials which can be applied in the ferromagnetic body include, for example,
NiFe alloys, FeNiCo alloys, Sendust, etc.
It should be noted that the inventive apparatus is also suitable for
detecting over-discharging of a battery, since this is also accompanied by internal
gas generation and attendant expansion of the battery. In such a scenario, the
measured expansion of the battery beyond a certain point can be used as a signal
to terminate further discharging.
The invention and its attendant advantages will be further elucidated
with the aid of exemplary embodiments and the accompanying schematic Figures,
- Figure 1 depicts an aparatus in accordance with the invention;
- Figure 2 depicts electrical means suitable for use in monitoring the magnetic
permeability of the ferromagnetic body employed in the apparatus in Figure 1.
Figure 1 shows an apparatus 1 in accordance with the present invention.
This apparatus 1 comprises a compartment 3 into which a battery 5 can be placed.
The battery 5 is thus positioned that its terminals are in contact with a pair
of electrodes 7, 9. These electrodes 7, 9 are in turn connected to a source 11
of electrical energy, which can be switched with the aid of a switch 13.
Also depicted is a ring 15 of ferromagnetic material, which is located
between the electrode 7 and a plate 17. Elastic means 19 (such as a spring) are
located between the plate 17 and a wall of the compartment 3. These elastic means
19 are preferably relatively rigid, so that both the ring 15 and the battery 5
are tightly pressed against the plate 17.
A coil 21 of insulated copper wire is wound around at least part of
the ring 15, and is further connected to electrical means 23. In conjunction with
these means 23, the coil 21 forms part of an electrical oscillator circuit. This
circuit can be used to measure the self-inductance L of the coil 21, which is in
turn dependent upon the permeability µ of the ferromagnetic material in the ring
In order to charge the battery 5, the switch 13 must be closed. As
charging approaches completion, the pressure and temperature within the battery
5 tend to rise relatively sharply, causing it to expand slightly. As a result of
this expansion, pressure is exercised on the ring 15, leading to a change in the
value of µ via magnetostriction.
A relatively sharp change in L can therefore be employed as an indicator
that the charging procedure is approaching its completion. If so desired, the means
23 can be connected to the switch 13 via a signal carrier 25; dependent
on the measured value of L, an appropriate signal from the means 23 can then be
used to open the switch 13 automatically.
Figure 2 depicts a suitable embodiment of the electrical means 23
employed to monitor the magnetic permeability µ of the ferromagnetic body 15 in
the apparatus of Figure 1. Corresponding features in Figures 2 and 1 are denoted
by the same reference symbols.
The coil 21 in Figure 1 is connected across an oscillator 22, in parallel
with a pair of series-connected capacitors 24a, 24b which are saddled about ground,
thereby forming a resonance circuit. The oscillator 22 is further connected to
a frequency-to-voltage converter 26, whose output signal is passed to an output
210 via a low-pass filter 28.
Changes in the inductance L of the coil 21 cause corresponding changes
in the frequency f of the oscillator 22. The instantaneous value of this frequency
f is converted into a corresponding voltage by the converter 26. The voltage V
measured at the output 210 of the filter 28 is therefore a function of f, which
is itself a function of L, and therefore also of µ.
If so desired, the value of the measured voltage V at the output 210
may be passed to a comparator (not depicted), where it can be compared with a reference
voltage value Vo. Referring to Figure 1, the condition V = Vo
can then be used to trigger a relay (via the carrier 25) so as to open (or
close) the switch 13.