The present invention relates to a method for controlling the charg¬ ing of multi-cell batteries, for example NiCd cells, in that the battery is coupled to a battery charger for impressing a current through the battery.

A plurality of various types of principles are hitherto known in the art for charging batteries with, for example, NiCd cells. Examples of this are constant current charging, constant voltage charging, pressure and temperature charging and pulse charging. The major ad- vantage inherent in constant current charging is that the charger may be of extremely simple design, while its drawback is a restric¬ tion to a temperature range of between 0 and 40°C and an extremely long charging time at low temperatures, since permitted mean current in cold conditions is considerably lower than at room temperature. Furthermore, charging takes place in a completely uncontrolled man¬ ner, without any adaptation whatever to the capacity of the cells to accept the charging energy. Constant voltage charging is also un¬ controlled, but in this process somewhat better use is made of the properties of the cell, but also in this case charging time will be long, in particular at low temperatures. Because of the difficulties involved in providing pressure or temperature sensing elements, pressure and temperature charging occurs only in extremely special and rare circumstances. The reason for this is that, in this parti¬ cular case, the practical difficulties are as good as insurmount- able. In per se conventional pulse charging, the charging is ef¬ fected in cycles of, for example, 1 Hz, the charging current being, for instance, twice as large as the discharging current. Pulse charging has proved to be more efficient than many other charging concepts, in particular at low temperatures. In conventional pulse charging, the only compensation made is for poor regulation of the charging current of the battery. However, conventional pulse charg¬ ing involves considerable charging times and relatively poor control of the various parameters.

The object forming the basis of the present invention is to devise a novel method for controlling charging of a multi-cell battery, for example NiCd cells, which gives a thorough charging of the battery in a considerably shorter time than has hitherto been possible, both at room temperature and in particular at lower temperatures, and also an apparatus for reducing the method into practice.

This object is attained according to the present invention in the method disclosed by way of introduction by employing therein the characterising features as set forth in one or more of the appended method claims. An apparatus for carrying out the method according to the present invention is given the characterising features as set forth in one or more of the appended apparatus claims.

The major advantage inherent in the method and apparatus according to the present invention resides in the possibility for a thorough charging of a battery in a considerably shorter time than has hith¬ erto been possible without any risk whatever of undesirable pressure build-up in the individual cells as a result of gas formation. While a charger according to the present invention may appear to be rela¬ tively complicated, its complexity is without any doubt motivated by the extraordinary advantages attained with the charger which gives as good as 50% and more shorter charging times than do prior art chargers, this without any risk whatever of undesirable gas for- mation in the cells. Moreover, the method according to the present invention takes into account the capacity of the individual cells to accept charging energy, whereby all cells in a battery will be given substantially the same charging level irrespective of whether any of the cells becomes fully charged before any of the others.

One embodiment of the present invention will be described in greater detail hereinbelow with reference to the accompanying Drawings. In the accompanying Drawings, Figs. 1A-1C show, highly schematically and not to scale, the process in diagram form for illustrating one embodiment of the method according to the present invention. Fig. 2

is a block diagram showing an apparatus for carrying out the method according to the present invention. Fig. 3 is a coupling diagram of a prototype final step for a battery charger according to the pres¬ ent invention. Fig. 4 is a coupling diagram of a prototype CPU unit for a battery charger according to the present invention with a final step according to Fig. 3. Figs. 5-8 are diagrams showing charging of batteries according to the present invention.

The pertinent embodiment of the method according to the present in- vention will now be illustrated with reference to Figs. lA-lC. It might, in this instance, be emphasised that the procedural cycles illustrated in these Figures 1A-1C are by no means to scale or ex¬ act, serving merely to facilitate an understanding of the method ac¬ cording to the present invention. This will be more obvious to the skilled reader on observation of Figs. 1A-1C in the light of the non-restrictive exemplifying values of voltage, current and time which will be set out in the following discussion.

In the following description, the expression 'rest voltage' is taken to signify the voltage measured across the connecting poles of a battery at a given point in time when no current flows either to or from the battery. The term 'pole voltage' is taken to signify the voltage measured across the connecting poles of the battery at a given point in time when current flows to or from the battery. The abbreviation EMK is taken to signify the rest voltage after a relat¬ ively long rest period of more than 10 min. This is also considered as a stable state for the battery.

After coupling-in of a battery which is to be charged and which com- prises, for example ten NiCd cells each of 1.2V, connected in series for realising a 12V battery, to a battery charger of the type schem¬ atically illustrated in Fig. 2, the apparatus will measure the rest voltage U3 of the battery. As soon as a voltage is impressed upon the battery for driving a current intended for charging of the bat- tery therethrough of, for example, 10A, the pole voltage rises im¬ mediately with the value U2 in order thereafter to rise more slowly

to the value Ul. The voltage increase U2, which may also be desig¬ nated offset-voltage, in all probability derives substantially from the voltage drop over input conductors and the internal resistance of the battery. Consequently, the voltage Ul can be set at a voltage level which is to be substantially equal to the so-called critical voltage of an NiCd cell of 1.52V or 1.55V at room temperature. At or above this critical voltage, there is a risk of gas formation in the cell and this critical voltage has been established purely chemi¬ cally. This value should not, however, be considered as an absolute value, since it varies somewhat with the ambient temperature and, thereby, the temperature in the cell proper.

Thus, the voltage value Ul constitutes the critical voltage added with the voltage drop U2 across input conductors and the internal resistance of the battery. In many cases, the voltage drop U2 proves to be approximately 0.1V, for which reason Ul will be approximately 1.62V-1.65V. This voltage value or this pole voltage Ul may not be exceeded after a power energization, as is illustrated in Fig. 1A in that the power energization is broken as soon as the voltage reaches the level Ul. After the power de-energization, the voltage falls to a rest voltage U6 which is slightly higher than the rest voltage U3 prior to the time period tl. The voltage for driving the charging current is energized during at most the time period t4 of, for ex¬ ample, Is., while the power break amounts to the time period t2 of, for example, 100 ms. During a certain established time period tl, a number nl of energizations and de-energizations of the voltage take place for driving the charging current through the battery and, at the beginning of each energization, measurement takes place of the rest voltage Ul and the voltage drop U2 for possible adjustment of the voltage value Ul. If the time period t4 of, for example, Is., has been exceeded without the voltage level Ul having been reached, the energization is nevertheless discontinued.

If the permitted voltage value Ul is reached very quickly, for ex- ample several times within a fraction of 1 min., for example within a few hundred ms, a reduction of the charging current is immediately

effected. The charging current is reduced at levels of for example, 32 st or bits, which depends upon the digital construction of the apparatus for carrying out the method according to the invention.

The number of energizations and de-energizations during the time period tl of 1 min is registered in a register and if a certain num¬ ber is exceeded, the charging current is reduced, while if the volt¬ age value Ul is not reached during a number of energizations and de- -energizations during the time period tl, an increase of the charg- ing current is effected. The smaller the charging current, the greater the number of accepted de-energizations because of the fact that the voltage level Ul is reached. At maximum charging current of 10A, for example 40 energizations and de-energizations are accepted during the time period tl, while up to as many as 150 energizations and de-energizations are accepted when the charging current is small, for example of the order of one or a few amperes, while the number of energizations and de-energizations with maximum time period t4 or longer is only 30 irrespective of the size of the charging current.

After each time period tl, a discharging of the battery takes place in accordance with Fig. 1C, with a discharging current 12 of, for example, 0.150A during a time period t3 of approx. 2 s, whereafter the rest voltage U3 is once again measured and the charging contin- ues in accordance with Fig. 1A.

Thereafter, charging continues in accordance with Fig. IB until such time as the rest voltage U3 shows a tendency to fall from a maximum value. When U3 has passed its maximum value or the elbow in Fig. IB, the normal charging cycle is discontinued during a time period t5 of, for example, 15 s, when the rest voltage U7 is measured. After a further time period t6 of 5-10 min, the rest voltage U4 is measured, which is then slightly lower than the rest voltage U7, the differ¬ ence U5 being calculated between the rest voltage U7, time period t5, after the charge discontinuation, and the rest voltage U4 after time period t6. The voltage level Ul is reduced by this voltage dif¬ ference U5, whereafter charging continues in the same way as before,

until a new elbow occurs in the rest voltage, when the cycle is re¬ peated. This latter charging is considered as a retro-charging in order that as many cells as possible be charged to maximum level without any of the fully charged cells being damaged. Once the volt- age difference U5 has become ery slight or quite simply zero, merely maintenance charging of the battery takes place, during which the voltage and current values correctly adapt to one another. This maintenance charging continues as long as the battery is connected to the charger.

Figs. 5-8 show examples of charging of different batteries, it being emphasised in this context that the charging is commenced in all cases with a maximum current of 32 bits corresponding to 10A. During the initial phase of the charging, the EMK curve is to be considered as the rest voltage U3, in which event the EMK proper is not ob¬ tained until after the charging when the battery has had time to stabilise, for example after 10-60 min.

This method may be realised employing the apparatus illustrated in Fig. 2 which is controlled by means of a microprocessor 1 which may include a CPU unit IC1/68HC11 and a number of additional IC cir¬ cuits, eg., IC3 which is an EPROM circuit for storing the program proper for executing the above-described method, one example of this being shown in Fig. 4. However, it should be pointed out that the unit is a prototype and embodies a number of functions which may possibly be dispensed with for an efficient battery charging. The microprocessor 1 has a number of current setting outputs leading to a digital/analog converter 2, one output which determines whether charging is to be effected, one output which determines whether dis- charging is to be effected and two supply inputs for measured cur¬ rent through the battery and measured voltage across the battery. The current is measured by an OP amplifier 3 via a resistor Rl, while the voltage is measured by means of an OP amplifier 4. From the converter 2, an analog signal is obtained corresponding to that charging current which it is desired to impress through the battery and which passes to the battery via an OP amplifier 5, an AND gate 6

and a transistor Tl. The OP amplifier 5 receives a current feedback via the OP amplifier 3. The AND gate 6 allows the passage of a sig¬ nal to the transistor Tl on condition that it has received a signal from the OP amplifier 5 and a signal from the AND gate 7 which emits a signal if there is a signal on the charge output from the micro¬ processor 1 and a response signal to the effect that there is no signal on the discharge output via the inverter 8, whose ouput is high when the discharge output is low. When the AND gate 6 emits a signal to the transistor Tl , this becomes conductive and conducts a charging current through the battery, the resistor Rl and an in¬ duction coil L, to earth, and the transistor Tl becomes non- conductive, the charging current passes through the battery, the resistor Rl, the induction coil L and a diode Dl. The induction coil L equilibriates the charging current through the battery.

Discharge of the battery, which takes place after each time period tl of one min, is ordered by the microprocessor 1 whose discharge output will, in such event, be high for back-up voltage of a diode D2 such that the transistor T2 becomes conductive by the output sig- nal from the OP amplifier 10 via a resistor R3. When the transistor T2 becomes conductive, a discharge current is led through the batt¬ ery via the resistors Rl and R2.

A separate charging voltage source of +24V is connected to the pos- itive pole of the battery. Such a power unit may be of conventional type and should be able to provide a desired current of 10A and even more when large batteries are to be charged, since it appears to be of importance that, in the initial phase of the charging of a bat¬ tery, the charging current should be large. On charging of batteries of 0.5 Ah in trials using the present invention, the charging cur¬ rent was initially 10 A and was thereafter reduced relatively quickly to a slight level, as is apparent from the charges exempli¬ fied in Figs. 6-8. As a result of limitations in the trial appara¬ tus, the charging current in the trial according to Fig. 8 was limited to 10 A.

In the diagrams shown in Figs. 3 and 4, the points denoted by the same eference numerals are interconnected and the voltage inputs, without any direct voltage indication, are coupled to a special mains unit for supply of the charging current. A person skilled in this art is unlikely to find any difficulty whatever in reading the two different diagrams after having studied the present functional description.

In this description, and in Fig. 1, a plurality of different desig- nations U3, U4, U6 and U7 occur for the rest voltage which is the voltage measured at a given point in time across the poles of the battery when no current flows to or from the battery. The rest volt¬ age U3 is measured between the established time periods tl, this im¬ mediately before a voltage energization and thereby at the end of Ml, in which time period the discharge takes place. The rest voltage U7 is measured after the time period t5 and 15s, and the rest volt¬ age is measured after the time period t6 of 5-10 min, and is sub¬ tracted from the rest voltage U7 for obtaining the voltage dif¬ ference U5. The rest voltage U6 is the rest voltage measured in the time period tl.