GB2189951A

GB2189951A - Charging electrochemical storage cells

Info

Publication number: GB2189951A
Application number: GB08709639A
Authority: GB
Inventors: Friedrich Wilhelm G Mollendorf; Johan Coetzer
Original assignee: Lilliwyte SA
Current assignee: Lilliwyte SA
Priority date: 1986-04-30
Filing date: 1987-04-23
Publication date: 1987-11-04
Also published as: JPS62268325A; DE3714511A1; GB8709639D0

Description

SPECIFICATION Electrochemical cells THIS INVENTION relates to the charging of secondary electrochemical power storage cells. In particular it relates to a method of charging said cells and to a protective device for use in charging of cells. According to one aspect of the invention there is provided a method of charging at least one secondary electrochemical power storage ce 11 which comprises supplying charging power to the cell so as to cause a charging current to pass through the cell and discontinuing the charging when the potential across the cell rises to a predetermined value, the method including monitoring the potential across the cell and establishing a low impedance path across the cell when the potential across it rises to a predetermined voltage, to bypass the charging current applied to the cell through the low impedance path. The charging may be discontinued when the potential across the cell rises to a predetermied value which may correspond to the fully charged state of the cell or to a preselected value. The potential across the cell may be monitored by sensing the potential across the cell and comparing the voltage sensed with a reference voltage. The potential across the cell may be sensed and compared with the reference voltage by means of a voltage comparator connected across the cell which compares the cell voltage via a voltage divider connected across the cell to a precision voltage reference. An output signal from the comparator may be arranged to activate switch means to establish the low impedance path across the cell when the cell voltage rises to the predetermined voltage. The voltage divider conveniently includes a variable resistor to provide for adjustment and two fixed resistors in series across the cell. The precision voltage reference is conveniently connected in series with a current limiting resistor. The precision voltage reference and the current limiting resistor may also be connected across the cell. Initially, when the bypass low impedance path is established, this causes a drop in the voltage across the cell to below the predetermined voltage. The method may then include cyclically establishing and disabling the low impedance path, the period during which the low impedance path is established in each cycle being controllably varied dependent upon the potential across the cell. An output signal from the comparator may thus be connected to the switch means which may be in the form of a transistor, the transistor being connected across the cell and being switched on when the output voltage from the comparator drops below a certain predetermined value. When the cell is bypassed the voltage across it drops below the reference voltage. This is sensed by the comparator, causing an increase in the comparator output voltage which switches off the transistor. This leads, if the charging continues, to a further increase in the voltage across the cell followed by a further on/off switching cycle from the transistor, which is repeated intermittently. A chopping effect is thus obtained with the charging of the cell being periodically interrupted by the bypassing. Typically the period for which the cell is bypassed by the transistor is shorter than the interval between bypasses, so that the cell will in effect be trickle charged. As the cell becomes progressively further charged, the intervals between bypasses will reduce progressively, so that the proportion of the time that the charging current is bypassed will increase. When a plurality of cells are connected in series, any number of the cells in the series can undergo the abovedescribed bypassing simultaneously, leaving each other and those cells which are not being bypassed essentially unaffected. By the time the series as a whole is substantially fully charged and the potential across the series rises to the predetermined value in response to which the charging is discontinued, none of the cells will have been overcharged.In this regard it will be appreciated that when a series of nominally identical discharged cells is charged by a potential applied across the series, differences in internal resistance of the cells, caused eg by different cell temperatures arising from their different locations in a battery which has a temperature profile, can lead to some cells, becoming fully charged while others still require charging and while the potential across the series is still below that predetermined value corresponding to the series as a whole being fully charged. In this situation a charger which automatically discontinues charging when the potential across the series reaches said predetermined value will continue to charge, and the charging current will continue to pass through the cells which are prematurely fully charged. The prematurely fully charged cells will become overcharged, and this can lead to a rapid increase in their internal resistances, leading potentially to catastrophic overheating thereof with resultant cell destruction. For convenience the output of the comparator may act, via a switching transistor connected in series with a resistor across the cell, on a power transistor connected directly across the cell and which effects the actual bypassing. The output of the switching transistor will thus activate the power transistor and this output may also activate an indicator transistor connected in series with a light emitting diode and resistor across the cell. For safety reasons a reverse biassed protective diode may also be connected across the cell. The charging power may be supplied from a suitable source which may be a constant current supply or have a current limiting facility. The invention extends also to a protective device for use in charging at least one secondary power storage cell which device comprises at least one protective circuit connectable across the cell, the protective circuit comprising monitoring means operable to monitor the voltage across the cell and switch means responsive to the monitoring means and operable to establish a low impedance path to bypass charging current applied to the cell through the low impedance path when the monitored voltage rises to a predetermined value. The monitoring means may include a comparator connectable across the cell, a precision voltage reference for supplying a reference voltage to the comparator, and a voltage divider circuit which may include a variable resistor connected across the cell. An output of the comparator may be connected to the switch means and the switch means may be connected across the cell to bypass the charging current. The switch means, which may comprise a transistor, may be arranged automatically to disable the bypass when the monitored cell voltage, in response to the bypass, drops to below the predetermined value.The switch means may comprise a switching transistor connected, conveniently in series with a resistor, across the cell, the output of the comparator being connected to the switching trans istor and the output of the switching transistor being connected to a power transistor which effects the bypassing and which is connected directly across the cell. The output of the switching transistor may also be connected to an indicator transistor connected in series with a light emitting diode and conveniently a resistor, across the cell. The protective circuit may also include a protective diode connected across the cell. The protective device may be permanently connected to a plurality of cells connected in series, to form part of a battery of cells, but where battery mass is a consideration the protective device preferably forms part of a battery charger.In either case the protective circuit can be a printed circuit on a printed circuit board having an edge connector, to form a unit which can be plugged into a subframe which eg forms part of the battery charger. A protective circuit is conveniently provided for each cell. Each protective circuit may have a pair of input terminals which are connectable to a charging power source and a pair of output terminals which are connectable to the terminals of the respective cells. The switch means may then be connected across the output terminals. It should be noted that the indicator transistor can provide a digital signal for a computer used to monitor the charging as regards the states of charge of individual cells in the series; and this digital signal can be used in calculating a desirable output current for the battery charger. The circuit can also provide an analogue output for a chart recorder or the like, eg from the cell terminals. The invention also extends to a battery charging system for charging a battery comprising a plurality of rechargeable electrochemical power storage cells connected in series, which system comprises a battery made up of said plurality of cells connected in series, and, connected across each cell a protective device as described above. The protective devices may then form part of and be incorporated in a battery charger for charging the battery and connected across the battery. The invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings, in which: Figure 1 shows a circuit diagram of a protective circuit forming part of a protective device according to the invention; Figure 2 shows a plot of cell voltage against battery capacity for a selected charge/discharge cycle for a series of cells being charged according to the method of the present invention; Figure 3 shows a plot similar to Figure 2 for a later charge/discharge cycle; and Figure 4 shows a similar plot for the series of cells as a whole for selected charge and discharge half cycles. In Figure 1 of the drawings, reference numeral 10 generally designates a protective circuit formig part of a protective device according to the invention and for use in accordance with the method of the invention for protecting, against overcharging, one of the cells of a series of interconnected secondary power storage electrochemical cells undergoing charging. The circuit 10 has input terminal s 11 connectable to a battery charger (not shown) and output terminals 14 and 16 connectable to the positive and negative terminals of a cell (also not shown). The battery charger is directly connected to the cell via lines 18 and 20. Two input terminals (7,4) of a TL331CD comparator 22 are connected respectively to the lines 18 and 20 by lines 24 and 26. A further input terminal (2) of the comparator 22 is connected respectively by line 28 to a 10k variable resistor 30 connected in series with 56k and 47k fixed resistors 32, 34 between the lines 18 and 20; and a further input terminal (3) of the comparator 22 is connected by line 36 between a LM385 Zener diode 38 forming a precision voltage reference and a 2k2 fixed current limiting resistor 40. The output (6) of the comparator is connected by line 42 to the input of a 2N3053 switching transistor 44, the line 42 being also connected via a 3k3 fixed biassing resistor 46 to the line 18, and the transistor 44 being connected to the line 18 and via a line 48 and a 10k fixed resistor 50 to the line 20. The output from the transistor 44 on line 52 leads to the input of an MJ802 power transistor 54 connected directly between the lines 18 and 20. A 2N2222 indicator transistor 56 has its input connected to the line 52 and is connected in series with a light emitting diode 58 and a 220 ohm resistor 60 between the lines 18 and 20. A reverse biassed protective diode 62 is connected between the lines 18 and 20 to prevent damage caused by connection of the battery charger to the imput terminals 11 with incorrect polarity. The circuit 10 is selected to prevent the voltage between the cell terminals 14, 16 from ever exceeding 2,695V. The comparator 22 is selected to have an operating voltage of 2V and the voltage reference 38 is set to 2,695V. In use, when the cell is in a discharged state, the comparator 22 will initially sense a voltage between the terminals 14, 16 of. less than that of the 2,695V setting of the reference 38 and will transmit a voltage signal along line 42 which keeps transistor 44, and hence transistor 54, switched off. As the cell becomes progressively more charged the voltage between its terminals 14, 16 will approach 2,7V and if, before the battery is charged, this voltage equals the setting of the reference 38 then the output voltage signal along line 42 from the comparator goes low and the transistor 44 is switched on. The output signal along line 52 from the transistor 44 then switches on the transistor 34 which establishes the low impedance path to bypass the charging current applied to the cell through the transistor 34. As soon as the charging current to the cell is bypassed the voltage across the terminals 14, 16 drops and the signal from the comparator 22 along line 42 increases to a value which switches off the transistor 44 which in turn switches off transistor 54 to break the bypass. Voltage across the terminals 14, 16 then again starts to climb progressively until the switching cycle is repeated, as long as the series of cells is being charged. A chopping effect in the charging current to the cell is created and, as the state of charge of the cell increases, successive bypasses will follow one another after progressively shorter intervals, until the bypassing is more or less continuous, with the result that the cell will be trickle charged until charging of the series of cells is discontinued. In the particular case where the battery charger employed is designed automatically to discontinue charging as soon as the voltage across the series of cells (battery) rises to a predetermined value, the method and device of the invention have the particular advantage that all the cells of the series can become substantially fully charged. The charging current applied to those cells which become fully charged before the others is bypassed as described above and charging thereof is effectively stopped (limited to trickle charging) while those that have become charged more slowly continue to be charged. Once all the cells are substantially fully charged the voltage across the series will rise to the value (when the charging current to none of the cells is instantaneously bypassed) which causes the charger automatically to discontinue the charging. In the absence of the method of the present invention certain cells in the series which become fully charged before others, eg because of differences in internal resistance arising from temperature differences if the battery has a temperature profile, can become overcharged before the others are fully charged. Unless charging is discontinued prematurely, ie before all the cells are fully charged, certain cells will be overcharged, leading to a number of potential problems, such as solid electrolyte poisoning, liquid electrolyte decomposition and indeed gross cell damage or destruction arising from overheating, bearing in mind that a fully charged or overcharged cell must take through it the full charging current in the absence of the bypass. The invention thus promotes full capacity utilization of the battery, as each cell can be fully charged without damaging any cells by overcharging, and maintenance is reduced as damage caused by overcharging is reduced, if not eliminated. Where battery mass is a consideration the protective device can form part of the charger. Instead the protective device can form part of the battery, and the comparator operating voltage can be selected to have a value, and the reference voltage set to a value (eg respectively 2V and 2,695V in the circuit shown in Figure 1) which renders the protective circuit substantially transparent to the cell, if it remains permanently connected thereto. The circuit shown in Figure 1 was tested with a series of six nominally identical interconnected high temperature power storage electrochemical cells operated at about 250[deg]C and each having an open circuit voltage of about 2,3-2,6V. The cells each had a molten sodium anode, a beta-alumina separator whose inteinal resistance is temperature sensitive, NaAICI4 liquid electrolyte separated by the separator from the sodium, and Fe/FeCl2 as active cathode material, in contact with the liquid electrolyte. The cells were charged and discharged for several cycles without using the method of the present invention. As is typical in such cases, certain cells exhibited different internal resistances leading to different rates of charge thereof, when charged in series with a common charging current passed therethrough by a potential of about 12-15V across the series. During the 5th charge cycle the method of the present invention was initially applied to the cells and was continued for the 6th charge cycle, each cell being connected to a protective circuit as shown in Figure 1. Figure 2 shows a plot of cell voltage against capacity (AH) of the battery as a whole for the 6th charge cycle of these cells and also the 6th discharge cycle. From this it is apparent that four of the cells became fully charged before the remaining two (after about 25 AH). The protective circuits however kept the cell voltage of the prematurely charged four cells at a safe value below 2, 75V, until the remaining two cells were fully charged (as shown by a rise in their cell voltage after about 40 AH). The method of the present invention was further applied for the 7th and 8th charge cycles, the latter being plotted in Figure 3 together with the 8th discharge cycle.From Figure 3 the surprising advantage becomes apparent that after a few charge cycles according to the method of the present invention, the six cells showed substantially more similar charging characteristics to one another, approaching the ideal case where these characteristics are identical. Differences of any consequence manifested themselves only after about 35 AH (of about 25AH in Figure 2). Furthermore, battery capacity was dramatically improved from the 6th discharge cycle to the 8th discharge cycle. On the basis that the battery was fully discharged and required recharging when the discharge voltage dropped to 9V (see Figure 4), the 6th discharge cycle delivered about 33 AH whereas the 8th discharge cycle delivered about 40 AH. Figure 4 shows a plot similar to Figures 2 and 3 for the battery as a whole for the 4th discharge cycle (before the method of the invention was used), the 8th discharge cycle, and the 6th and 8th charge cycles, battery voltage being plotted against battery capacity. From this plot it is apparent that for the battery as a whole, use of the method and protective device of the present invention leads to a lower charging voltage and to a substantial increase in capacity, as well as to the advantages inherent in prevention of overcharging of individual cells, ie reduction in cell damage and an increase in cell life.Finally, it will be appreciated, although in the aforegoing the invention has been described with reference to a plurality of cells connected in series, the invention applies equally to cells connected in series/parallel, eg where a plurality of batteries are connected in parallel between the terminals of the battery charger and are charged simultaneously. Furthermore, it will be appreciated that a protective circuit in accordance with the present invention may be used to charge a single cell, for the purpose of trickle-charging that cell up to its full capacity.

Claims

1. A method of charging at least one secondary electrochemical power storage cell, which comprises supplying charging power to the cell so as to cause a charging current to pass through the cell and discontinuing the charging when the potential across the cell rises to a predetermined value, the method including monitoring the potential across the cell and establishing a low impedance path across the cell when the potential across it rises to a predetermined voltage, to bypass the charging current applied to the cell through the low impedance path.

2. A method as claimed in Claim 1, in which the potential across the cell is monitored by sensing the voltage across the cell and comparing the sensed voltage with a reference voltage.

3. A method as clained in Claim 2, in which the comparing is achieved by means of a voltage comparator arranged to compare the cell voltage with a precision voltage reference.

4. A method as claimed in Claim 3, in which an output signal from the comparator is arranged to activate switch means thereby to establish the low impedance path across the cell.

5. A method as claimed in any one of the preceding claims, in which the low impedance path is cyclically established and disabled and in which the period during which the low impedance path is established in each cycle is controllably varied dependent upon the potential across the cell.

6. A method as clained in any one of the preceding clains, in which a plurality of series connected cells are charged simultaneously, and which includes selectively establishing a low impedance path for individual cells dependent upon the state of charge of the individual cells.

7. A protective device for use in charging at least one secondary power storage cell, which device comprises at least one protective circuit connectable across the cell, the protective circuit comprising monitoring means operable to monitor the voltage across the cell and switch means responsive to the monitoring means and operable to establish a low impedance path to bypass charging current applied to the cell through the low impedance path when the monitored voltage rises to a predetermined value.

8. A protective device as claimed in Claim 7, in which the monitoring means includes a comparator connectable across the cell, a precision voltage reference for supplying a reference voltage to the comparator and a voltage divider circuit connectable across the cell to provide an input signal to the comparator pro- portional to the voltage across the cell.

9. A protective device as claimed in Claim 8, in which the voltage divider includes a variable resistor for providing an adj ustable variable input signal proportional to the voltage of the cell.

10. A protective device as claimed in in any one of the preceding claims 7 to 9, in which the switch means is controlled cyclically to establish and disable the low impedance path over varying periods in each cycle dependent upon the monitored cell voltage relative to the predetermined value.

11. A protective device as claimed in any one of the preceding claims 7 to 10, in which the switch means comprises a switching transistor responsive to the comparator and a power transistor responsive to the switching transistor and forming portion of the low impedance path.

12. A protective device as claimed in any one of the preceding claims 7 to 11, which further includes indicator means operable to give an indication of when the cell is being charged.

13. A protective device as clained in any one of the preceding claims 7 to 12, which further includes a reversed biassed protective diode operable to mininise damage caused by connection of the device to the cell with incorrect polarity.

14. A battery charging system for charging a battery comprising a plurality of rechargeable electrochemical power storage cells connected in series, the system comprising a battery made up of said plurality of cells connected in series, and, connected across each cell, a protective device as claimed in any one of the preceding clains 7 to 13.

15. A battery charging system as claimed in Claim 14, which further include s a battery charger operable to supply charging current to the cells, the protective devices being incorporated in the battery charger.

16. A method of charging at least one secondary electrochemical power storage cell, substantially as described herein with reference to the accompanying drawings.

17. A protective device for use in charging at least one secondary power storage cell, substantially as described and as illustrated herein.

18. A battery charging system substantially as described herein with reference to the accompanying drawings.