WO1998016870A1

WO1998016870A1 - Control of electrochromic devices

Info

Publication number: WO1998016870A1
Application number: PCT/AU1997/000697
Authority: WO
Inventors: Igor Lvovich Skryabin; Marcus John Bell
Original assignee: Sustainable Technologies Australia Limited
Priority date: 1996-10-16
Filing date: 1997-10-16
Publication date: 1998-04-23
Also published as: AUPO303096A0

Abstract

A control system for electrochromic devices is disclosed that is adapted to ensure the safe charging of such devices as they age. The control system is designed to limit current flow while the voltage across the cell is low, to limit voltage across the cell as the cell resistance increases and to terminate current flow after it has fallen to a predetermined level. In one form the control system (12) includes separate current-limiting and voltage-limiting sources (34 and 36) selectable by a relay (46) that is operated from a voltage sensor (58). A current sensor (52) limits the maximum charge rate while a charge sensor (50) limits the total charge deliverable to the device. As the device ages and its effective internal impedance rises, the voltage limiter (58) effectively reduces the total charge which can be delivered to the device.

Description

CONTROL OF ELECTROCHROMIC DEVICES TECHNICAL FIELD

This invention relates to apparatus and methods for controlling electrochromic devices. It is particularly, but not exclusively, concerned with devices commonly referred to as 'smart glass' or 'smart windows' used as switchable glazing in buildings. The transparency of such devices can be electrically controlled to regulate the solar-thermal load on a building. The invention also relates to systems comprising electrochromic devices and their associated controls.

Electrochromic devices are electrolytic cells which have transparent electrodes, at least one of which changes transparency when it receives or releases ions (via an electrolyte) when external DC power of appropriate polarity is applied to the cell. In this specification, the reduction and increase of electrode transparency is called 'colouration' and 'bleaching' respectively. An electrode which changes transparency is called the 'working' electrode and an opposing electrode which does not change transparency is called the 'counter' electrode. An electrochromic cell which colours when ions are inserted into the working electrode is said to be 'charged' when coloured and 'discharged' when bleached. An example of such a cell is one having a W0₃ working electrode which changes oxidation state [W⁵⁺ <== W⁶⁺] upon the insertion or extraction of Li⁺ ions, a polymer electrolyte and a VO counter electrode. In this case, the working (W0₃) electrode forms the cathode of the cell when it is being charged. Typically in such cells, the working and counter electrodes are applied onto transparent ionicly-conducting oxide (TCO) electrodes which are formed on the inner faces of a juxtaposed glass panes, the electrodes being applied by sol- gel and/or sputtering techniques, for example.

A useful figure of merit for electrochromic cells designed for use as smart windows is the 'contrast ratio' of the cell; that is, the ratio of the light-transmittance of the cell when fully discharged (or bleached) to the light-transmittance of the cell when fully charged (or coloured). Contrast ratios of between 3:1 and 6:1 are desirable. Unlike electrochromic devices designed for display purposes, the switching time of a smart window is not critical, switching times of a few minutes being satisfactory. BACKGROUND TO THE INVENTION

As it is well known that electrochromic cells can be damaged by overcharging, various methods have been proposed to limit the maximum charge which can be applied to such devices. It has also been appreciated that, since the insertion of ions into a working electrode during charging is diffusion-limited, damage may also occur due to localised excessive charge-density even though total average charge is not excessive. The use of charge-current limiters is therefore also known. However, despite the use of charge and current limiters, it is known that the contrast ratio of electrochromic cells decreases increasing rapidly as the cell is cycled. This is recognised as the 'aging' problem.

The problem of over-charging is, however, further complicated when the colouration is varied incrementally because successive charge increments can easily result in a total charge which is excessive. This problem is addressed by US patent 4,512,637 to Rallmer which teaches (i) the use of separate constant-current sources for charge and discharge, the discharge current being set higher than the charge current, and (ii), the use of interval counters to regulate the number of charge or discharge increments. While a series of random charge and discharge increments will not result in over charge, it is will result in gradual discharge which is not convenient for the user. Rallmer also teaches that over-charging when switching from a fully bleached to a fully coloured state can be avoided by limiting the total charge to a safe level.

US patent 5,365,365 to Ripoche takes a different approach to charge control and incremental adjustment. A reference capacitor is employed to simulate the cell charge status so that the quantum of charge or discharge required to achieve a desired increment of transparency can be gauged. The voltage across the capacitor indicates the charge level of the device (ie, its transparency), a settable reference voltage is used to indicate the degree of transparency required, and the difference between these voltages is used to determine the quantum of charge to be inserted or removed to achieve the desired transparency. There are, however, three major problems with such a control system: it is highly unlikely that a capacitor can be found to closely mimic the charge-discharge characteristics of an electrochromic cell over its life; the proposed control circuit does not limit charging rates; and, there is no safeguard on the total charge which can be acquired by a cell.

Despite recognition by the prior art that both total charge and charge rate need to be limited to reduce the rate of cell degradation, the useful life of a typical electrochromic cell is still less than that desirable for a smart window. The useful life of a cell can be measured, for example, by the number of cycles it takes to reduce its contrast ratio by 50%.

OBJECTIVES OF THE INVENTION

It is therefore an object of the present invention to provide improved apparatus and methods for controlling electrochromic devices such as 'smart windows' and to provide improved systems incorporating such apparatus and methods. More particularly, it is desirable to reduce the danger of overcharging and cell damage as the cell ages and thus extend the useful working life of the device.

OUTLINE OF INVENTION

This invention is based upon the realisation that the increasing fall-off of the contrast ratio of an electrochromic cell during use indicates that the safe charge capacity of the cell decreases as it is cycled. Thus, even the best prior art controllers will overcharge the device more frequently and more severely as it ages. It is therefore not sufficient to simply limit the total charge delivered according to some preset level. The maximum charge deliverable must be somehow automatically reduced as the cell ages. This desirable feature is characteristic of the present invention. While reduction of the preset charge limit with cycle-count is envisaged, it is not preferred as it cannot take account of the variability of the large-area cells used for smart windows. According to this invention, the charge limit is automatically adjusted by voltage-limiting (in addition to current-limiting) during charging and, preferably, discharging.

Furthermore, the present invention is based upon the realisation that, in addition to excessive charge and/or current, an electrochromic cell is damaged by excessive voltage. Indeed, when a constant current source is used to deliver (or remove) a safe total charge, the voltage-drop across an old cell will reach damaging levels because cell resistance increases toward the end of the charge and discharge processes. This over-voltage causes a reduction of charge capacity because it leads, initially, to the loss of the sites in an electrode for the mobile charge carriers (eg, Li⁺) and, then, to loss of charge carriers to parasitic side reactions within the electrolyte.

From one aspect, therefore, the present invention involves the use of a charging and/or discharging procedure for electrochromic cells wherein both the maximum rate of charge addition or removal and the maximum voltage across the cell are limited. In addition, the maximum charge which can be delivered to the cell — when new — is desirably also limited. The voltage (and, possibly, the current) limits will desirably be set differently for charging and discharging. Normally, as a new cell commences charging from a fully discharged state, current limitation will be effective and charging will occur at substantially constant current until charging is terminated by the charge-limiter. However, with older cells, cell resistance is likely to rise to the point where the preset voltage limit is reached before the preset charge limit is reached, and the charging current will thereafter will fall off, effectively reducing the total charge delivered to the cell. A similar mechanism applies to discharging, except that discharge is not normally terminated upon the removal of a fixed amount of charge. Finally, to reduce the adverse effects of extended periods of applied voltage during charge or discharge (albeit at the nominated safe voltage level and at very low current), it is desirable to effectively disconnect the external power source when the charging or discharging current falls to a low level. Variation of the current level at cut-off above the minimum can be used to vary the charge delivered to a cell and, therefore, its transmittance or contrast ratio. Of course, variation of the charge limit to levels below the preset maximum will achieve the same result.

As a cell ages it will reach the voltage limit(s) earlier and earlier as it charges and discharges. And, since the charging or discharging process is terminated according to the fall-off in current, the average charge rate will first be reduced and then to total charge delivered will decrease over the life of the cell. In an old cell, the preset maximum charge would never be reached. The invention is, of course, concerned with both circuit means and methods of achieving this effect. From another aspect, the invention is also based upon the realisation that it is not safe practice to attempt to adjust the colour of a cell which has been left partially charged for some time. That is, it is always preferable to start with a fully discharged or a fully charged cell before setting the degree of desired colouration. As indicated above, the fully charged or discharged state of the cell is signalled when the preset cut-off current is reached. We have found a minimum current limit of between 10% and 2%a the maximum safe current flow to be satisfactory. It is generally more reliable and convenient to use the discharged or bleached state of an electrochromic cell as the reference state from which charge is added to achieve the desired level of colouring. That is, we prefer to always fully bleach the cell before initiating any charging or colouration. Counter electrodes such as vanadium oxide are, in general, more tolerant to excessive charge than working electrodes such as tungsten oxide.

The safe cell-voltage levels will depend upon the particular materials chosen for the electrodes, the electrolyte and the mobile ions. Experimental evidence and theoretical analysis of ion-transfer and associated electrode polarisation/depolarisation phenomena show that there is a stage toward the end of the charging and discharging process where the cell voltage, after plateauing, increases sharply. If the cell voltage is allowed to increase beyond these plateaus, cell damage will occur. While the actual voltages will differ according to the electrode and the electrolyte materials, this general phenomenon sets the desirable limits on the respective discharge and charge voltages for most practical electrochromic cells with application as smart windows.

DESCRIPTION OF EXAMPLE

Having broadly portrayed the nature of the present invention, one particular example will now be described by way of illustration only. In the following description, reference will be made to the accompanying drawings in which:

Figure 1 is a diagrammatic representation of an electrochromic window and its associated control circuit formed in accordance with the present invention. Figure 2A is graph depicting the variation of current density and voltage, and Figure 2B is a graph depicting the variation of charge density and contrast ratio, for a cell of medium age used as a smart window.

Figures 3A, 3B and 3C are simplified current and voltage graphs for young, middle-aged and old cells used as smart windows.

Referring to Figure 1 , the 'smart window' system of the chosen example essentially comprises an electrochromic window or cell 10 and a control circuit apparatus 12. Window 10 comprises two closely-spaced panes of glass 14 and 16, the window being intended for mounting in a building so that pane 14 is on the outside and pane 16 is on the inside. TCO (transparent conducting oxide — eg, indium tin oxide) filmlike conductors 18 and 20 are formed on the opposing faces of panes 14 and 16 (respectively), conductor 18 being in turn coated with W0₃ to form a thin working electrode 22 and conductor 20 in turn being coated with V₂0₅ to form a thin counter electrode 24. The manner in which such conductors and electrodes are applied is known in the art, normally being via vacuum-sputtering or via sol-gel deposition. A liquid polymer electrolyte 26 fills the narrow gap between electrodes 22 and 24 so that it forms a thin central layer of the cell. As already noted, suitable electrolytes are known in the art and available commercially. Finally, a peripheral seal 28 is formed between the panes to stop leakage of electrolyte, and electrical leads 30 and 32 are attached to conductors 18 and 20 respectively. Lead 30 is connected to ground and lead 32 carries the output of control circuit apparatus 12.

Control apparatus 12 basically comprises a voltage-limiting source 34 and a current- limiting source 36 supplied from a balanced DC power supply 38. Voltage-limiting source 34 and current-limiting source 36 can be manually or automatically switched, via control input 40 from a master controller unit 41 , to generate positive or negative outputs on their respective power output lines 42 and 44, either of which is selectable by a two-position relay 46 having a pole 46a which normally (ie, when not energised) selects current-limiting output line 44. Master controller 41 generates a signal on line 40 indicating whether cell 10 is to be coloured or bleached (charged or discharged). Controller 41 itself may be manually or automatically controlled by an input indicated at 43, the controller having the logic necessary to ensure that, if an input 43 requires cell 10 to be coloured to a certain level, the output on line 40 will initially be such as to effect complete cell discharge before the desired charge is applied. [Cell charge or discharge is determined by the polarity of power-supply 5 outputs 42 and 44.]

Relay 46 is connected in series with a double-pole off/on relay switch 48, one pole 48a of which being connected to electrode lead 32 of window cell 10 via a charge- measuring or detecting circuit 50 and a current measuring or sensing circuit 52, the

10 other pole 48b of which being connected to ground so as to signal the state of switch 48 to master controller 41 via line 53. Master controller 41 can, via an output on line 55, itself cause the operation of switch 48. Charge detector 50 is arranged to measure the total amount of charge (by integrating current with respect to time) delivered to cell 10 and to energise relay switch 48 via control line 54 to open switch

15 48 when a preset (safe maximum) charge has been delivered. Similarly, current sensor 52 is arranged to measure the electric current flowing to cell 10 and to energise relay switch 48 via control line 56 to open switch 48 when the current falls to a few percent, preferably about 5%, of the maximum current deliverable by source 36 for charging and for discharging. Finally, a voltage measuring or sensing circuit

20 58 is connected between lead 32 and ground and is arranged to measure the voltage applied across cell 10 and to energise relay 46, via line 59, to switch from current source 36 to voltage source 34 when the voltage across cell 10 reaches a preset maximum during cell charging or discharging, the voltage delivered by source 34 during charging or discharging being substantially equal to this maximum voltage.

25

It will be appreciated that, as indicated above, the preset values for charge, current and voltage may differ (i) between the charge and discharge cycles (that is, for positive and negative currents and voltages), (ii) according to the area of cell 10 and (iii) according to the electrode and electrolyte materials. It is desirable, therefore, that

30 voltage-limiting supply 34, current-limiting supply 36, charge detector 50, current detector 52 and voltage detector 58 have adjustable presets, indicated by inputs 60, 62, 64, 66 and 68 respectively. Additionally, input 64 which presets the charge to be delivered to window cell 10, and/or input 66 which sets the cut-off current, may be used (by controller 41 or by manual adjustment) to determine the degree of colouration of window 10.

In operation, presets 60 to 68 are set to the levels appropriate to the cell to be controlled. Power supply 38 is energised to supply regulated and balanced DC to voltage-limiting and current-limiting circuits 34 and 36. Assuming that a signal on input 43 to controller 41 indicates that colourisation is required and that the degree of colourisation is indicated by adjustment of preset 64, controller 41 first effects the total discharge of cell 10. This is done by (i) placing a signal on its output line 40 to cause sources 34 and 36 to generate negative outputs (ie, make to counter- electrode 24 negative with respect to working-electrode 22), and (ii) applying a signal to output line 55 to close switch 48 and connect current supply 36 to counter electrode 24 of cell 10 so that bleaching current flows through cell 10 at a maximum safe value determined by the current-limiting supply 36. After a little while, the increasing resistance of cell 10 will cause the voltage dropped across it to increase to the preset maximum safe magnitude (for bleaching) of voltage sensor 58, whereupon sensor 58 energises relay switch 46 via line 59 to connect the voltage- limiting source 34 to cell 10 instead of the current-limiting source 36. Discharge (bleaching) of the cell then continues under constant voltage conditions until current detector 52 detects that the current has dropped to 5% of the current level to which source 36 is limited, whereupon current sensor 52 opens switch 48 (via line 56) to terminate the discharge process. The fall in voltage across cell 10 is detected by sensor circuit 58 and causes the de-energisation of relay 46 so that it once again selects the output 44 of current source 36. It should be noted that the charge-limiter (circuit 50) is not active during the discharge phase.

Upon receiving the signal, via line 53, indicating that the discharge process is complete (ie, that switch 48 has opened), controller 41 (which is still programmed to effect cell colouration) applies a signal on line 40 to effect reversal of the polarity of the outputs of voltage and current circuits 34 and 36 and then closes switch 48 (via control line 55) to connect the positive (charging) current flow to cell 10. If the cell is new, current flows through the cell at the maximum preset rate of current-limiting circuit 36 until the charge limit set by circuit 50 is reached when charging is terminated by the opening of switch 48 via a signal on line 54. It the cell is old, the voltage dropped across the will rise to the maximum positive preset value of sensor circuit 58 which then energises relay 46 to connect the voltage-limiting source 34 to cell 10. Charging (colourisation) of the old cell then continues under constant voltage conditions until one of the following alternative events occurs first: master controller 41 receives a feedback signal (via input 43) from a separate light sensor (not shown) to indicate that colourisation is sufficient, • charge detector 50 indicates (via line 54) that the preset charge has been delivered, or ^♦ current sensor 52 indicates that the charge current has fallen to, say, 2% of the level to which current-limiting supply 36 was set. Whereupon switch 48 is opened, by a signal on the respective control line (55, 54 or 56) to disconnect the cell from the power supply. Again, the removal of voltage from the cell causes voltage detector circuit to operate relay 46 via line 59 to reconnect current supply 36 ready for the next discharge (bleaching) command.

Where master controller 41 receives a command via input 43 to bleach cell 10, it again switches supplies 34 and 36 to deliver negative-polarity voltage and current and again energises switch 48 to deliver constant current to cell 10. Again, this continues until voltage detector 58 energises relay 46 to select the voltage-limiting supply 34. Again, the discharge process continues until the discharge falls so low that current detector 50 de-energises switch 48 and disconnects the voltage supply from the cell. This time, however, the logic of master controller 41 does not signal subsequent charging or colourisation. Preferably, whenever the fall of discharge current indicates that the cell is fully bleached, controller 41 re-sets the charge counter 50 to zero.

The manner in which current density, voltage, charge density and contrast-ratio vary over a full colouring and bleaching cycle for a normal cell is shown in Figures 2A and 2B. Assuming that charging starts with cell voltage at zero and the cell fully bleached (contrast ratio = 1), closure of switch 48 causes the preset maximum current to flow from current supply 36 while the voltage across the cell steadily rises for about 160 seconds, at which time the voltage preset (sensed by circuit 58) is reached and switch 46 switches to the voltage-limited supply. Charging continues for another 80 seconds as the current steadily falls until, at about 240 seconds the charge limit is reached and the voltage-limiting supply is disconnected. At this stage, the cell is fully charged, its voltage remains near maximum, the current is zero, charge density is at its limit and the contrast ratio is about 4:1. After a few seconds (in this example), controller 41 reverses the polarity of supplies 34 and 36 ready for discharge and re- closes switch 48, whereupon cell voltage rapidly falls and cell current rapidly increases to its (negative) maximum preset magnitude. At about 400 seconds (from the start of the cycle), the magnitude of the cell voltage has increased to the (negative) limit set by sensor 58 and relay switch 46 is energised to connect the voltage-limited supply 34. The magnitude of the current then falls away until the minimum preset level (for discharge) is reached at about 460 seconds (from the cycle start), whereupon switch 48 is opened. At this stage, the cell voltage is slightly negative, the current is zero, the charge density is zero and the contrast ratio is about 1.

Figures 3A, 3B and 3C show, in a diagrammatic and comparative manner, the voltage and current curves for a new cell which has completed 83 cycles, a middle- aged cell which has completed 4,207 cycles and an old cell which has completed 10,105 cycles.

Since the young cell (Figure 3A) readily accepts its charge because of its low effective impedance, its voltage (solid line) rises slowly and charging current (broken line) flows at the maximum rate until the full charge has been delivered and charge- limiter circuit 50 terminates the colouring stage. After a delay (during which the cell may have self-discharged a little), discharge (bleaching) is initiated. Again discharge current flows at its maximum level until the voltage limit for discharge is reached, whereupon the current supply is disconnected and the cell voltage falls to near zero.

The higher effective impedance of the middle-aged cell (Figure 3B) causes the charging voltage (solid line) to rise more sharply, reaching the voltage limit before the full charge can be delivered under constant current. The consequent switch to voltage-limiting operation causes the current to fall off fairly rapidly. However, in this example, full-charge is reached just before the current minimum so that, once again, charge limiter 50 causes termination of colouration. While full charge (and full colouration) has been delivered, it has taken longer than for the young cell of Figure 3A. A similar situation applies to bleaching where the voltage-limiter is switched-in toward the end of the bleaching stage, causing the discharge current to rapidly fall to its minimum cut-off magnitude.

The effective impedance of the old cell (Figure 3C) is so high and the voltage limiter on charging switches in so early that the current (broken line curve) has time to fall to its cut-off level well before the maximum charge has been delivered. While the colouration is certainly not so intense as for the young or middle-aged cells, the cell is not severely damaged by over-charging as would be the case in prior art. The bleaching stage is similarly curtailed because the voltage (solid-line curve) quickly increases to its limit, causing the switch to the voltage-limited source and a rapid fa.ll- off in current until the preset minimum magnitude is reached.

In the above examples, the current-limiting source was set at 5 mA for both bleaching and colouration, while the voltage-limiting source was set at -1.5 V for bleaching and +1.8 V for colouration. The minimum current trip level was set at 250 μA (2% of the 5 mA maximum current); and, the maximum safe charge limit was set at 14 mC/cm².

While the cut-off current levels can be set in an arbitrary fashion at between 1% and 10% of maximum current, the total charge and voltage limits may need to be determined experimentally for each type of cell. As a rule of thumb, we have found the maximum safe charge for a cell of the type described in the example to be between 12 and 18 mC/cm². The safe cell voltage (V_d) may be determined by subtracting the safe voltage (V_c) of the counter electrode upon cell discharge from the safe voltage (V of the working electrode upon cell charge and adding the effective voltage drop due to the electrolyte itself, as indicated in the following formula:

V_d = V_w - V₀ + (j.p.d) where j is the current density through the electrolyte, p is the resistivity of the electrolyte and d is the distance between the working and counter electrodes across the electrolyte.

To determine V_w, the working electrode is placed close to a reference electrode (voltage probe) and both are immersed in bulk electrolyte loaded with charge carriers (eg Li⁺ ions). A third metal electrode is also immersed in the electrolyte, but well away from the reference and working electrodes. Charging current is then passed between the working and the third electrode while the voltage of the reference electrode is monitored. It will be found that the voltage sensed by the reference probe will rise rapidly and then plateau before rising again quite steeply. The safe voltage of the working electrode is then the plateau voltage or slightly higher. V_c is determined in the same manner, except that the counter electrode is substituted for the working electrode and current flow in the cell is reversed (to effect transfer of the charge carriers to the counter electrode). Once again, the plateau voltage sets the approximate safe voltage.

Though it has been found that substantial increases in cell or window life are achieved using the method, system and circuits described in the chosen examples, it will be appreciated by those skilled in the art that many modifications and alterations can be made without departing from the scope of the invention as defined by the following claims.

Claims

1 Apparatus for controlling the charging and discharging of an electrochromic cell, comprising: voltage-limiting circuit means adapted to limit the maximum voltage supplied to the cell during both charging and discharging, and cut-off circuit means adapted to cut-off current to the cell when the magnitude of the current falls below a predetermined magnitude during the charging or discharging of the cell.

2 Apparatus according to claim 1 wherein: current-limiting circuit means are provided to limit the magnitude of current flow through the cell when the cell is being charged and discharged, voltage sensor means are provided for sensing the voltage across the cell and for activating said current-limiting circuit means when the sensed voltage is below a predetermined magnitude and for activating said voltage-limiting circuit means when the sensed voltage is above said predetermined magnitude, and said cut-off means is operative to disconnect the cell from both current and voltage supply when the current to or from the cell falls below a predetermined magnitude.

3 Apparatus according to claim 1 or 2 having charge limiting-circuit means adapted to sense the total charge delivered to a cell during charging and to cut-off current supply to the cell when the total charge delivered reaches a predetermined magnitude.

4 Apparatus according to any preceding claim having control means for effecting the charging and discharging of the cell, the control means being adapted to first effect complete discharge of the cell before the cell is charged.

5 An electrochromic window system comprising an electrochromic cell suitable for use in a building as glazing and a power supply connected to the cell for effecting the colouring and bleaching of the cell by the insertion and extraction of electric charge, wherein said power supply comprises the apparatus as claimed in any preceding claim.

6 An electrochromic system comprising an electrochromic cell connected to a power supply for effecting the colouring and bleaching of the cell by the insertion and extraction of electric charge, wherein said power supply comprises: current-limiting circuit means adapted to supply DC power of variable voltage and magnitude-limited current, voltage-limiting circuit means adapted to supply DC power of variable current and magnitude-limited voltage, switch means operative to connect either said current-limiting means or said voltage- limiting means to the cell, and voltage sensor means adapted to sense the voltage applied to the cell, said voltage sensor means being connected to said switch means and being adapted to effect the connection of the current-limiting circuit means to the cell when the sensed voltage is below a first preset magnitude and to effect the connection of the voltage-limiting circuit means to the cell when the sensed voltage is above said first preset magnitude.

7 An electrochromic system according to claim 6 wherein: said switch means is adapted to disconnect both said current-limiting circuit means and said voltage-limiting circuit means from the cell, current-sensor circuit means are provided for sensing the magnitude of the current flowing to or from the cell, said current-sensor circuit means being connected to the switch means to effect disconnection of both the current-limiting means and the voltage-limiting means from the cell when (i) charging current flowing to the cell falls below a second preset magnitude and (ii) discharging current flowing from the cell falls below a third preset magnitude.

8 An electrochromic system according to claim 6 or 7 wherein: control circuit means are provided and connected both to said current-limiting circuit means and to said voltage limiting circuit means for controllably effecting the charging and discharging of the cell, charge-sensor means are provided for sensing the total charge inserted into the cell during charging thereof, said charge-sensor means being connected to said switch means and being adapted to cause the switch means to disconnect the current-limiter means and the voltage-limiter means from the cell when the charge inserted into the cell reaches a fourth preset magnitude, and wherein said control circuit means, upon receipt of a command to increase the charge inserted in a cell, is adapted to first effect the extraction of charge from the cell until said current-sensor circuit means detects that the current has fallen to said third preset magnitude, and to then effect the insertion of charge into the cell until either said charge-sensor means senses that the charge inserted into the cell has reached said fourth preset magnitude, or said current-sensor circuit means detects that the charging current has fallen to said second preset magnitude.

9 An electrochromic system according to any one of claims 5 to 8 wherein: the electrochromic cell comprises a working electrode including tungsten oxide, a counter electrode including vanadium oxide and an electrolyte including lithium ions, and wherein each of said second and third preset current magnitudes is less than 5% of the current level at which said current-limiting means is limited for cell charging and cell discharging, respectively.

10 A method of controlling the charge in an electrochromic cell comprising the steps of: connecting the cell to a power supply, charging the cell under current-limiting conditions until the cell voltage reaches a first preset magnitude, continuing to charge the cell under voltage-limiting conditions until either the charging current falls to a second preset magnitude or a preset charge has been delivered, and then disconnecting the power supply from the cell.

11 A method according to claim 10 including the step of monitoring the charge delivered to the cell and terminating charging of the cell when the total charge delivered to the cell reaches a third preset magnitude, even if the current has not fallen to said second preset magnitude. 12 A method according to claim 10 or 11 including the steps of: discharging the cell under current-limiting conditions until the cell voltage reaches a fourth preset magnitude, continuing to discharge the cell under voltage-limiting conditions until the discharge current falls to a fifth preset magnitude, and then disconnecting the power supply from the cell.

13 A method of charging an electrochromic cell including the steps of: first discharging the cell under current-limiting conditions until the cell voltage reaches a fourth preset magnitude, continuing to discharge the cell under voltage-limiting conditions until the discharge current falls to a fifth preset magnitude, and then charging the cell as claimed in claim 10 or 11.

14 A method according to any one of claim 10 to 13 wherein said preset voltage magnitude for charging and for discharging is near the safe cell voltage as defined herein.

15 A method of charging and discharging an electrochromic cell suitable for use as a smart window for buildings, characterised by the step of reducing the maximum charge which can be delivered to or extracted from the cell with increasing cell cycling or with cell aging.