TWI492482B - Master-slave type battery management system for accurate capacity gauge of battery pack - Google Patents

Description

Master servant battery management system for accurate battery capacity measurement

The present invention relates to a battery management system; and more particularly to a master servant battery management system that can be used to accurately measure the capacity of a battery formed by a plurality of battery packs.

Press, notebooks, tablets, mobile phones, and even large electric vehicles or robots, including a rechargeable battery with a series of rechargeable battery packs connected in parallel with a certain operating capacity. The foregoing rechargeable battery pack is composed of a plurality of rechargeable battery cells in series/parallel. As far as battery assembly plants are concerned, the rechargeable battery cells they purchase may come from different suppliers (or even self-produced), and the specifications may not be much different (discharge power, voltage). However, due to the difference in materials used, detailed design or processing precision, and even batch materials, the performance of rechargeable battery cells, such as charge and discharge, will vary. Mixing different incoming materials often leads to imbalance in the assembled rechargeable battery. However, it is necessary to avoid charging the battery when it is not mixed with different sources or using the same characteristics. The core of the pool has practical difficulties. For example, electric vehicles powered by large battery packs, because of the larger number of battery cores, battery packs are often repaired and repaired in a way that replaces the rechargeable battery cells to increase their life. It is difficult to require a replacement rechargeable battery cell to have the same characteristics as an existing rechargeable battery cell. Battery management and accurate capacity measurement for the above problems are not easy.

Master servant control is a method of using a device as a master device to control or manage other connected slave devices. Using this method in battery management, you can achieve the results of classified battery monitoring and unified data organization. It is a good method to divide the rechargeable battery cells into different main servant battery packs according to different characteristics for management and measurement. A related prior art is disclosed in U.S. Patent No. 7,453,236, which is incorporated herein by reference. Figure 1 depicts a multi-battery system of the technology. The system includes a main battery pack 100 and first to Nth slave battery packs 101-10N. The main battery pack 100 is connected in parallel with the first to Nth slave batteries 101 to 10N to the power output terminals V+ and V-, and outputs predetermined power to the power output terminals V+ and V-. The main battery pack 100 has the same configuration as the first to Nth slave battery packs 101 to 10N, except that the operation is different from the main battery, so only one battery pack will be described in detail below.

Referring to FIG. 2, when set as the main battery pack, one of the battery pack controllers 200 receives the total voltage from the first to the Nth slave battery packs 101 to 10N, and operates from a battery voltage and current detecting circuit. 202 receives the total voltage from a battery string 204 (containing 8 cells in series) itself. Next, the controller 200 calculates the target total voltage of the entire battery pack, and calculates the calculated target total voltage. It is sent to the first to Nth slave battery packs 101 to 10N, and the switch unit 206 is controlled in accordance with the target total voltage. Further, when set as the servant battery pack, the controller 200 sends its own total voltage according to the requirements of the main battery pack 100, and controls the switch unit 206 according to the target total voltage supplied from the main battery pack 100. A switching element controller 208 in the switching unit 206 controls the switching of a charging switching element 2081 and a discharging switching element 2082 according to a target total voltage in a charging and discharging phase, respectively. The system uses a drive signal to operate the controller 200, which is coupled to another battery pack via a communication path to conduct RS232 compliant sequence communication.

The most special feature of ‘236 is its way of choosing the main battery pack. The method is to let each battery pack determine whether the voltage signal in the communication line is high or low in a first time when a set main battery signal is just started or received. If it is low, it means that other battery packs want to be the main battery pack, then set to the servant battery pack when receiving another set main battery signal indicating which is the main battery pack; if it is high, it means other battery If the group does not want to be the main battery pack, it will send a low voltage signal in a second time, and after the second time expires, a high voltage signal will be issued. If a battery pack detects that the voltage signal from the communication line is high after a high voltage signal is issued, indicating that the other battery packs do not want to be the main battery after the inquiry, the battery pack is set as the main battery pack by itself. And issue the aforementioned set main battery signal for indicating which one has been the main battery pack to the other battery pack. It is conceivable that if each battery pack emits a high voltage signal at the end of the second time, it is not impossible to confirm which is the main one. Battery pack? In particular, the patent specifies a different second time for different battery packs. This means that the choice of the main battery pack has a prioritized difference regardless of environmental variation and signal transmission problems.

The above patents provide good master control technology. Using the role of the main servant battery pack, a single ordered window is provided, so that the voltage calculation of each battery pack is completed in the main battery pack, and can be most efficiently transmitted back to each servant battery pack so that the battery pack system can be charged. During discharge, charge and discharge of a battery pack are controlled to avoid problems such as excessive charge and discharge.

However, according to the battery system of the patent, the role of the main battery pack is not to take a turn of each battery pack, and it is easy to cause excessive dependence on resources in a battery pack, causing a burden on the relevant management components in the battery pack, affecting its use. Life and performance. In addition, for the selection of the main battery pack, the order is already fixed. If there is a problem with the management component of a battery pack, it will drag down the operation of the entire system. In addition, the system can only detect voltage, and the remaining power related information cannot be obtained by the system. The above listed shortcomings, the solution, is urgently needed by the industry to provide a more complete battery system and battery management system.

It is known that the battery pack controlled by the main servant does not effectively utilize its management components, resulting in a burden on the relevant management components in the battery pack, affecting its service life and performance. In addition, there is no requirement for the priority of the management component to be used, and it is easy for the management component of the problem to continue to be the master. The role of the system, dragging down the operation of the entire system. Finally, the measurement range of the battery pack is only a voltage value, and the range is too narrow.

Therefore, it is necessary to develop a battery management system using master and servant management, effectively utilize all battery management components, and replace the main battery management components in time to solve the problem of excessive use. In addition, battery management components must not have a preset order of use. Finally, the battery management system should be able to effectively measure a variety of physical data about the battery power, preferably calculating the application values derived from the physical data.

The main servant battery management system for accurately measuring the capacity of a battery composed of a plurality of rechargeable battery packs includes a plurality of battery management units connected to each other by a communication line, and each battery management unit is respectively A specific rechargeable battery pack is connected to manage the specific rechargeable battery pack, detect physical measurement data of the specific rechargeable battery pack, and calculate a battery pack capacity value, wherein each battery management unit is given a unique one. The battery ID, by designating the battery management unit with a specific battery ID as the main battery management unit, the remaining battery management units become the servant battery management unit according to the respective battery IDs; the servant battery management unit will detect the detected physical quantity Data and calculated battery capacity value are transmitted from the communication line to the main battery management unit; the main battery management unit detects physical measurement data of the corresponding rechargeable battery pack and calculates the battery capacity value, and accumulates from each battery management unit Physical measurement data for a battery to detect physical data, calculate the battery capacity from each battery management unit A numerical value of the battery capacity, and physical properties of the cell and detecting The battery capacity value is transferred to a host connected to the battery management system.

According to the present concept, the host can manage the rechargeable battery pack connected to the battery management unit by identifying the battery ID and communicating with the battery management unit having a specific battery ID.

According to the concept of the present invention, the host can communicate with the battery management unit having a specific battery ID by identifying the battery ID, and obtain physical measurement data or calculated battery capacity value of the rechargeable battery pack connected to the battery management unit.

According to the present concept, the main battery management unit is specified by the host.

According to the present concept, the designated main battery management unit may decide to make a certain battery management unit a new main battery management unit, and reset itself to the servant management unit, and notify the host of the decision.

According to the present concept, the physical measurement data is a voltage value, a current amount, or a coulomb count.

According to the concept of the case, the battery capacity value is the full charge amount, the remaining power, the estimated power supply time or the estimated charge time.

According to the present concept, the battery detection physical data is an accumulated value of physical measurement data from each battery management unit.

According to the present concept, the battery capacity value is the cumulative value of each battery pack capacity value or the maximum value of each battery pack capacity value.

According to the concept of the present invention, the battery management unit further includes: a measuring component connected to the corresponding battery pack for measuring the rechargeable battery pack Physical measurement data; a management component electrically connected to the measurement component for calculating and/or transmitting the physical measurement data, the battery detection physical data and the battery capacity value, and accepting the command of the host to control the Recharge the battery pack, and when designated as the main battery management unit, according to a restriction condition, decide to make a certain battery management unit a new main battery management unit, and reset itself to the servant battery management unit, and decide this. Notifying the host; a memory component electrically connected to the management component for memorizing and providing the battery ID; and a communication component electrically connected to the management component and connected to the communication line for transmitting and receiving the physical measurement Data, battery capacity values, battery detection physical data and battery capacity values and communication with the host.

According to the concept of the present invention, the restriction condition is the number or time of use of the transmission battery to detect physical data or battery capacity values.

The battery management system of the invention can replace the main battery management unit in time to avoid the life decay caused by excessive use. The replacement of the main servant battery management unit does not have a preset order of use. Each battery management system should be able to effectively measure a plurality of physical measurement data about the battery power, accumulating the physical measurement data of each battery management unit as a battery to detect physical data, and calculating the battery capacity value of each battery management unit as one. The battery capacity value, and the battery detection physical data and battery capacity value are transmitted to a host connected to the battery management system for battery management data collection.

100‧‧‧Main battery pack

101‧‧‧Server battery pack

10N‧‧‧Server battery pack

200‧‧‧ controller

202‧‧‧Battery voltage and current detection circuit

204‧‧‧Battery string

206‧‧‧Switch unit

208‧‧‧Switching element controller

2081‧‧‧Charging switch components

2082‧‧‧Discharge switch components

30‧‧‧Battery

301‧‧‧First Battery Management Unit

3011‧‧‧Measurement components

3012‧‧‧Management components

3013‧‧‧Memory components

3014‧‧‧Communication components

302‧‧‧Second Battery Management Unit

303‧‧‧ Third Battery Management Unit

311‧‧‧First rechargeable battery pack

3111‧‧‧Charged battery core

312‧‧‧Second rechargeable battery pack

313‧‧‧ Third rechargeable battery pack

340‧‧‧Communication lines

350‧‧‧Power lines

40‧‧‧Power management device

50‧‧‧ internal load

Figure 1 is a block diagram showing a conventional technique for applying a multi-battery system for master-servant control.

Fig. 2 is a block diagram showing the internal structure of the battery pack.

FIG. 3 is a block diagram of a battery management system according to an embodiment of the present invention.

Figure 4 is a block diagram showing the connection mode of the rechargeable battery cells in a rechargeable battery pack.

Figure 5 is a block diagram showing the internal structure of a battery management unit.

Please see Figures 3 to 5. 3 is a battery management system according to an embodiment of the present invention, and a power management device inside the notebook computer using the battery management system, and FIG. 4 is a charging battery core connection in a rechargeable battery pack in the embodiment. Mode, FIG. 5 is an internal structure of a battery management unit in the embodiment.

A battery 30 includes a first rechargeable battery pack 311, a second rechargeable battery pack 312 and a third rechargeable battery pack 313, and a first and a second rechargeable battery pack 311, a second rechargeable battery pack 312 and a third rechargeable battery. The battery unit 313 is connected to a first battery management unit 301, a second battery management unit 302 and a third battery management unit 303. The first rechargeable battery pack 311, the second rechargeable battery pack 312, and the third rechargeable battery pack 313 are connected in parallel with each other via a power line. The 350 provides DC power to an internal load 50 inside the notebook to drive the operation of the notebook. The first battery management unit 301, the second battery management unit 302, and the third battery management unit 303 are connected to each other by a communication line 340, and are connected to a power management device 40 inside the notebook computer for battery management data transmission. Each of the rechargeable battery packs 311, 312, and 313 is composed of a rechargeable battery cell of the same specification. The same specifications here mean that the factory-rated charge and discharge specifications of each rechargeable battery cell, such as discharge voltage, power, etc., are consistent. However, since the supplier of the rechargeable battery core may use different rechargeable battery cells to manufacture materials, and the actual characteristics of the batches of the same manufacturer may be different (or the difference caused by the storage time), according to the present invention, each charging in the embodiment The rechargeable battery cells of the battery pack need not be close to the incoming material characteristics, but the rechargeable battery cells of each rechargeable battery pack must use the same incoming material characteristics for battery management. The connection manner of the internal rechargeable battery cells of each of the rechargeable battery packs 311, 312 and 313 is the same, and the first rechargeable battery pack 311 is taken as an example, see FIG. The figure shows that the inside of the first rechargeable battery pack 311 is composed of two rechargeable battery cells 3111 connected in series.

The foregoing communication line 340 conforms to the internal integrated circuit (I2C) specification, but is not limited thereto, and conforms to the system management busbar specification (SMBus) sequence peripheral interface (SPI) specification or the communication protocol of the American Electronics Industry Association RS232 specification. 340 can also be applied to the present invention. It should be noted that although the communication line 340 is illustrated by a single line in FIG. 3, the communication line 340 may actually include two lines (the serial data used by the I2C and the serial arrangement). Pulse) or more lines. In understanding, it should not be restricted by illustration. The power management device 40 is a host. Due to the integration of the host functions, the integrated CPU has a power management function. Therefore, any device having a power management function, particularly a battery power management function, can be used as the power management device mentioned in the present invention.

In the embodiment of the present invention, it is necessary to cooperate with a designated main and servant battery management unit for battery management. The specified method is as follows: each battery management unit is given a specific battery ID, and the battery management unit with a certain battery ID is designated as the main battery management unit, and the remaining battery management units become the battery management according to the respective battery IDs. unit. Here, the battery ID of the first battery management unit 301 is HYC001, the battery ID of the second battery management unit 302 is HYC002, and the battery ID of the third battery management unit 303 is HYC003. After being set, the first battery management unit 301 having the HYC001 battery ID is the main battery management unit, and the second battery management unit 302 and the third battery management unit 303 are the slave battery management units. It is worth noting that a feature of the present invention is that the designated main battery management unit itself can decide to make a certain battery management unit a new main battery management unit due to certain factors, such as timeout use, and re-establish itself. Set as the servant battery management unit. For example, if the battery management unit is responsible for the operation data transmission time of the main battery management unit is one million times, after the hundredth data transmission, the first battery management unit 301 selects the second battery management unit 302 as the main battery management. The unit can be retired as a servant battery management unit to reduce the burden and extend the service life.

According to the present invention, the first battery management unit 301, the second battery management unit 302, and the third battery management unit 303 have the same configuration but the specified work contents are different. When the battery management system is in operation, the second battery management unit 302 and the third battery management unit 303 (servant battery management unit) respectively measure the physical measurement data detected by the second rechargeable battery pack 312 and the third rechargeable battery pack 313. And the calculated battery pack capacity value is transmitted from the communication line 340 to the first battery management unit 301 (main battery management unit). The first battery management unit 301 accumulates the physical measurement data of each battery management unit (including the main and the servant battery management unit) as a battery detection physical data, calculates the battery capacity value of each battery management unit as a battery capacity value, and The battery detects physical data and battery capacity values and transmits them to a host connected to the battery management system.

The power management device 40 can further manage the rechargeable battery pack connected to the battery management unit by identifying the battery ID and communicating with the battery management unit having a specific battery ID. For example, the power management device 40 can recognize the rechargeable battery pack imbalance from the battery ID-HYC002 by the communication line 340, and wants to temporarily let the second rechargeable battery pack 312 balance the charged battery cells, as long as an indication is given. The above objects can be achieved by the operation of the battery management system.

In addition, the power management device 40 can obtain the physical measurement data of the rechargeable battery pack connected to the battery management unit and/or the calculated battery capacity by identifying the battery ID and communicating with the battery management unit having a specific battery ID. Value. For example, the power management device 40 wants to know the current third charge. The discharge voltage value and the remaining power of the battery pack 313 can be issued as long as the corresponding battery ID is identified as HYC003, and the third battery management unit 303 having the battery ID-HYC003 is commanded to report the latest measured third rechargeable battery pack 313. The discharge voltage value is calculated with the remaining charge.

The physical measurement data referred to above refers to physical data measured directly by electronic components, such as voltage value, current amount or coulomb count. The present invention is not limited to the first three, and other physical quantities such as temperature may also be physical measurement data; for the present embodiment, the physical measurement data includes the foregoing three. Measuring a single physical measurement data or to partially specifying a measured voltage value and a current amount is in accordance with the requirements of the present invention. The battery capacity value is calculated according to the physical measurement data of different detections, and the value related to the power measurement is calculated according to a specific calculation method. According to the embodiment, the calculated battery capacity value includes the full charge amount, the remaining power amount, the estimated power supply time, the estimated charging time, the average voltage value, or the average current amount. The aforementioned battery pack capacity values may be individually transmitted, partially transmitted, or even fully transmitted in accordance with the requirements of the power management device 40.

After all the physical measurement data and the battery capacity value are transmitted from the second battery management unit 302 and the third battery management unit 303 to the first battery management unit 301 via the communication line 340, the first battery management unit 301 together with its physical quantity The measured data and the battery pack capacity value are calculated for the relevant data of the battery 30 itself. According to the present invention, battery detection physical data and battery capacity values can be obtained. In this embodiment, the amount of current of the battery 30 is each battery. The sum of the snap-ins. If at a certain time, the current amount of the first rechargeable battery pack 311 is 700 mA, the current amount of the second rechargeable battery pack 312 is 710 mA, and the current amount of the third rechargeable battery pack 313 is 705 mA, and the total current amount of the entire battery 30 is It is 2115mA. Similarly, the total coulomb count of battery 30 is also accumulated from the coulomb count of each battery management unit. In addition, each battery management unit can also detect the average current amount as the summed physical quantity. In other embodiments, the total voltage value is obtained from a series of rechargeable battery packs.

However, regarding the battery capacity value, the calculation method is not necessarily the cumulative value transmitted by each battery management unit. In the example of the embodiment, if each battery management unit transmits the calculated full charge amount of the corresponding rechargeable battery pack, the first battery management unit 301 (main battery management unit) calculates all the full charge amounts. Obtaining a total full charge of the battery 30; if each battery management unit transmits the calculated remaining power of the corresponding rechargeable battery pack, the first battery management unit 301 adds up all remaining power to calculate the total remaining power of the battery 30; Each battery management unit transmits an estimated required charging time of the corresponding rechargeable battery pack, and the first battery management unit 301 selects the maximum value of each estimated charging time as the estimated charging time of the battery 30; The management unit transmits the estimated power supply time of the corresponding rechargeable battery pack calculated by the management unit, and the first battery management unit 301 selects the maximum value of each estimated power supply time as the estimated power supply time of the battery 30. In addition, the first battery management unit 301 can further calculate a relative state of charge of the battery, which is a value obtained by dividing the total remaining power of the battery 30 by the total full charge of the battery 30, expressed as a percentage, and the result is also The battery capacity value is output.

The battery management system provided by the present invention is composed of a plurality of battery management units connected to each other by a communication line. The number of battery management units is not limited to three in the embodiment, and may be increased or decreased according to actual needs. Each battery management unit is the same, and each may exist in the form of a separate integrated circuit or in a manner that an integrated circuit includes several battery management units. This embodiment uses three independent integrated circuits, each of which includes a first battery management unit 301, a second battery management unit 302, and a third battery management unit 303. In order to understand the composition and operation mode of the battery management unit, please refer to FIG. 5, and the first battery management unit 301 is taken as an example for explanation.

In FIG. 5, the first battery management unit 301 includes a measuring component 3011, a management component 3012, a memory component 3013, and a communication component 3014. The measuring component 3011 is connected to the first rechargeable battery pack 311 to measure physical measurement data of the rechargeable battery pack 311. A management component 3012 is electrically connected to the measuring component 3011 for calculating and/or transmitting the physical measurement data, the battery detection physical data and the battery capacity value, and the command management charging battery pack 311 of the power management device 40. . When the first battery management unit 301 is designated as the main battery management unit, the management component 3012 may decide to make a certain battery management unit (the second battery management unit 302 or the third battery management unit 303) new according to a restriction condition. The main battery management unit is itself reset to the servant battery management unit, and the decision is notified to the power management device 40. As mentioned above, the number of operation data transmissions when each battery management unit is responsible for the main battery management unit is One million times, after the hundredth data transmission, the main battery management unit (first battery management unit 301) selects another servant management unit as a new main battery management unit, and can be retired as a servant battery management unit. To reduce the burden and extend the service life. Here, the hundredth data transmission is the constraint. Similarly, in terms of usage time, if the continuous use of 24 hours is a threshold, once the main battery management unit continuously operates for more than 24 hours, the main battery management unit replacement operation can be performed.

A memory component 3013 is electrically coupled to the management component 3012 for memorizing and providing the battery ID. The communication component 3014 is electrically connected to the management component 3013 and connected to the communication line 340 for transmitting and receiving the physical measurement data, the battery capacity value, the battery detection physical data, and the battery capacity value. Additionally, communication component 3014 can communicate with power management device 40 to receive commands for battery management.

It should be noted that the present invention is not limited to a small battery management system, such as a battery management system for a notebook computer battery; and a battery that requires a large number of rechargeable battery packs to be powered, the battery management system provided by the present invention is applicable. For example, the battery of an electric vehicle is applicable. In addition, an advantage of the application of the present invention is that when a battery having the battery management system of the present invention is to be replaced and repaired, it is only necessary to replace the rechargeable battery cells of the same source or characteristics with a rechargeable battery pack. It is not necessary to consider whether the rechargeable battery cells are similar to those in other rechargeable battery packs. That is to say, the rechargeable battery cells used between rechargeable battery packs can vary, including power, impedance or composition. The difference in things.

Although the present invention has been disclosed in the above embodiments, it is not intended to limit the invention, and any one of ordinary skill in the art can make some modifications and refinements without departing from the spirit and scope of the invention. The scope of the invention is defined by the scope of the appended claims.

30‧‧‧Battery

301‧‧‧First Battery Management Unit

302‧‧‧Second Battery Management Unit

303‧‧‧ Third Battery Management Unit

311‧‧‧First rechargeable battery pack

312‧‧‧Second rechargeable battery pack

313‧‧‧ Third rechargeable battery pack

340‧‧‧Communication lines

350‧‧‧Power lines

40‧‧‧Power management device

50‧‧‧ internal load

Claims (11)

A master servant battery management system for accurately measuring the capacity of a battery composed of a plurality of rechargeable battery packs, comprising a plurality of battery management units connected to each other by a communication line, each battery management unit and a specific charging The battery pack is connected to manage the specific rechargeable battery pack, detect physical measurement data of the specific rechargeable battery pack, and calculate a battery pack capacity value, wherein each battery management unit is given a unique battery ID By designating the battery management unit with a specific battery ID as the main battery management unit, the remaining battery management units become the servant battery management unit; each servant management unit compares the detected physical measurement data with the calculated battery capacity. The value is transmitted from the communication line to the main battery management unit; the main battery management unit detects the physical measurement data of the corresponding rechargeable battery pack and calculates the battery capacity value, and accumulates the physical measurement data from each battery management unit as one The battery detects physical data and calculates the battery capacity value from each battery management unit as a battery capacity Value, and the battery detects physical data and battery capacity values to a host connected to the battery management system. The battery management system of claim 1, wherein the host can communicate with the battery management unit having a specific battery ID by identifying the battery ID, and performing charging on the rechargeable battery unit connected to the battery management unit. management. The battery management system of claim 1, wherein the host can communicate with the battery management unit having a specific battery ID by identifying the battery ID to obtain the physicality of the rechargeable battery pack connected to the battery management unit. Measure data or calculated battery capacity values. The battery management system of claim 1, wherein the main battery management unit is specified by the host. The battery management system of claim 1, wherein the designated main battery management unit determines to make a servant battery management unit a new main battery management unit, and resets itself to a servant battery management unit, and Notify the host of this decision. The battery management system of claim 1, wherein the physical measurement data is a voltage value, a current amount, or a coulomb count. The battery management system of claim 1, wherein the battery capacity value is a full charge amount, a remaining power amount, an estimated power supply time, or an estimated charge time. The battery management system of claim 1, wherein the battery detection physical data is an accumulated value of physical measurement data from each battery management unit. The battery management system of claim 1, wherein the battery capacity value is a cumulative value of each battery pack capacity value or a maximum value of each battery pack capacity value. The battery management system of claim 1, wherein the battery The management unit further includes: a measuring component connected to the corresponding rechargeable battery pack for measuring physical measurement data of the rechargeable battery pack; a management component electrically connected to the measuring component for calculating and/or Transmitting the physical measurement data, the battery detection physical data and the battery capacity value, accepting the command of the host to control the rechargeable battery pack, and when designated as the main battery management unit, according to a restriction condition, determining to give a certain A servant battery management unit becomes a new main battery management unit, and is itself reset to the servant battery management unit, and notifies the host of the decision; a memory component is electrically connected to the management component for memorizing and providing the battery ID And a communication component electrically connected to the management component and connected to the communication line for transmitting and receiving the physical measurement data, the battery capacity value, the battery detection physical data and the battery capacity value, and communicating with the host. The battery management system of claim 10, wherein the restriction condition is a number of times or a time of use for transmitting the battery to detect physical data or a battery capacity value.