CN101216538A - Lithium battery capacity measuring apparatus and determination method - Google Patents

Abstract

The invention provides a lithium cell capacity measurement device and a determination method are a device and a method for calculating the capacity of a lithium cell according to the constant current and the service life of the cell obtained by the measurement device. The device comprises a lithium cell (2), an A/D converter (3), an LCD (4) and a microprocessor (5), wherein a constant-current load consists of a voltage regulating chip (1) and a load resistor (RL) and is serially connected with the lithium cell (2) and a sampling resistor (Rrest) to form a loop to achieve constant current discharge of the lithium cell (2); the sampling resistor (Rrest) is respectively connected with a first sampling end (AD_In1) and a second sampling end (AD_In2) of the A/D converter (3) at both ends thereof; the digital port (A/DC I/O) of the A/D converter (3) is connected with the first input/output port (I/O 1) of the microprocessor (5); a parallel pull-down resistor (Rdown) of the lithium cell (2) is connected with the second input/output port (I/O 2) of the microprocessor (5); and the third input/output port (I/O 3) of the microprocessor (5) is connected with the digital port (LCD I/O) of the LCD (4).

Description

The measurement mechanism of lithium battery capacity and the method for determining

Technical field

The present invention relates to mensuration and definite method, belong to the technical field of lithium battery parameter determination method lithium battery capacity.

Background technology

Outstanding advantage is widely used in the mobile electronic devices such as mobile phone, MP3, digital camera and notebook computer lithium battery because of its output voltage height, capacity density be big etc.

The capacity of determining lithium battery is significant for the use of lithium battery.On the one hand, battery capacity is the important indicator that the user selects lithium battery, and the user tends to be chosen in given shape and weight requires the bigger battery of capacity down.Definite employing usually of lithium battery nominal capacity is a kind of more coarse method in the reality, and promptly capacity equals the life-span of lithium battery under the partial discharge electric current and the product of this electric current.In fact, the numerical value of this product is with different under the different discharge currents, thereby the battery capacity that this method obtains is inaccurate.On the other hand, mobile electronic device is along with the raising of the increase of its function and performance and consume increasing energy, and to provide the capacity of the lithium battery of energy to gather way slow relatively for these equipment, and energy requirement that mobile device increases fast and the lithium battery technology slowly contradiction between the development become increasingly conspicuous.Improving energy content of battery service efficiency is to alleviate an important means of contradiction between the two, and concrete technology has dynamic power management and dynamic electric voltage adjusting etc.In order to make management decision accurately farthest to improve mobile device energy service efficiency, these power managed technical requirements comprise that each system element of lithium battery provides component information accurately, and be even more important as the information of the lithium battery of mobile device energy source, the energy sum that on behalf of equipment, battery capacity can use, whether the battery capacity that provides accurately will produce material impact to the effect of power managed strategy.

The imperfect flash-over characteristic of lithium battery (as speed-capacity effect, it is big more to show as discharge current, and the lithium battery output capacity is more little) makes and has complicated nonlinear relationship between the output capacity of lithium battery and battery discharge current.Different battery models is described both relations with different modes.For the relation between cell load current i (τ) and life-span (equipment service time) L is carried out accurate modeling, Rakhmatov D (referring to RakhmatovD.A Model for Battery Lifetime Analysis for Organizing Applications ona Pocket Computer[J] .IEEE Transaction on VLSI systems, 2003,11 (6): 1019-1030) proposed an accurate lithium battery analytic model, its expression formula is:

$\mathrm{\α}={\∫}_0^L\text{i}\left(\mathrm{\τ}\right)\mathrm{d\τ}+\underset{\mathrm{\ϵ}\→0^{+}}{\lim }2\underset{m=1}{\overset{\∞}{\mathrm{\Σ}}}{\∫}_0^{L-\mathrm{\ϵ}}i\left(\mathrm{\τ}\right)e^{-{\mathrm{\β}}^2{m}^2(L-\mathrm{\τ})}\mathrm{d\τ}$ Formula (1)

Wherein α and β are battery parameters, and α represents the theoretical capacity of lithium battery, and unit is mA-min, and β is illustrated in the speed that the movable charge carrier of electrode surface is compensated, and also represent the imperfect degree that lithium battery capacity consumes, and unit is s ^-1

When cell load current i (τ) was the constant electric current I, formula (1) can be reduced to not influencing under the computational accuracy situation:

$\mathrm{\&alpha;}\&ap;I\left[\begin{array}{cc}L+2\underset{m=1}{\overset{10}{\mathrm{\&Sigma;}}}& \frac{1-e^{-{\mathrm{\&beta;}}^2{m}^{2}L}}{{\mathrm{\&beta;}}^2{\mathrm{<figure-callout\; id="2"\; label="m"\; filenames="S2008100193386E00011.png"\; state="\{\{state\}\}">m</figure-callout>}}^2}\end{array}\right]$ Formula (2)

According to formula (2), lithium battery capacity α can determine by the following method: according to formula (2) a plurality of constant electric current I and corresponding battery life L are carried out data fitting.But because formula (2) expression formula more complicated, the result is undesirable usually when carrying out match with mathematical softwares such as SAS, and the parameter beta that obtains may cause this parameter not have physical significance less than 0, and this will influence the accuracy of the parameter alpha that obtains simultaneously.

Summary of the invention

Technical matters: the objective of the invention is to provide a kind of measurement mechanism and the method for determining of lithium battery capacity, the cell constant electric current and the battery life counting cell capacity that obtain according to measurement mechanism, thus accurately determine lithium battery capacity.

Technical scheme: the measurement mechanism of lithium battery capacity of the present invention comprises lithium battery, analog to digital converter, LCD and microprocessor, wherein Constant Current Load is made up of voltage stabilizing chip and pull-up resistor, it and lithium battery, sampling resistor are connected into a loop, realize that lithium battery is with the constant current discharge; The two ends of sampling resistor connect first sampling end and second sampling end of analog to digital converter respectively, and the digital port of analog to digital converter connects first I/O port of microprocessor; Connect second I/O port of microprocessor behind the lithium battery pull down resistor in parallel; The 3rd I/O port of microprocessor connects the digital port of LCD.

Definite method of the lithium battery capacity of the measurement mechanism of lithium battery capacity makes lithium battery discharge with different constant electric currents by pull-up resistor resistance in the measurement mechanism that changes lithium battery capacity, establishes lithium battery respectively with the constant electric current I ₁, I ₂..., I _nDischarge, the corresponding battery life-span is respectively L ₁, L ₂..., L _n, n is the positive integer greater than 2; Make J _i=1/I _i, i=1,2 ..., n, then battery capacity α is calculated as

$\mathrm{\&alpha;}=\frac{\underset{i=1}{\overset{n}{\mathrm{\&Sigma;}}}{J}_i{L}_i-n\stackrel{\&OverBar;}{J}\stackrel{\&OverBar;}{L}}{\underset{i=1}{\overset{n}{\mathrm{\&Sigma;}}}{J}_i^2-n{\stackrel{\&OverBar;}{J}}^2}$ Wherein $\stackrel{\&OverBar;}{J}=\frac{1}{n}\underset{i=1}{\overset{n}{\mathrm{\&Sigma;}}}{J}_i,$ $\stackrel{\&OverBar;}{L}=\frac{1}{n}\underset{i=1}{\overset{n}{\mathrm{\&Sigma;}}}{L}_i.$

Determining step is:

1) first sampling end of analog to digital converter sampling lithium battery output voltage,

2) first sampling end of analog to digital converter and the voltage difference between second sampling end are exactly sampling resistor both end voltage V _Test, battery discharge current I=V _Test/ R _Test,

3) when lithium battery does not have place in circuit, first sampling end is a low level, shows not discharge of lithium battery; First sampling end becomes high level behind the lithium battery place in circuit, shows that lithium battery begins discharge; Microprocessor starts timer when first sampling end becomes high level, and control analog to digital converter timing sampling lithium battery voltage, when lithium battery voltage smaller or equal to shutoff voltage V _CutoffThe time, timer stops, and the result of timer is exactly lithium battery life-span L,

4) after microprocessor obtains lithium battery discharge current I and life-span L, show by LCD.

Beneficial effect: measurement mechanism of the present invention is realized lithium battery with the constant current discharge, and accurately measures battery life under battery discharge current and this constant electric current; Lithium battery capacity determines that method is based on accurate battery model, and be foundation with the accurate battery data that obtains by measurement mechanism, it can accurately determine lithium battery capacity, this method provides a kind of accurate lithium battery capacity scaling method, accurate battery capacity information not only provides an important indicator for consumer evaluation's lithium battery quality, can also help the power managed strategy to improve lithium battery energy service efficiency, prolong mobile device service time.

Description of drawings

Fig. 1 is the measurement mechanism structural drawing of lithium battery capacity.

Fig. 2 is the measurement mechanism embodiment schematic diagram of lithium battery capacity.

Embodiment

Embodiment at first provides definite method of lithium battery capacity, it comprises battery model that this method adopts and determines method based on the capacity of this battery model, provide the measurement mechanism of the required lithium battery data of this method of acquisition then, provide example at last according to lithium battery discharge current and Life Calculation lithium battery capacity.

Obtain the lithium battery model by in the error allowed band, Rakhmatov D model being simplified.Order $\mathrm{\&gamma;}=2\underset{m=1}{\overset{10}{\mathrm{\&Sigma;}}}\frac{1}{{\mathrm{\&beta;}}^2{\mathrm{<figure-callout\; id="2"\; label="m"\; filenames="S2008100193386E00011.png"\; state="\{\{state\}\}">m</figure-callout>}}^2},$ $A\left(L\right)=2\underset{m=1}{\overset{10}{\mathrm{\&Sigma;}}}\frac{e^{-{\mathrm{\&beta;}}^2{m}^{2}L}}{{\mathrm{\&beta;}}^2{\mathrm{<figure-callout\; id="2"\; label="m"\; filenames="S2008100193386E00011.png"\; state="\{\{state\}\}">m</figure-callout>}}^2},$ Then formula (2) can equivalence be expressed as:

$\frac{\mathrm{\&alpha;}}{I}=L+\mathrm{\&gamma;}-A\left(L\right)$

Notice exponential function f (x)=e ^-x, when x (x＞0) increased, functional value descended very fast.According to the functional value of exponential function this characteristics that descend fast, because A (L) is a series of exponential function sums, so when battery life L is enough big, an A (L) can make the A (L)=0 in the following formula at this moment with very little and can ignore.Thus, when the lithium battery load was the constant electric current I, battery life L satisfied following relational expression:

$L=\frac{\mathrm{\&alpha;}}{I}-\mathrm{\&gamma;}$ Formula (3)

Wherein lithium battery capacity α and parameter γ are constants, and be only relevant with battery.The analysis of formula (3) scope of application can be referring to being permitted ginseng. a kind of lithium-ions battery Life Prediction model [J]. and applied science journal, 2006,24 (4): 368-371.

According to formula (3), when known 2 Battery pack constant electric current I and corresponding life-span L, α and γ just can be determined.For example, suppose that known given lithium battery is in the constant electric current I ₁The time life-span be L ₁, in the constant electric current I ₂The time life-span be L ₂, I ₁≠ I ₂, then $\mathrm{\&alpha;}=\frac{(L_1-L_2)I_1{I}_2}{I_2-I_1}.$ But, may there be measuring error in these constant electric current I and corresponding life-span L data, thereby influence the accuracy of the battery capacity α that obtains by this formula, the influence that this measuring error causes can reduce by statistical method, concrete grammar is to measure many Battery packs constant electric current I and corresponding life-span L, then these measurement data is done linear regression analysis.

If lithium battery is respectively with the constant electric current I ₁, I ₂..., I _nDischarge, the corresponding battery life-span is respectively L ₁, L ₂..., L _n, n is the positive integer greater than 2.If J _i=1/I _i, i=1,2 ..., n.According to the theory (referring to " engineering mathematics method (the 3rd fascicle) ", publishing house of Southeast China University) of linear regression analysis, lithium battery capacity α and parameter γ may be calculated:

$\left\{\begin{array}{c}\mathrm{\&alpha;}=\frac{\underset{i=1}{\overset{n}{\mathrm{\&Sigma;}}}{J}_i{L}_i-n\stackrel{\&OverBar;}{J}\stackrel{\&OverBar;}{L}}{\underset{i=1}{\overset{n}{\mathrm{\&Sigma;}}}{J}_i^2-n{\stackrel{\&OverBar;}{J}}^2}\\ \mathrm{\&gamma;}=-\stackrel{\&OverBar;}{L}+\mathrm{\&alpha;}\stackrel{\&OverBar;}{J}\end{array}\right.$ Formula (4)

Wherein $\stackrel{\&OverBar;}{J}=\frac{1}{n}\underset{i=1}{\overset{n}{\mathrm{\&Sigma;}}}{J}_i,$ $\stackrel{\&OverBar;}{L}=\frac{1}{n}\underset{i=1}{\overset{n}{\mathrm{\&Sigma;}}}{L}_i.$

Determine that from above-mentioned lithium battery capacity method can see, determine that lithium battery capacity α needs the measurement data of many group lithium battery constant discharge current I and corresponding life-span L, these data will obtain by measurement mechanism.

Because lithium battery voltage constantly descends in the discharge process, if directly a resistance as cell load, battery discharge current will descend and constantly minimizing can not keep constant with cell voltage so.In order to make lithium battery carry out the constant current discharge and to measure battery life under this discharge current, the present invention has provided a measurement mechanism, and its structure as shown in Figure 1.The measurement mechanism of lithium battery capacity of the present invention comprises lithium battery 2, analog to digital converter 3, LCD4 and microprocessor 5, and wherein Constant Current Load is by voltage stabilizing chip 1 and pull-up resistor R _LForm it and lithium battery 2, sampling resistor R _TestBe connected into a loop, realize that lithium battery is with the constant current discharge; Sampling resistor R _TestTwo ends meet the first sampling end AD_In1 and the second sampling end AD_In2 of analog to digital converter 3 respectively, the digital port A/DC I/O of analog to digital converter 3 meets the first I/O port I/O 1 of microprocessor 5; Lithium battery 2 pull down resistor R in parallel _DownAfter meet the second I/O port I/O 2 of microprocessor 5; The 3rd I/O port I/O 3 of microprocessor 5 meets the digital port LCD I/O of LCD4.

The job step of this measurement mechanism is:

1) voltage that samples of the first sampling end AD_In1 of analog to digital converter 3 is exactly lithium battery 2 output voltages,

2) first sampling end AD_In1 of analog to digital converter 3 and the voltage difference between the second sampling end AD_In2 are exactly sampling resistor R _TestBoth end voltage V _Test, battery discharge current I=V _Test/ R _Test,

3) when lithium battery 2 does not have place in circuit, the first sampling end AD_In1 is a low level, shows not discharge of lithium battery 2; The first sampling end AD_In1 becomes high level behind lithium battery 2 places in circuit, shows that lithium battery 2 begins discharge; Microprocessor 5 starts timers when the first sampling end AD_In1 becomes high level, and the voltage of control analog to digital converter 3 timing sampling lithium batteries 2, when lithium battery 2 voltages less than shutoff voltage V _CutoffThe time, timer stops, and the result of timer is exactly lithium battery life-span L.

4) after microprocessor 5 obtains the discharge current I and life-span L of lithium battery 2, show by LCD4.

The measurement mechanism embodiment of lithium battery capacity is seen Fig. 2.

Constant Current Load makes lithium battery with the constant current discharge.Constant Current Load requires current stabilization, and the stable here influence that had both referred to that size of current was not changed by both end voltage refers to that also electric current is subjected to influence of thermal effect little.Be Constant Current Load in the frame of broken lines among Fig. 2, it is by voltage stabilizing chip TPS72518 and pull-up resistor R _LRealize.When input end IN voltage was in 2.8V～5.5V scope, chip TPS72518 can guarantee that the voltage of output terminal OUT remains on 1.8V.The GND end electric current of TPS72518 can be ignored in the microampere magnitude, thereby can think that the electric current of IN and two ports of OUT equates that size of current is I=1.8/R _L, by the regulating load resistance R _LResistance can regulate electric current I.In addition, Constant Current Load in use will produce a large amount of heats, and this will require the thermal stability of Constant Current Load to get well, and electric current I is subjected to the Constant Current Load influence of temperature variation little.The TPS72518 influence of temperature change of originally experiencing is little, and when input voltage was 2.8V, temperature changed to 120 ℃ from 20 ℃, and its output voltage changes only 10mV, and the electric current that causes thus changes very little; Metalfilmresistor temperature coefficient commonly used is bigger, thereby pull-up resistor R _LNeed to adopt the little precision resistance of temperature coefficient, and dispel the heat for the ease of pull-up resistor, the pull-up resistor of a certain resistance is realized by a plurality of identical resistance are in parallel, heat is dispelled the heat by a plurality of thermals source, avoid single heat source temperature too high and influence the pull-up resistor resistance, cause the lithium battery discharge current to change.

Voltage sample realizes that by analog to digital converter chip MAX1301 it is 16 analog to digital converters.The voltage reference 4.096V voltage that reference voltage adopts chip to provide, because lithium battery voltage variation range is generally 2.9～4.1V in the discharge process, this reference voltage can satisfy the requirement of sampling lithium battery voltage basically.The precision of sampled voltage is $\frac{4.096}{2^{16}-1}\&ap;\frac{1}{16}\mathrm{mV}.$ Simultaneously, be not subjected to the electromagnetic interference (EMI) of external environment, the circuit of sampling section is added a radome in order to make voltage sample.The voltage that sample port CH1 samples is exactly the lithium battery output voltage.

Sampling resistor R _Test=50m Ω, the voltage difference V at its two ends _TestEqual voltage difference between MAX1301 sample port CH1 and CH2, then lithium battery discharge current I=V _Test/ 0.05.

Chip C8051F040 is a kind of SOC of 8051 micro-processor kernels at a high speed that adopts, and it is two functions of main realization in measurement mechanism: 1) control MAX1301 sampling lithium battery output voltage and sampling resistor R _TestBoth end voltage is poor, to determine lithium battery output voltage and discharge current; 2) control LCD shows and lithium battery discharge current and life-span is shown on LCD.Whether C8051F040 begins when judging the lithium battery discharge gage by the state of inquiry port P0.4: when lithium battery did not have place in circuit, P0.4 voltage was low level, and timer does not start; When the lithium battery place in circuit began to discharge, P0.4 voltage was high level, timer initiation, and C8051F040 begins timing sampling lithium battery output voltage.Lithium battery shutoff voltage V is set _CutoffFor 3.0V (when the lithium battery output voltage drops to V _CutoffThe time, think that lithium battery exhausts), when microprocessor samples arrived lithium battery voltage smaller or equal to 3.0V, timer stopped so, and timer numerical value is exactly lithium battery life-span L.

LCD adopts 128 * 64 dot matrix black/whites, two gray scale LCD MODULE, and the digital port of LCD links to each other with the input/output end port of microprocessor C8051F040.C8051F040 shows information such as lithium battery discharge current and life-span by LCD.

When utilizing measurement mechanism to obtain a series of given lithium batteries behind the life-span L under the constant electric current I, the capacity of lithium battery just can calculate according to formula (4).

For example: in the time will determining the capacity of lithium battery YOKU045058, at first this battery of full capacity is inserted the measurement mechanism of lithium battery capacity, measurement mechanism will make this battery with constant current discharge and life-span of measuring this battery under this constant electric current, and the constant electric current equals 1.8/R _L, its size can be by changing pull-up resistor R _LResistance is adjusted.Table 1 has provided the life-span of lithium battery YOKU045058 under the different constant electric currents.

The life-span of lithium battery YOKU045058 during table 1 constant current discharge

Electric current I (mA)	105.3	?194.5	?309.8	?410.2	?493.9	?587.4	?690.6	?761.1
									Life-span L (min)	554.6	?281.7	?167.7	?119.5	?92.2	?74.3	?57.4	?48.6

Utilize the data in the table 1, calculate capacity α=61356mA-min of lithium battery YOKU045058, parameter γ=30.8min according to formula (4).

Claims (3)

1. the measurement mechanism of a lithium battery capacity is characterized in that this device comprises Constant Current Load, lithium battery (2), analog to digital converter (3), LCD (4) and microprocessor (5); Wherein Constant Current Load is by voltage stabilizing chip (1) and pull-up resistor (R _L) form it and lithium battery (2), sampling resistor (R _Test) be connected into a loop, realize that lithium battery (2) is with the constant current discharge; Sampling resistor (R _Test) two ends connect first sampling end (AD_In1) and second sampling end (AD_In2) of analog to digital converter (3) respectively, the digital port of analog to digital converter (3) (A/DC I/O) connects first I/O port (I/O 1) of microprocessor (5); Lithium battery (2) pull down resistor (R in parallel _Down) after connect second I/O port (I/O 2) of microprocessor (5); The 3rd I/O port of microprocessor (5) (I/O 3) connects the digital port (LCD I/O) of LCD (4).

2. definite method of the lithium battery capacity of the measurement mechanism of a lithium battery capacity as claimed in claim 1 is characterized in that, by pull-up resistor (R in the measurement mechanism that changes lithium battery capacity _L) resistance makes lithium battery (2) discharge with different constant electric currents, establishes lithium battery (2) respectively with the constant electric current I ₁, I ₂..., I _nDischarge, exhaust up to lithium battery (2), measurement mechanism obtains the corresponding lithium battery life-span and is respectively L ₁, L ₂..., L _n, n is the positive integer greater than 2; Make J _i=1/I _i, i=1,2 ..., n, then lithium battery capacity α is calculated as:

$\mathrm{\&alpha;}=\frac{\underset{i=1}{\overset{n}{\mathrm{\&Sigma;}}}{J}_i{L}_i-n\stackrel{\&OverBar;}{J}\stackrel{\&OverBar;}{L}}{\underset{i=1}{\overset{n}{\mathrm{\&Sigma;}}}{J}_i^2-n{\stackrel{\&OverBar;}{J}}^2}$ Wherein $\stackrel{\&OverBar;}{J}=\frac{1}{n}\underset{i=1}{\overset{n}{\mathrm{\&Sigma;}}}{J}_i,$ $\stackrel{\&OverBar;}{L}=\frac{1}{n}\underset{i=1}{\overset{n}{\mathrm{\&Sigma;}}}{L}_i.$

3. definite method of lithium battery capacity according to claim 2 is characterized in that determining step is:

1) output voltage of first sampling end (AD_In1) of analog to digital converter (3) sampling lithium battery (2),

2) first sampling end (AD_In1) of analog to digital converter (3) and the voltage difference between second sampling end (AD_In2) are exactly sampling resistor (R _Test) both end voltage V _Test, lithium battery (2) discharge current I=V _Test/ R _Test,

3) when lithium battery (2) when not having place in circuit, first sampling end (AD_In1) is a low level, shows not discharge of lithium battery (2); First sampling end (AD_In1) becomes high level behind lithium battery (2) place in circuit, shows that lithium battery (2) begins discharge; When first sampling end (AD_In1) when becoming high level microprocessor (5) start timer, and the voltage of control analog to digital converter (3) timing sampling lithium battery (2), when lithium battery (2) voltage smaller or equal to shutoff voltage V _CutoffThe time, timer stops, and the result of timer is exactly lithium battery life-span L,

4) after microprocessor (5) obtains lithium battery discharge current I and life-span L, show by LCD.

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