Inaccuracies Estimating Remaining Cell Capacity with Voltage Measureme

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Inaccuracies Estimating Remaining Cell Capacity with Voltage Measureme

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AN121
Inaccuracies Estimating Remaining Cell Capacity with Voltage Measurements Alone www.dalsemi.com Determining remaining capacity cell simply measuring voltage level less expensive less computing power host compared coulomb counting, under real world conditions, voltage measurements alone very misleading. While true that given cell's voltage level will continually drop during discharge, voltage level's relationship remaining charge varies greatly over cell temperature discharge rate. Figure shows Nickel-Metal Hydride (NiMH) cell's voltage curve during discharge different temperatures different discharge rates.
Figure NiMH Discharge over Temperature (0.1C/0.7C)
Visible stairsteps data caused voltage measurement resolution. Resolution these measurements 10mV. Cell charged ambient temperature rate. Charge terminated when cell voltage fell from peak.
60°C 0.1C Voltage
60°C 0.7C -20°C 0.1C -20°C 0.7C
5000 10000 15000 Time 20000 25000 30000 35000
important point made Figure relationship between cell's voltage it's remaining charge. remaining charge cell graph above calculated multiplying it's discharge rate remaining seconds before being fully discharged. Tables show remaining charge values milliamp-hours versus cell's voltage. Values higher voltage levels available Table because higher discharge rate immediately lowers cell's starting voltage level.
Table Remaining NiMH Capacity Voltage Temperature (0.1C Discharge Rate) Temperature (°C) 1.40 3418.46 3361.71 3321.66 3271.60 3205.04 3141.66 3031.61 2828.14 2391.26 1.35 3245.02 3181.60 3124.88 3068.15 3011.60 2968.23 2914.87 2761.44 2357.90 1.30 2921.51 2828.09 2754.69 2687.96 2651.40 2641.39 2641.39 2581.34 2271.19 1.25 1177.27 1113.89 1023.83 953.79 920.48 940.49 1150.60 2127.77 2047.73 1.20 463.57 506.92 480.24 450.22 420.22 403.54 420.22 570.29 1354.04 1.15 273.47 290.14 280.14 263.46 246.79 230.12 220.12 260.13 490.26 1.10 146.74 170.09 166.75 156.74 146.74 136.73 130.07 143.40 226.78 1.05 53.36 66.70 66.70 66.70 63.37 60.03 56.70 63.36 90.05 1.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Voltage
111700
AN121
Table Remaining NiMH Capacity Voltage Temperature (0.7C Discharge Rate) Temperature (°C) 1.40 1.35 2823.67 2706.99 2520.30 2030.24 1.30 3010.36 2940.35 2847.01 2753.66 2636.98 2473.63 2006.91 1096.80 420.05 1.25 2823.67 2730.33 2660.32 2566.97 2473.63 2356.95 1960.24 1073.47 412.28 1.20 2450.29 2356.95 2286.94 2240.27 2240.27 2170.26 1866.89 1050.13 404.48 1.15 746.75 630.07 583.40 630.07 910.11 1750.21 1633.53 956.79 396.71 1.10 140.02 116.68 93.35 116.68 163.35 256.70 1003.46 746.76 350.04 1.05 23.33 23.34 23.34 23.33 46.67 46.68 116.68 280.04 233.36 1.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
data these tables shows that remaining capacity cell vary widely given measured voltage. Figure shows maximum percentage change remaining capacity that encountered over indicated temperature range, given discharge rate cell voltage, using 20°C remaining capacity number baseline. This maximum percentage change maximum error voltage-based fuel gauge over indicated temperature range. example, cell voltage 1.2V 0.7C discharge rate, maximum error over -20°C 60°C percentage change from 20°C number 20°C baseline, (2240.27-404.28)/2240.27 82%. data separated into -20° temperature ranges show that even normal operating conditions error substantial. Notice also that greater discharge current causes greater error measurement.
Figure NiMH Maximum Error Different Discharge Rates Temperature Ranges
Voltage
200.00% 0.1C, 180.00% 160.00% 140.00% 120.00% Error 100.00% 80.00% 60.00% 40.00% 20.00% 0.00% 1.40 1.35 1.30 1.25 Cell Voltage 1.20 0.1C, -20° 0.7C, 0.7C, -20°
1.15
1.10
1.05
1.00
Similar data Lithium-Ion (Li+) cells also collected. results shown tables figure below. Note that nominal capacity Lithium-Ion cell roughly third that NickelMetal Hydride cell. percentage errors cell types similar.
Table Remaining Capacity Voltage Temperature (0.1C Discharge Rate) Temperature (°C) 1240.17 1216.83 1195.83 1177.16 1157.33 1152.66 1100.17 1004.50 953.17 1053.50 1032.49 1016.16 1007.99 1009.17 1041.83 1069.83 1003.33 951.25 824.83 801.49 786.33 780.49 789.93 842.33 952.00 994.00 949.33 670.83 646.33 626.49 610.16 607.83 627.66 756.00 932.17 947.33 581.00 556.49 535.49 519.16 513.33 513.33 565.83 802.67 924.00 505.17 480.66 460.83 444.49 436.33 432.83 466.67 632.33 866.83 438.67 416.49 396.66 380.33 372.17 367.49 394.33 488.83 753.67 380.33 359.33 340.66 324.33 315.00 311.49 333.67 397.83 583.33 326.67 306.83 289.33 274.16 264.83 261.33 281.17 329.00 432.83 277.67 258.99 242.66 227.49 218.17 216.99 234.50 271.83 330.17 228.67 212.33 197.16 184.33 175.00 176.16 192.50 220.50 256.67 179.67 165.66 152.83 141.16 133.00 136.49 154.00 175.00 198.33 129.50 118.99 110.83 100.33 93.33 99.16 117.83 154.00 148.17
Voltage
AN121
Table Remaining Capacity Voltage Temperature (0.7C Discharge Rate) Temperature (°C) 1157.34 1148.00 1129.34 1082.67 1036.00 980.00 989.33 1026.67 1045.34 1008.00 924.00 672.00 756.00 774.67 830.67 933.34 961.33 914.67 668.83 214.67 588.00 588.00 616.00 746.67 868.00 896.00 665.75 212.80 494.67 485.33 494.67 560.00 700.00 877.33 662.67 210.94 420.00 410.67 410.67 438.67 513.33 830.67 648.67 209.07 354.67 345.33 345.33 354.67 392.00 653.33 634.67 207.21 298.67 289.33 289.33 298.67 317.33 457.33 611.34 205.34 242.67 242.67 242.67 242.67 252.00 317.33 588.00 186.67 196.00 196.00 196.00 186.67 196.00 233.33 420.00 177.34 158.67 149.33 149.33 149.34 149.33 168.00 289.33 163.34
Voltage
37.33 35.46 33.60 31.73 29.87 28.00 18.66
Figure Maximum Error Different Discharge Rates Temperature Ranges
140.00% 0.1C, 0.1C, -20° 120.00% 0.7C, 0.7C, -20° 100.00%
Error
80.00%
60.00%
40.00%
20.00%
0.00% 4.10 4.00 3.90 3.80 3.70 3.60 3.50
3.40
Cell Voltage
3.30
3.20
3.10
3.00
2.90
This error corrected system cell's temperature discharge rate known. However those measurements included, process becomes more complicated expensive than coulomb counting approach does provide significant advantages. Please refer Dallas Semiconductor's application note AN131 "Li+ Fuel Gauging with Dallas Semiconductor Devices" information increase accuracy Dallas coulomb counting devices even further. Contact Battery Management Marketing copy document. Data this experiment collected from 4/3A size Nickel-Metal Hydride cell with rated capacity 3500mAH 4/3A size Lithium-Ion cell with rated capacity 1250mAH. NiMH cell charged with rate ambient temperature until cell's voltage fell from peak value. charged rate until cell voltage attained. Charging completed constant volts until charge current fell below C/20 70mA. During discharge various rates temperatures cells' voltage levels were recorded 30-second intervals. NiMH cell considered fully discharged volts volts. Measurements were taken with Keithley 2304A DVM/power supply. cells' temperature regulated with Tenney environmental chamber.

Inaccuracies Estimating Remaining Cell Capacity with Voltage Measureme

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