Abstract: DS278x series stand-alone fuel gauges accurately estimate the remaining capacity of rechargeable lithium ion batteries or lithium polymer batteries. This series of fuel gauges are factory calibrated to meet the high precision standards specified in the data. On the other hand, the user can reset the current measurement gain factor (RSGAIN) to improve the current detection accuracy after modular design or assembly. Adjust RSGAIN to correct the nominal value of the external current-sense resistor, allowing the use of low-cost, non-precision current-sense resistors. This application note details the RSGAIN calibration procedure in the DS278x series of stand-alone fuel gauges.
Introduction The DS278x series of stand-alone fuel gauges, including the DS2780, DS2781, DS2782, DS2784, and DS2788, can accurately estimate the remaining capacity of rechargeable lithium ion batteries or lithium polymer batteries. The accuracy of the ammeter depends on the battery characteristic parameters stored in the EEPROM, the application parameters, and the accuracy of the current reading.
Each device is calibrated at the factory and meets the current accuracy specifications specified in the data. On the other hand, the user can read or reset the current measurement gain factor (RSGAIN) in order to improve the current measurement accuracy after modular design or assembly. The user can adjust RSGAIN to correct the nominal value of the external current-sense resistor, thereby allowing the use of low-cost, non-precision current-sense resistors.
It indicates that RSGAIN is a proportional coefficient, and the DS278x series fuel gauges use it to accurately adjust the current measurement value of the shunt resistor. The current measurement value of the device is multiplied by the RSGAIN scale factor. This value is sent to the current register and accumulated in the accumulated current register (ACR) to provide accurate current measurement.
Indication current (mA) = measurement current (mA) × RSGAIN
(Formula 1)
Figure 1 shows the format of the RSGAIN register. RSGAIN is 11-bit data, which is stored in the parameter memory EEPROM and occupies 2 bytes. The RSGAIN register value can be set between 0 and 1.999 in steps of 0.001 (2-10). The MSb weight is 1, and the LSb weight is 1/1024 (or 2-10).
Figure 1. The calculated RSGAIN value should be written to addresses 78h and 79h according to the above RSGAIN register format.
To ensure accurate current measurement without the need for gain adjustment, the device needs to select an RSGAIN value of 1. Typically, the RSGAIN value of the DS278x series fuel gauges is shipped from 0.990 to 1.100.
Table 1. Examples of RSGAIN and corresponding register values
Calculating RSGAIN Users must set RSGAIN carefully to ensure accurate current measurement. When the device leaves the factory, the gain calibration value is stored in two different areas of the parameter memory EEPROM: RSGAIN (resettable) and FRSGAIN (read only). RSGAIN determines the gain of the current measurement. The read-only FRSGAIN is only used to save factory settings and is not used in current measurement. If the wrong value is written to the RSGAIN register, FRSGAIN can restore the original RSGAIN.
To calculate RSGAIN, an accurate reference current must be loaded on the sense resistor. Divide the reference current by the current value indicated by the device, and the ratio of the reference current and the device indicated current is multiplied by the current RSGAIN to determine the new RSGAIN value.
For example, if the precision current loaded on the current-sense resistor is 500 mA, the current value reported by the DS278x is 495 mA, and the RSGAIN is 1.02637, the new RSGAIN value is 1.03674.
Write the calculated RSGAIN value to addresses 78h and 79h, as shown in Figure 1, and then copy to EEPROM. This ensures that the current value reported by the DS278x is consistent with the reference current loaded on the current-sense resistor.
Conclusion The calibration function allows the DS278x series of stand-alone fuel gauges to use low-cost, low-accuracy current-sense resistors, while also ensuring current measurements are as accurate as possible.
Introduction The DS278x series of stand-alone fuel gauges, including the DS2780, DS2781, DS2782, DS2784, and DS2788, can accurately estimate the remaining capacity of rechargeable lithium ion batteries or lithium polymer batteries. The accuracy of the ammeter depends on the battery characteristic parameters stored in the EEPROM, the application parameters, and the accuracy of the current reading.
Each device is calibrated at the factory and meets the current accuracy specifications specified in the data. On the other hand, the user can read or reset the current measurement gain factor (RSGAIN) in order to improve the current measurement accuracy after modular design or assembly. The user can adjust RSGAIN to correct the nominal value of the external current-sense resistor, thereby allowing the use of low-cost, non-precision current-sense resistors.
It indicates that RSGAIN is a proportional coefficient, and the DS278x series fuel gauges use it to accurately adjust the current measurement value of the shunt resistor. The current measurement value of the device is multiplied by the RSGAIN scale factor. This value is sent to the current register and accumulated in the accumulated current register (ACR) to provide accurate current measurement.
Indication current (mA) = measurement current (mA) × RSGAIN
(Formula 1) Figure 1 shows the format of the RSGAIN register. RSGAIN is 11-bit data, which is stored in the parameter memory EEPROM and occupies 2 bytes. The RSGAIN register value can be set between 0 and 1.999 in steps of 0.001 (2-10). The MSb weight is 1, and the LSb weight is 1/1024 (or 2-10).
Figure 1. The calculated RSGAIN value should be written to addresses 78h and 79h according to the above RSGAIN register format.
To ensure accurate current measurement without the need for gain adjustment, the device needs to select an RSGAIN value of 1. Typically, the RSGAIN value of the DS278x series fuel gauges is shipped from 0.990 to 1.100.
Table 1. Examples of RSGAIN and corresponding register values
| RSGAIN | Register Value |
| 0.000 | XXXX X000 0000 0000 |
| 0.001 | XXXX X000 0000 0001 |
| 0.990 | XXXX X011 1111 0110 |
| 1.000 | XXXX X100 0000 0000 |
| 1.010 | XXXX X100 0000 1010 |
| 1.020 | XXXX X100 0001 0100 |
| 1.030 | XXXX X100 0001 1111 |
| 1.040 | XXXX X100 0010 1001 |
| 1.050 | XXXX X100 0011 0011 |
| 1.999 | XXXX X111 1111 1111 |
Calculating RSGAIN Users must set RSGAIN carefully to ensure accurate current measurement. When the device leaves the factory, the gain calibration value is stored in two different areas of the parameter memory EEPROM: RSGAIN (resettable) and FRSGAIN (read only). RSGAIN determines the gain of the current measurement. The read-only FRSGAIN is only used to save factory settings and is not used in current measurement. If the wrong value is written to the RSGAIN register, FRSGAIN can restore the original RSGAIN.
To calculate RSGAIN, an accurate reference current must be loaded on the sense resistor. Divide the reference current by the current value indicated by the device, and the ratio of the reference current and the device indicated current is multiplied by the current RSGAIN to determine the new RSGAIN value.
For example, if the precision current loaded on the current-sense resistor is 500 mA, the current value reported by the DS278x is 495 mA, and the RSGAIN is 1.02637, the new RSGAIN value is 1.03674.
Write the calculated RSGAIN value to addresses 78h and 79h, as shown in Figure 1, and then copy to EEPROM. This ensures that the current value reported by the DS278x is consistent with the reference current loaded on the current-sense resistor.
Conclusion The calibration function allows the DS278x series of stand-alone fuel gauges to use low-cost, low-accuracy current-sense resistors, while also ensuring current measurements are as accurate as possible.
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