In the present time, more and more devices are using rechargeable battery as a backup source during a power supply failure. These devices should monitor battery state of charge. For certain types of devices, such as remote alarm on passenger goods lifts, monitoring is mandatory (EN81-28, §4.1.3. Battery check).
Charge/discharge current creates voltage drop Vsense across the sense resistor. This voltage is applied to an auto-zeroed differential analogue integrator. Integration time depends on current, sensing resistor, programmable pre-scaler M and internal REHHI, REFLO thresholds. For M=1, AC register is incremented during charging or decremented during discharging after integrator voltage make one cycle REFLO-REFHI-REFLO. For M> 1, M cycles are required to increment/decrement register.
Analogue integrator uses a similar principle as dual slope integrating ADC, which allows great reduction of offset voltages influence. The relative integration error due to offset (CEov) can be expressed by: CEov = (Vos/Vsense)^2.
Many other devices convert Vsense to digit by ADC and calculate integral in digital domain. In such architecture, the offset Vos causes relative integration error of Vos/Vsense. For small Vsense values, Vos can be the main source of error. As an example, at 1mV input signal and voltage offset of Vos = 20µV, digital integration results in a 2% error, whereas the error is only 0.04% (a factor of50 times smaller!) using the analogue integration approach of LTC2944.
LTC2944 is suitable for all battery chemistry and voltages in range 3.6 to 60V. External current sense resistor and programmable pre-scaler allow using a wide range of battery capacities.
LTC2944 measures battery voltage, current flowing through sense resistor and chip temperature by 14-bit no latency delta-sigma ADC. Host reads measured values over an I²C/SMBus-compatible 2-wire interface that is also used to configure chip. The host can program high and low thresholds for all measured parameters, which if tripped, signal an alert using either the SMBus alert protocol or by setting a register flag.
LTC2944 is pin and function compatible with LTC2943 and LTC2943-1 with integrated sense resistor.
3.6V to 60V operating range without external level shifting circuitry.
Suitable for all battery chemistry and a wide range of capacity.
Measures battery charge, battery voltage, current through sense resistor and temperature.
High side sense, ±50mV sense voltage range.
1% voltage, current and charge accuracy.
Configurable alert output/charge complete input.
Quiescent current less than 150μA.
8-Lead 3mm × 3mm DFN package.
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Date of publishing 10.08.2017 When you issue article on your website, please give its source: http://www.soselectronic.com/articles/linear-technology/how-to-easily-monitor-the-battery-state-of-charge-2024