AN3992| Application Note
Maxim/Dallas > App Notes > BATTERY MANAGEMENT
POWER-SUPPLY CIRCUITS
Keywords: OCV, Open Circuit Voltage, Fuel Gauge, OCV Fuel Gauge
Jan 26, 2007
APPLICATION NOTE 3992
Interpreting the Open-Circuit-Voltage (OCV) Fuel Gauge of the DS2786
Abstract: The DS2786 is an open-circuit-voltage (OCV) based fuel gauge that reports the total energy that is stored in a Li+ cell. The OCV is a good indicator of the stored energy in a Li+ cell over normal operating temperatures. However, because the impedance of the cell varies with temperature, the amount of energy that can actually be delivered to the application will differ from the energy that is stored in the cell. This application note describes how to enhance the DS2786's OCV-based fuel gauge to accurately estimate the amount of energy that can be delivered to an application over various temperatures and loads.
Introduction
The DS2786 stand-alone OCV-based fuel gauge estimates the available capacity of rechargeable Li+ batteries based on the cell voltage in an open-circuit state following a relaxation period. The open-circuit voltage is used to determine relative cell capacity based on a lookup table stored in the IC. This capability makes accurate capacity information available immediately after a battery pack is inserted. However, because the cell's impedance varies with temperature, deliverable capacity may differ from the remaining capacity reported by the DS2786, depending upon the temperature and load. This application note describes how to interpret the DS2786's OCV-based fuel gauge readings and accurately estimate the amount of energy that can be delivered to an application at a specific temperature and load.
Characterizing a Cell
To illustrate how to enhance the OCV-based fuel gauge, this application note provides an example that uses the OCV profile for an 1100mAh cell that has been characterized by Dallas Semiconductor. The cell was charged at 20 C under a constant current of 950mA until the cell reached 4.2V. The cell was then charged at a constant voltage of 4.2V. When the charge current tapered off to 50mA, the cell was considered Full (100%). The cell was then discharged under a 220mA load (0.2C) until the cell voltage reached 2.5V. At this point, the cell was considered Empty (0%). The profile was then summarized into an 8-segment linear approximation that can be stored in the DS2786. Table 1 shows the Capacity and Open-Circuit-Voltage pairs for this particular cell, and Figure 1 charts its OCV profile.
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