Characterizing a Battery for use with a Battery Fuel Gauge

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The cell under test should be connected to the test system (Fig. 1) using good Kelvin sense connections and placed in an environmental chamber that can be set to 40°C, 20°C and 0°C. Additionally, the test system temperature probe should be placed in close contact with the skin of the cell under test.

Next, a Cell Characterization Request (CCR) form should be created and filled with the charge/discharge parameters for this application and submitted to the fuel-gauge chip supplier to create a custom profile that will be used to customize the fuel-gauge chip for that specific battery pack. For example, Maxim has a CCR form that can be provided to designers, once filled out and returned to Maxim, a custom project number is assigned to ensure the fuel-gauge chips are properly configured.

CELL CHARACTERIZATION PARAMETERS

The battery pack should be cycled with the charging and discharging levels that will be used in the application. Following are example values for a single-cell li-ion battery pack:

Charge current: C rate /2

Charge voltage: 4.2V

Terminating current: 50mA

Heavy load: 1.5 × Typical Active Current

Medium load: 1 × Typical Active Current

Light load: Typical Active Current/4 or C rate/10

Empty voltage: 3.0V

(Note: Empty voltage should be measured at the cell, not outside the pack)

Hot: +40°C

Room: +20°C

Cold: 0°C

There are three basic routines that must be implemented in order to properly characterize a cell for use with a Model Gauge device. Those routines are: charge to full, constant current discharge, and stepped discharge.

The Charge to full routine is charging the battery pack at the charge current until the pack voltage reaches the charge voltage. At that time, the voltage is held at the charge voltage until the current tapers below the terminating current. It is recommended that during characterization all charges occur at room temperature and the cell is allowed to “relax” for at least 60 minutes after each charge. Fig. 2 shows a sample charge cycle.

Discharge is a constant-current loss of charge at the specified rate until the voltage of the battery pack drops to the empty voltage level. It is recommended that the cell relax for at least 60 minutes after each discharge. Constant current discharges should be performed with heavy, medium and light loads at room temperature. The constant current discharge should be repeated at hot and cold temperatures as well. Fig. 3 is an example discharge from full to empty with a constant current. Please note that after the cell reaches the empty voltage, the cell voltage will recover to a higher voltage as the cell is allowed to “relax”.

Stepped discharge is discharging the cell at the heavy load for approximately 20% of the capacity of the battery, and then allowing the cell to relax for 60 minutes (Fig. 4). For a C/2 discharge rate, a stepped discharge of 24 minutes is recommended. Continue these 20% discharge steps until the voltage reaches the empty voltage level.

To allow the battery to relax between the relaxed states of the first stepped discharge, three additional stepped discharge cycles should be run with a first step of 5%, 10%, and 15% of the battery's capacity to create an offset of the relaxed states. For a C/2 discharge rate, a 6-minute discharge will provide the 5% offset, a 12-minute discharge will provide the 10% offset, and an 18-minute discharge will provide the 15% offset. Continue with the original 20% discharge steps until the voltage reaches the empty voltage level. The stepped discharges are described in Steps 16-27 of the following procedure.

PROCEDURE TO CHARACTERIZE A BATTERY

The procedure to characterize a battery encompasses the following steps. Steps 16 through 27 cover the procedure to perform a stepped discharge.

1. Set the environmental chamber to 20°C.

   (Allow the cell to dwell for 30 minutes at each temperature change)

2. Charge the cell to Full and allow the cell to relax.

   (Each relax should be approximately 1 hour)

3. Set the environmental chamber to 40°C.

4. Discharge to the Empty Voltage with a medium discharge rate and allow the cell to relax.

   The medium rate is the typical active discharge current.

5. Set the environmental chamber to 20°C.

6. Charge to Full and allow the cell to relax.

7. Discharge to the Empty Voltage with a medium discharge rate and allow the cell to relax.

8. Charge to Full and allow the cell to relax.

9. Set the environmental chamber to 0°C.

10. Discharge to the Empty Voltage with a medium discharge rate and allow the cell to relax.

11. Set the environmental chamber to 20°C.

12. Charge to Full and allow the cell to relax.

13. Discharge to the Empty Voltage with a heavy discharge rate and allow the cell to relax.

    The Heavy rate is 1.5 × typical active discharge current.

14. Charge to Full and allow the cell to relax.

15. Discharge to the Empty Voltage with a light discharge rate and allow the cell to relax.

    The Light rate is typically the typical active discharge current / 4 or C-rate/10.

    Procedure to perform the stepped discharge:

16. Charge to Full and allow the cell to relax.

17. Discharge the cell 20% under a C-rate/2 and allow the cell to relax for 1 hour.

18. Repeat step 17 discharging 20% at a time until the Empty Voltage is reached.

    (This allows the observation of OCV at 100%, 80%, 60%, 40%, 20%, and 0%)

19. Charge to Full and allow the cell to relax.

20. Discharge the cell 5% under a C-rate/2 and allow the cell to relax for 1 hour.

21. Repeat step 17 discharging 20% at a time until the Empty Voltage is reached.

    (This allows the observation of OCV at 100%, 95%, 75%, 55%, 35%, 15%, and 0%)

22. Charge to Full and allow the cell to relax.

23. Discharge the cell 10% under a C-rate/2 and allow the cell to relax for 1 hour.

24. Repeat step 17 discharging 20% at a time until the Empty Voltage is reached.

    (This allows the observation of OCV at 100%, 90%, 70%, 50%, 30%, 10%, 0%)

25. Charge to Full and allow the cell to relax.

26. Discharge the cell 15% under a C-rate/2 and allow the cell to relax for 1 hour.

27. Repeat step 17 discharging 20% at a time until the Empty Voltage is reached.

    (This allows the observation of OCV at 100%, 85%, 65%, 45%, 25%, 5%, and 0%)

28. Charge to Full and allow the cell to relax.

Fig. 5 shows the complete cycle of charging and discharging the cell. The waveforms show what happens during each step in the just described procedure.

Note: Any of the rest periods can be extended indefinitely to accommodate changing temperature of the chamber.

MEASUREMENTS

To generate a strong reference state-of-charge (for determining error), a minimum of the following data must be collected:

Reference Data:

Charge & Discharge coulomb-counters:

These may be two registers or may be combined into one.

Capacity should be measured with <1mA coulomb-counter drift for accurate characterization and performance verification

Battery Voltage

Charge Discharge Current

Temperature

Time:

If reference data and silicon data are collected on different systems, then they should both have synchronized system clocks for accurate comparison.

To prevent the data file from become too large, yet still capturing enough information, the data should be recorded once every 15 seconds. All data should in one continuous file, including any extended time delays.

Once the procedure has completed, the data should be sent to the fuel-gauge vendor, clearly identifying the project number for this data. The vendor then processes the data and provides a recommended model for the cell under test based on the charge/discharge parameters defined by the application. The model is then embedded in the fuel-gauge chip to characterize battery performance.

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