8.4.5.4 HIBERNATE Mode

HIBERNATE mode should be used for long-term pack storage or when the host system must enter a low-power state, and minimal gauge power consumption is required. This mode is ideal when the host is set to its own HIBERNATE, SHUTDOWN, or OFF mode. The gauge waits to enter HIBERNATE mode until it has taken a valid OCV measurement (cell relaxed) and the magnitude of the average cell current has fallen below Hibernate Current. When the conditions are met, the fuel gauge can enter HIBERNATE due to either low cell voltage or by having the [HIBERNATE] bit of the CONTROL_STATUS register set. The gauge will remain in HIBERNATE mode until any communication activity appears on the communication lines and the address is for bq27545. In addition, the SE pin SHUTDOWN mode function is supported only when the fuel gauge enters HIBERNATE due to low cell voltage.

When the gauge wakes up from HIBERNATE mode, the [HIBERNATE] bit of the CONTROL_STATUS register is cleared. The host is required to set the bit to allow the gauge to re-enter HIBERNATE mode if desired.

Because the fuel gauge is dormant in HIBERNATE mode, the battery should not be charged or discharged in this mode, because any changes in battery charge status will not be measured. If necessary, the host equipment can draw a small current (generally infrequent and less than 1 mA, for purposes of low-level monitoring and updating); however, the corresponding charge drawn from the battery will not be logged by the gauge. Once the gauge exits to NORMAL mode, the IT algorithm will take about 3 seconds to re-establish the correct battery capacity and measurements, regardless of the total charge drawn in HIBERNATE mode. During this period of re-establishment, the gauge reports values previously calculated before entering HIBERNATE mode. The host can identify exit from HIBERNATE mode by checking if Voltage() < Hibernate Voltage or [HIBERNATE] bit is cleared by the gauge.

If a charger is attached, the host should immediately take the fuel gauge out of HIBERNATE mode before beginning to charge the battery. Charging the battery in HIBERNATE mode will result in a notable gauging error that will take several hours to correct. It is also recommended to minimize discharge current during exit from Hibernate.