9.3.23 Battery Detection

For applications with removable battery packs, BQ2461x provides a battery-absent detection scheme to reliably detect insertion or removal of battery packs.

Figure 17. Battery Detection Flow Chart

Once the device has powered up, an 8-mA discharge current is applied to the SRN terminal. If the battery voltage falls below the LOWV threshold within 1 second, the discharge source is turned off, and the charger is turned on at low charge current (125 mA). If the battery voltage rises above the recharge threshold within 500 ms, there is no battery present and the cycle restarts. If either the 500-ms or 1-second timer times out before its respective threshold is hit, a battery is detected and a charge cycle is initiated.

Figure 18. Battery Detect Timing Diagram

Ensure that the total output capacitance at the battery node is not so large that the discharge current source cannot pull the voltage below the LOWV threshold during the 1-second discharge time. The maximum output capacitance can be calculated as follows:

Equation 10.

where

• C_MAX is the maximum output capacitance.
• I_DISCH is the discharge current.
• t_DISCH is the discharge time.
• R₂ and R₁ are the voltage feedback resistors from the battery to the VFB pin.

The 0.5 factor is the difference between the RECHARGE and the LOWV thresholds at the VFB pin.

Example

For a 3-cell Li+ charger, with R2 = 500 kΩ, R1 = 100 kΩ (giving 12.6 V for voltage regulation), I_DISCH = 8 mA, t_DISCH = 1 second,

Equation 11.

Based on these calculations, no more than 2.7 mF should be allowed on the battery node for proper operation of the battery detection circuit.