8.3.1 Battery Voltage Regulation

The BQ24650 uses a high accuracy voltage regulator for the charging voltage. The charge voltage is programmed through a resistor divider from the battery to ground, with the midpoint tied to the VFB pin. The voltage at the VFB pin is regulated to 2.1 V, giving Equation 1 for the regulation voltage:

Equation 1.

where

• R2 is connected from VFB to the battery
• and R1 is connected from VFB to GND.

Li-Ion, LiFePO4, and sealed lead acid are widely used battery chemistries. Most commercial Li-ion cells can now be charged to 4.2 V/cell. A LiFePO4 battery allows a much higher charge and discharge rate, but the energy density is lower. The typical cell voltage is 3.6 V. The charge profile of both Li-Ion and LiFePO4 is preconditioning, constant current, and constant voltage. For maximum cycle life, the end-of-charge voltage threshold could be lowered to 4.1 V/cell.

Although the energy density is much lower than Li-based chemistry, lead acid is still popular due to its low manufacturing cost and high discharge rates. The typical voltage limit is from 2.3 V to 2.45 V. After the battery has been fully charged, a float charge is required to compensate for the self-discharge. The float charge limit is 100 mV to 200 mV below the constant voltage limit.

Figure 9. Typical Charging Profile