SLUAAT5 December   2023 BQ25756

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2LiFePO4 Charging Profile
  6. 3BQ25756 Settings For LiFePO4
    1. 3.1 Set Charging Voltages
      1. 3.1.1 Regulation Voltage
      2. 3.1.2 Recharge Voltage
      3. 3.1.3 Battery Low Voltage
    2. 3.2 Set Charging Current
      1. 3.2.1 Charging Current
        1. 3.2.1.1 ICHG Pin Pull Down Resistor
        2. 3.2.1.2 ICHG_REG Register Bits
      2. 3.2.2 Pre-Charge Current
      3. 3.2.3 Termination Current
    3. 3.3 TS Pin Setting
      1. 3.3.1 JEITA Enabled
      2. 3.3.2 JEITA Disabled
      3. 3.3.3 NTC Monitoring Disabled
  7. 4Supercapacitor Charging Profile
  8. 5SBQ25756 Settings For Supercapacitor
    1. 5.1 Charge Current Setting
    2. 5.2 Charge Voltage Setting
    3. 5.3 TS Pin Setting
  9. 6Test Results From EVM
    1. 6.1 Charging LiFePO4 Battery
    2. 6.2 Charging Supercapacitor
  10. 7Summary
  11. 8References

2 LiFePO4 Charging Profile

LiFePO4 battery has some unique features compared to a Li-ion battery such as high thermal runaway temperatures, very high discharge current capability and high charge current. LiFePO4 batteries follow a similar charging profile to the Li-ion batteries. The device charges the battery in four phases.

Phase 1 – Trickle charge: When the battery heavily depleted, the battery voltage is very low and below the threshold VBAT_SHORT. The device employs low charge current called trickled charge to bring the battery voltage up above VBAT_SHORT.

Phase 2 – Pre-charge: When the battery voltage is above VBAT_SHORT but below VBATLOWV, the device charges the battery with pre-charge current in to bring the battery voltage above VBATLOWV. The pre-charge current is usually 20% of the fast charge current. For LiFePO4 battery, VBATLOWV is 55% of the fully charged voltage. For example, a one cell battery has charge voltage of 3.6 V and VBATLOWV = 1.98 V.

Phase 3 – Constant current: When the battery voltage is above VBATLOWV, the device charges the battery with a full fast charge current. This phase continues until the battery voltage reaches the charge regulation voltage. A one cell LiFePO4 battery typically has a regulation voltage 3.5 V to 3.65 V.

Phase 4 – Constant voltage: When the battery voltage reaches the regulation voltage, the charge current is tapered down and the battery voltage is held constant at the regulation voltage. When the battery voltage is above a recharge threshold and the current is below termination current, the device terminates charging. The recharge threshold VRECHG = 93% of the battery regulation voltage and termination current is typically 10% of the fast charge current.


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Figure 2-1 Typical LiFePO4 Charging Profile