Lithium Iron Phosphate (LiFePO₄) batteries offer good thermal stability and low risk of thermal runaway. LiFePO₄ batteries also offer longer cycle life and high discharge current. Charging LiFePO₄ batteries is similar process as Li-Ion batteries. However, there are few important voltage thresholds to be considered. Apart from the batteries, supercapacitors are a popular choice for backup power. Picking the correct charging device can maximize the charge life of both LiFePO₄ batteries and supercapacitors. The BQ25756 buck-boost charge controller can be programmed to support both LiFePO₄ batteries and supercapacitors.

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 V_{BAT_SHORT}. The device employs low charge current called trickled charge to bring the battery voltage up above V_{BAT_SHORT}.

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

Phase 3 – Constant current: When the battery voltage is above V_BATLOWV, 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 V_RECHG = 93% of the battery regulation voltage and termination current is typically 10% of the fast charge current.