Battery Charger IC Selection Guide: Li-Ion, LiFePO4 and NiMH Charging Circuits

A lithium-ion battery stores roughly 250 Wh/kg and, if charged incorrectly, releases that energy as fire. Selecting the right charger IC, configuring charge current and voltage correctly, and adding the safety features the chemistry demands is not optional. This guide covers the three battery chemistries most common in embedded products and how to design their charging circuits.

Li-Ion and Li-Polymer: the CC/CV profile

Every Li-Ion cell charges in three phases: precharge, constant current (CC) and constant voltage (CV).

Precharge: if the cell voltage is below roughly 3.0 V (varies by IC), the charger applies a reduced current, typically C/10 (where C is the cell capacity divided by 1 hour). A 2000 mAh cell receives 200 mA. This protects a deeply discharged cell from damage. Once the cell reaches 3.0 V, the charger transitions to CC mode.

Constant current: the charger delivers the full programmed charge current, typically 0.5C to 1C. At 1C, a 2000 mAh cell charges in roughly 70-80 minutes to about 70% capacity. Higher currents reduce charge time but increase cell heating and accelerate capacity fade over cycle life.

Constant voltage: the charger holds the terminal voltage at 4.20 V (±1%, roughly ±42 mV) and the charge current naturally decays. The charger terminates when current drops to C/10 or a programmed threshold. Float charging (holding 4.20 V indefinitely) accelerates degradation. Good charger ICs disconnect or reduce to a maintenance level after termination.

Overcharging above 4.25 V causes metallic lithium plating on the anode, permanently reducing capacity and creating internal short-circuit paths. Overcharging above 4.30 V risks thermal runaway: the cathode decomposes, releasing oxygen that ignites the organic electrolyte.

LiFePO4: lower voltage, longer life

Lithium iron phosphate (LiFePO4) cells charge to 3.60-3.65 V instead of 4.20 V. The lower voltage reduces energy density (roughly 90-120 Wh/kg vs 200-250 Wh/kg for Li-Ion) but delivers 2000-5000 cycles compared to 300-500 for standard Li-Ion, and is far more tolerant of abuse.

The charge profile is the same CC/CV sequence, but the terminal voltage is 3.60 V. A standard Li-Ion charger set to 4.20 V will overcharge and destroy a LiFePO4 cell. Select a charger IC with a programmable termination voltage or choose a LiFePO4-specific device.

LiFePO4 is the default choice for solar-powered equipment, electric vehicles, marine applications and any system where safety margins or cycle life outweigh the need for maximum energy density.

NiMH: delta-V termination

Nickel-metal hydride cells charge at constant current with termination based on a voltage drop (-ΔV) or a temperature rise (dT/dt). Unlike Li-Ion, NiMH tolerates continuous trickle charging at C/10 to C/40, making simple timer-based chargers feasible for low-cost products. However, fastest charging requires -ΔV detection: the charger monitors the cell voltage and terminates when it drops 5-10 mV per cell after reaching peak voltage during a 0.5C-1C charge.

NiMH chargers are simpler than Li-Ion chargers because the chemistry self-regulates overcharge through oxygen recombination at low currents. The trade-off is lower energy density (60-80 Wh/kg) and a higher self-discharge rate (10-20% per month for standard cells, 2-3% for low-self-discharge variants like Eneloop).

Key charger IC features

Power path management: separates the system load from the battery. Without power path, plugging in a deeply discharged battery and a USB charger simultaneously forces the system to wait until the battery reaches minimum voltage. With power path, the system runs immediately from USB power while the battery charges independently. ICs like the TI BQ2407x and Microchip MCP73871 include this feature. Essential for any product where the user expects instant-on when plugged in.

NTC thermistor input: monitors battery temperature during charge. Li-Ion cells must not be charged below 0°C (lithium plating) or above 45-60°C (accelerated degradation, thermal runaway risk). A 10 kΩ NTC thermistor (β = 3435) mounted on the battery pack or near the cell provides the temperature signal. The charger IC suspends charging outside a safe window, typically 0-45°C.

Input current limiting: prevents the charger from exceeding the USB port's current capability. A USB 2.0 port provides 500 mA; USB 3.0 provides 900 mA; USB-C with basic power delivery provides up to 3 A at 5 V. The charger IC should dynamically limit input current based on the detected source type (D+/D- detection or USB-C CC pin negotiation).

Charge status output: an open-drain STAT pin drives an LED (or connects to a microcontroller GPIO) to indicate charging, charge complete, fault or temperature suspend. Essential for user feedback.

USB-C PD charging integration

USB Power Delivery (PD 3.0/3.1) enables charging at up to 240 W (48 V, 5 A). A USB-C PD sink controller (e.g., STUSB4500, FUSB302) negotiates the power contract with the source, then configures a downstream buck or buck-boost charger IC for the negotiated voltage.

A typical USB-C PD charging architecture: USB-C connector → PD controller → buck-boost charger (e.g., TI BQ25790, 1-4 cell, 5 A, I2C-controlled) → battery + system. The PD controller requests a voltage that maximizes efficiency for the current battery charge state. For a 2S Li-Ion pack (8.4 V full), requesting 9 V from the PD source and bucking to 8.4 V is more efficient than requesting 5 V and boosting.

FAQ

Q: What happens if I charge a Li-Ion cell with a power supply set to 4.2 V directly?

A lab power supply has no current limiting unless you set it. If the cell is at 3.5 V and the power supply can deliver 5 A, the cell will draw that current, overheating and potentially venting. Always use a dedicated charger IC with current limiting. The power supply's constant-current mode can substitute in a lab environment with careful monitoring, but never in a product.

Q: Can I charge two Li-Ion cells in series without balancing?

Two cells in series (2S, 8.4 V full) develop voltage mismatches over cycles. One cell reaches 4.20 V while the other is at 4.15 V. The charger sees 8.35 V total and continues charging, pushing the higher cell past 4.25 V. A balance charger or a protection IC (e.g., TI BQ2920x) with cell-voltage monitoring and a bleed resistor prevents this. Charging 2S without balancing guarantees cell mismatch and shortened pack life.

Q: What is the difference between a charger IC and a fuel gauge?

A charger manages the CC/CV profile, current, voltage and safety timers. A fuel gauge (e.g., TI BQ27427, Maxim MAX17048) measures voltage, current and temperature, and uses a battery model to estimate state of charge (SoC) and remaining capacity. The two often work together: the fuel gauge reports to the host microcontroller, which configures the charger IC over I2C.

Select battery charger ICs, fuel gauges and protection circuits from Novapart's batteries and chargers category. Filter by chemistry, cell count, charge current and interface. Need a complete battery management solution? Upload your BOM for sourcing across all power-management components.