Designing a compact, reliable battery management breakout board isn’t just about cramming chips together — it’s about understanding the trade-offs behind every component choice.
Here’s the thinking behind the Battery Charger & Fuel Gauge Breakout Board and why I chose the ICs I did.
The TP4056 has earned its fame in the maker world — it’s cheap, easy to use, and well supported. But it hides a quirk that can cause headaches in certain designs.
The TP4056 quirk:
- Pre-charge threshold: ~3.0 V
- Fixed pre-charge (trickle charge) current: 10 mA
- If your battery is below 3 V and you have an LDO pulling ~10 mA from the charger output (to power 3.3 V logic, for instance), the TP4056 will happily supply that current to the LDO… and zero to the battery.
- Result? The battery never rises above 3 V, and the charger never switches to constant current mode. Stalemate.
How the BQ24040 solves it:
- Lower pre-charge threshold: ~2.5 V (battery starts charging earlier in its discharge)
- Configurable pre-charge current: e.g., 50 mA — plenty to both feed your LDO and still trickle energy into the cell.
- Additional perks:
- Power path management: can supply load and charge battery simultaneously.
- Status outputs for charge and fault indication.
- Thermal regulation built in.
This means your system boots faster from a dead battery, and you don’t get stuck in a “never charging” deadlock.
Battery fuel gauging is trickier than it looks. You can measure voltage, but it doesn’t tell the whole story about state of charge (SoC).
Two main approaches:
- Impedance Tracking — Highly accurate, but requires painful calibration and “training” cycles to match the gauge to your specific cell.
Great for high-volume products, less great for quick prototypes or small-run boards. - Voltage Correlation — Uses cell voltage, temperature, and discharge patterns to estimate SoC. Less accurate over extreme conditions, but easy to set up.
The BQ27621-G1 uses a dynamic voltage correlation algorithm:
- No complex calibration process.
- Good enough accuracy for most portable projects.
- Very low quiescent current — ideal for battery-powered devices.
- I²C interface for easy microcontroller integration.
For a breakout board aimed at prototyping and flexibility, avoiding impedance tracking’s long setup time is worth the small trade-off in accuracy.
This board isn’t just two chips and some passives — it’s designed to be flexible in different setups.
Key features:
- USB input for charging.
- Battery JST connector for Li-Ion/Li-Po cells.
- Load output that stays powered even while charging (thanks to the BQ24040’s power path management).
- I²C interface (SDA, SCL) for reading fuel gauge data.
- Configurable pre-charge current via resistor selection.
- Charge status pins for LED indicators or MCU monitoring.
The BQ24040 + BQ27621-G1 pairing offers:
- Robust charging under low-voltage conditions.
- Accurate-enough battery state reporting without lengthy setup.
- Support for both charging and powering your project at the same time.
- Minimal parts count without sacrificing essential battery management features.
Refer to the Design Document for the board configurations