A 40 to 200 mAh rechargeable coin cell has different charging requirements from a 1,000 mAh Li-Po pouch. The off-the-shelf charger IC catalogue is dominated by Li-Po-class parts; the few that support sub-100 mAh cells need careful configuration to avoid over-charge or thermal abuse.
The shortlist
| Part | Min charge current | Cell voltage support | NTC input | Quiescent | Package |
|---|---|---|---|---|---|
| Microchip MCP73831 | 15 mA | 4.20 V (LIR), 3.10 V (ML w/ resistor div) | No | 50 µA | SOT-23-5 |
| TI BQ24210 | 10 mA | 4.20 V (LIR), programmable to 3.6-4.4 V | Yes (10 kΩ) | 15 µA | WSON-10 (3×3) |
| Consonance CN3052 | 50 mA (limit) | 4.20 V (LIR only) | Yes | 30 µA | SOT-23-5 |
| TI BQ25040 | 5 mA | 3.6 to 4.4 V programmable | Yes | 10 µA | WSON-6 (2×2) |
| Skyworks AAT3681A | 10 mA | 4.20 V (LIR) | No | 40 µA | SC-70-5 |
For ML cells (3.0 V nominal, 3.10 V cutoff) the catalogue narrows further. Most charger ICs assume 4.20 V cutoff; you either select a programmable part (BQ24210, BQ25040) or use a resistor divider on the FB pin to fool a fixed-cutoff part into stopping at 3.10 V. The divider approach works but doubles the IR error on the cutoff voltage; we generally recommend the programmable parts.
Setting the charge rate
The charger IC's ISET resistor (sometimes called PROG) sets the charge current. For coin cells the rule is simple: start at 0.2C, drop to 0.1C if thermal headroom is tight.
| Cell | Capacity | 0.2C charge | 0.1C charge |
|---|---|---|---|
| LIR2032 | 40 mAh | 8 mA | 4 mA |
| LIR2450 | 120 mAh | 24 mA | 12 mA |
| ML2032 | 65 mAh | 13 mA | 6.5 mA |
| ML2430 | 110 mAh | 22 mA | 11 mA |
For an MCP73831 the formula is I_chg = 1000 / R_PROG. For an 8 mA target, R_PROG = 125 kΩ. Most engineers we work with default to a 10 kΩ resistor (100 mA) which is too aggressive for a 40 mAh LIR2032. Always do the math.
Termination current
The charger IC stops when charge current falls to a threshold (typically 10% of the programmed rate, sometimes called I_TERM). For an 8 mA programmed rate, termination at 0.8 mA. This works fine with most charger ICs for LIR cells, but ML cells need explicit verification: the LiMn2O4 cathode plateau is flat near full charge, so cells can sit at 90% SOC indefinitely without termination triggering. We recommend a firmware-level timeout (e.g. 4 hours from charge start) as a backstop.
NTC integration
Charge inhibit above 45 °C is the single biggest cycle-life saver for coin cells. The TI BQ24210 and BQ25040 have a dedicated NTC input that handles this in hardware — connect a 10 kΩ NTC between the cell and the IC's TS pin. The MCP73831 has no NTC input; you have to monitor temperature in firmware and gate the IC's enable line.
For sealed designs (SMD-mounted ML cell, no separate thermistor) the host MCU's internal temperature sensor is usually within 3-5 °C of the cell. That is good enough for a 45 °C inhibit threshold but not for thermal runaway protection — for that we still recommend a dedicated NTC at the cell.
Quiescent draw matters
For a coin-cell-powered device, the charger IC is on the battery rail at all times. A 40 µA quiescent draw on a 65 mAh ML cell costs 350 mAh / year — half the cell's annual capacity budget gone to the charger IC alone. The TI BQ25040 at 10 µA is the quietest option in the comparable price range. The MCP73831 at 50 µA is acceptable if the device has a recharge path that gets hit weekly.
Common mistakes
Programming charge current too high. The MCP73831 default reference designs assume 100 mA. For a 40 mAh LIR cell that is 2.5C — well above the cell's ratings, and reliable cycle life suffers.
Skipping the NTC. Without temperature protection the cell ages 2-3× faster in field deployments that hit warm enclosures during charging.
Floating the cell at 100% SOC. Continuous trickle from a USB-attached device. For LIR cells specifically this kills calendar life. Add a firmware-level "stop charging at 90% if device is on continuous power" rule.