The Pico Dual Battery Architecture: Source to Load
When designing a power system for an embedded microcontroller, you must map the exact path from the chemical source to the silicon load. A robust pico dual battery architecture follows this strict system block sequence: Source (Dual Cells) → Protection (BMS) → Charge Controller → DC-DC Conditioning → Distribution (Pico + Peripheral Loads). Skipping the BMS or placing the DC-DC converter before the charge controller's load output will result in unregulated voltage spikes that can permanently brick the Pico's RP2040 chip.Series vs. Parallel Consequences
The decision to wire your two cells in series (2S) or parallel (1S2P) fundamentally alters your voltage, amp-hour (Ah) capacity, and PCB trace requirements.- Parallel (1S2P): Voltage remains at the nominal cell voltage (3.7V for Li-ion, 3.2V for LiFePO4), while the Ah capacity doubles. Because the Pico requires 5V, you must use a boost converter. Boosting 3.2V to 5V at 500mA requires drawing nearly 900mA from the cells (accounting for 85% converter efficiency). This high current demands thick 18 AWG silicone wire and wide PCB copper pours to prevent voltage sag.
- Series (2S): Voltage doubles (7.4V Li-ion, 6.4V LiFePO4), while Ah capacity remains equal to a single cell. You use a buck converter to step down to 5V. Bucking 6.4V to 5V at 500mA only draws about 400mA from the pack. This halves the current stress on your battery tabs and BMS traces, making 2S the superior choice for embedded projects where space and trace width are limited.
| Configuration | Nominal Voltage | Typical Capacity | DC-DC Stage | Input Current @ 2.5W Load | Quiescent Loss |
|---|---|---|---|---|---|
| 1S2P Li-ion (NMC) | 3.7V | 6000mAh | Boost to 5V | ~790mA | High (Boost switching) |
| 2S Li-ion (NMC) | 7.4V | 3000mAh | Buck to 5V | ~390mA | Low (Buck efficiency) |
| 1S2P LiFePO4 | 3.2V | 6000mAh | Boost to 5V | ~920mA | Very High |
| 2S LiFePO4 | 6.4V | 3000mAh | Buck to 5V | ~440mA | Lowest (Recommended) |
Sizing Math: Peukert, DoD, and Charge Limits
To guarantee your remote sensor node survives a multi-day storm without sunlight, you cannot rely on nominal capacity labels. You must apply Peukert's Law and depth-of-discharge (DoD) derating.Peukert's Law and Efficiency Factors
Peukert's Law calculates the actual runtime of a battery under a specific load, accounting for internal resistance and chemical reaction limits. The formula is:t = (C / Ik) × ηWhile lead-acid batteries suffer from a high Peukert exponent (k ≈ 1.3), lithium chemistries are highly efficient, with lithium iron phosphate (LiFePO4) exhibiting a k value of roughly 1.05. Worked Example: Let's size a 2S LiFePO4 pack (6.4V nominal, 3000mAh / 3.0Ah rated capacity) powering a Raspberry Pi Pico W and a BME280 sensor drawing a combined 2.5W.
Where t = time (hours), C = rated capacity (Ah), I = discharge current (A), k = Peukert exponent, and η = DC-DC converter efficiency.
- Load Current (I) = 2.5W / 6.4V = 0.39A
- Peukert exponent (k) = 1.05
- DC-DC Buck Efficiency (η) = 0.90 (90%)
- t = (3.0 / 0.391.05) × 0.90
- t = (3.0 / 0.373) × 0.90 = 8.04 × 0.90 = 7.23 hours of continuous runtime at 100% discharge.
Depth of Discharge (DoD) and C-Rate Limits
You must never design a lithium system for 100% DoD. Discharging LiFePO4 below 2.5V per cell (5.0V for a 2S pack) causes copper dissolution in the anode, permanently destroying the cell. For a long-life IoT deployment, restrict your DoD to 80%. Applying the 80% DoD factor to our 7.23-hour theoretical runtime yields a usable runtime of 5.78 hours per full charge cycle. If your Pico uses deep-sleep modes and only wakes for 2 minutes every hour, your average current drops drastically, extending calendar life to several weeks. Regarding charge and discharge limits, always respect the manufacturer's C-rate. For standard cylindrical LiFePO4 cells, the maximum continuous discharge rate is typically 1C (3A for a 3000mAh cell), and the recommended charge rate is 0.5C (1.5A). Pushing a 3C discharge rate to feed a sudden inverter surge will trigger the BMS over-current protection and crash your Pico mid-write.Inverter, Charger, and BMS Sizing for the Pico Node
In advanced environmental monitoring, a Pico might need to power a 120V AC peripheral—such as a Dylos particulate counter or a heated sample tube—via a micro-inverter. Sizing this AC stage and its corresponding DC charger requires matching the surge profiles.Micro-Inverter and Charger Sizing
If your Pico triggers a 120V AC load that draws 60W continuously with a 120W startup surge, you must size a pure sine wave micro-inverter rated for at least 150W continuous / 300W surge. Modified sine wave inverters will introduce harmonic distortion that can reset the Pico's brownout detector via ground-loop noise. For the charger, a 2S LiFePO4 pack requires a Constant Current / Constant Voltage (CC/CV) profile peaking at exactly 7.2V to 7.3V (3.65V per cell). Do not use a standard 8.4V Li-ion NMC charger; the 1.1V overvoltage will vent the LiFePO4 safety valves. Size the charge controller to deliver a maximum of 0.5C (1.5A for a 3Ah pack) to balance charge speed with thermal safety.
⚠ LITHIUM FIRE-SAFETY & BMS CRITICAL WARNING
- Never parallel mismatched cells: Paralleling an old cell with a new cell, or mixing different chemistries (e.g., NMC and LiFePO4), creates infinite cross-currents that will melt 18 AWG wire and cause thermal runaway. Only parallel cells from the exact same manufacturing batch, matched to within 0.02V and 5mΩ internal resistance.
- Mandatory BMS: A 2S pack requires a 2S BMS with passive cell balancing. The BMS must be rated for at least 1.5x your maximum continuous load current to prevent nuisance tripping.
- Physical Isolation: Mount the battery pack in a fire-retardant LiPo-safe bag or a vented steel enclosure, physically separated from the Pico's PCB to prevent collateral damage in the event of a cell failure.
