Battery Sizing for LoRa Mesh Nodes
Correctly sizing the battery for a solar-powered LoRa node prevents two failure modes: undersizing (the battery dies overnight or during cloudy periods) and oversizing (wasted cost and weight). This page walks through a systematic methodology and provides worked examples for three common node types.
Step 1 - Measure Actual Current Draw
Never rely solely on datasheet figures. Real-world current draw depends on firmware configuration, peripherals, GPS lock cycles, LoRa transmit duty cycle, and whether deep sleep is used. Measure with a USB power meter (e.g., UM25C, AT34) or an inline current shunt (e.g., INA219 module on the 3.3 V rail).
Take measurements in three states:
- Transmit peak: Current during an active LoRa TX burst (typically 80 - 120 mA at 3.3 V for SX1276-based modules at +17 dBm).
- Receive / idle: Firmware running, radio in RX mode, no
TXTX. This is the whole-node draw (MCU + radio + regulator + peripherals), typically 30 - 80 mAdependingonplatform).ESP32-class boards — the SX1276 radio alone is only ~11 - 14 mA in RX, the rest is the MCU and supporting circuitry. Confirm against a board-level power measurement for your platform. - Deep sleep (if used): Microcontroller and radio in lowest power
statestate, highly design-dependent (0.01 - 10mAmA).dependingAnonnRF52840design).board with clean power management can reach low microamps, whereas an ESP32 board with a leaky regulator may sit in the milliamp range — measure your specific board (e.g. nRF52840 vs ESP32 T-Beam) rather than assuming.
Calculate a weighted average current based on the fraction of time spent in each state. The example below is illustrative, using an assumed duty cycle rather than measured T-Beam data — substitute your own measured currents and duty cycle. For a Meshtastic router node set to 5-minute heartbeat with 20-second sleep cycles:
Example:Illustrative example (assumed duty cycle): T-Beam v1.1 (ESP32 + SX1276 + NEO-6M GPS) TX (0.5% of time at 120 mA) = 0.6 mA average RX active (79.5% at 80 mA) = 63.6 mA average Deep sleep (20% at 3 mA) = 0.6 mA average ───────────────────────────────────────────────── Weighted average ≈ 64.8 mA
Step 2 - Calculate Daily Watt-Hours
Multiply the average current (in amps) by the system voltage and by 24 hours:hours. Use the voltage the node actually runs at when you measured the current — for a single-cell board that is ~3.7 V, even if the battery pack you eventually buy is a 12.8 V LiFePO4 pack. Energy (Wh) is conserved across the voltage conversion: you compute daily Wh at the node's running voltage, then later convert the required Wh to pack Ah using the pack's nominal voltage (Step 6). Do not mix the 3.7 V cell figure with a 12.8 V pack in the same multiplication.
Daily_Wh = I_avg(A) × V_system(V) × 24 h Example: 64.8 mA × 3.7 V × 24 h = 5.75 Wh/day
If your system runs at 5 V (e.g., USB-powered node) or 12 V (e.g., Raspberry Pi gateway), substitute the appropriate system voltage.
Step 3 - Determine Required Autonomy Days
Autonomy is the number of consecutive days with no solar input (full cloud cover, snow burial, north-facing shade) the battery must sustain the node. Select based on your climate and criticality:
| Deployment Type | Recommended Autonomy | Rationale |
|---|---|---|
| Sunny desert / Southwest US | 3 - 5 days | Extended low-sun periods are rare |
| Pacific Northwest / Northeast US | 5 - 7 days | Multi-day overcast events common Nov - Mar |
| High alpine / polar | 7 - 14 days | Snow burial possible; winter darkness |
| Non-solar (mains backup only) | 0.5 - 1 day | Bridge a brief power outage |
Step 4 - Calculate Raw Battery Capacity
Raw_Wh = Daily_Wh × Autonomy_days Example (5 days autonomy): 5.75 Wh × 5 = 28.75 Wh
Step 5 - Apply Derating Factors
Real batteries deliver less than their nameplate capacity due to temperature, aging, and depth-of-discharge limits. Apply the following derating multipliers:multipliers. The temperature factors below are conservative planning estimates for a −10 °C average low, not values pulled from a specific datasheet curve — check your battery's own capacity-vs-temperature curve where one is published:
| Factor | LiFePO4 | LiPo | Lead Acid |
|---|---|---|---|
| Max recommended DoD | 80% (use 0.80) | 50% (use 0.50) | |
| Temperature derating (cold climate, −10 °C avg | 0.85 | 0.70 | 0.65 |
| End-of-life capacity (design to still work at EOL) | 0.80 | 0.80 | 0.80 |
| Combined derating factor | 0. | 0.80 × 0.70 × 0.80 = 0.448 | 0.50 × 0.65 × 0.80 = 0.260 |
Cold-charge warning: Never charge any lithium chemistry — including LiFePO4 — below 0 °C (32 °F); sub-freezing charging causes lithium plating and permanent damage. In cold climates require a BMS with low-temperature charge cutoff or a charge controller with a battery temperature sensor. (LiFePO4 may still discharge down to about −20 °C.)
Required_Wh = Raw_Wh / Combined_derating_factor Example (LiFePO4, cold climate): 28.75 / 0.578544 =49.752.8 Wh → round up to5053 Wh
Step 6 - Add a 20% Safety Margin and Convert to Ah
Final_Wh = Required_Wh × 1.20 (20% safety margin) Final_Ah = Final_Wh / V_nominal_pack Example (LiFePO4, 12.8 V nominal pack): Final_Wh =49.752.8 × 1.20 =59.663.4 Wh Final_Ah =59.663.4 / 12.8 = 4.6595 Ah → use a 6 Ah pack
Worked Examples
Example A - ESP32 LoRa Repeater (T-Beam, indoor/outdoor enclosure)
| Platform | TTGO T-Beam v1.1 (ESP32 + SX1276 + AXP192 PMIC) |
| Measured average current | 65 mA at 3.7 V = 0.240 Wh/h |
| Daily consumption | 5.76 Wh/day |
| Target autonomy | 5 days (Pacific NW) |
| Raw requirement | 28.8 Wh |
| After derating (LiFePO4, cold) | 28.8 / 0. |
| With safety margin | |
| Recommended battery | Bioenno BLF-1206A (6 Ah, 12.8 V LiFePO4) or equivalent |
Example B - nRF52840 Ultra-Low-Power Node (RAK4631 + solar harvest)
Note on chemistry: the LiPo sizing below is shown only to illustrate the LiPo derating column. For an unattended outdoor or permanent solar deployment, LiPo is not recommended — use a LiFePO4 (or a protected Li-ion) cell instead, per the battery-chemistry, LiFePO4-vs-LiPo, and cold-weather pages. The 8 mA average is an assumed measurement; the duty cycle behind it (mostly deep sleep with brief RX) must be confirmed on your own board.
| Platform | RAK WisBlock Core RAK4631 + RAK12500 GPS (GPS duty-cycled off) |
| Measured average current | 8 mA at 3.7 V = 0.0296 Wh/h (assumed, with aggressive |
| Daily consumption | 0.71 Wh/day |
| Target autonomy | 7 days |
| Raw requirement | 4.97 Wh |
| After derating (LiPo, moderate | 4.97 / (0.80 × 0.80 × 0.80) = 9.71 Wh |
| With safety margin | 11.65 Wh → at 3.7 V = 3.15 Ah → use a 3.5 Ah |
| Recommended battery | EEMB LP905060 3.7 V 3500 mAh |
Example C - Raspberry Pi Zero 2W + SX1302 HAT Gateway
Gateway note: a gateway is the highest-value node in an incident (it bridges to internet/MQTT), so the 3-day / no-cold-derate figures below are a desert best-case, not a default. For a gateway, use at least 5-day autonomy even in sunny climates, and do not apply a 1.00 temperature factor unless you have confirmed the battery never sees sub-freezing nights. The 620 mA average is a representative estimate (Pi Zero 2W idles ~150 - 250 mA, the SX1302 HAT adds load) — measure your own build.
| Platform | RPi Zero 2W + RAK2287 SX1302 HAT + LTE modem |
| Measured average current | 620 mA at 5 V = 3.1 W = 3.1 Wh/h (representative estimate — measure your build) |
| Daily consumption | 74.4 Wh/day |
| Target autonomy | |
| Raw requirement | |
| After derating ( | |
| With safety margin | |
| Recommended battery | Battle Born BB10012 (100 Ah, 12 V LiFePO4) or |
Rule of Thumb Quick Reference
These figures are computed from the Step 1-6 methodology (5-day autonomy, LiFePO4, with DoD/cold/EOL derate and a 20% margin). They are planning floors — run your own numbers from your measured daily Wh.
| Node Type | Typical Daily Wh | Minimum Battery (5-day, LiFePO4) |
|---|---|---|
| nRF52840 sleepy node | 0.3 - 1.5 Wh | |
| ESP32 Meshtastic router (no GPS) | 3 - 5 Wh | |
| ESP32 + GPS always-on | 5 - 10 Wh | |
| Pi Zero 2W gateway | 60 - 90 Wh | |
| Pi 4 + LTE gateway | 100 - 150 Wh |