OCV Curve Configuration for nRF52840 Devices

Problem

The default OCV (Open Circuit Voltage) curve in Meshtastic firmware has a minimum voltage of 3100mV (3.1V):

// Default from power.h
#define OCV_ARRAY 4190, 4050, 3990, 3890, 3800, 3720, 3630, 3530, 3420, 3300, 3100
//            100%   90%   80%   70%   60%   50%   40%   30%   20%   10%    0%

This is NO-GOOD for nRF52840-based devices:

1. DC-DC Converter: Most board designs use DC-DC converters that require minimum input voltage around 3.3V for stable 3.3V output
2. E-ink Displays: E-ink controllers typically need stable 3.3V supply
3. LoRa Module (SX1262): While spec allows 1.8V minimum, in practice operates better with stable supply
4. Flash Memory: QSPI flash requires stable voltage for reliable operation
5. Peripheral ICs: RTC, sensors, and other ICs may malfunction below 3.3V

When battery voltage drops below 3.3V, the device may experience:

• Random resets
• Flash corruption
• Display glitches
• LoRa transmission failures
• I2C communication errors

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Solution

Set minimum OCV to 3400mV (3.4V) to ensure safe shutdown before voltage drops too low.

Modified OCV Curve

#define OCV_ARRAY 4190, 4050, 3990, 3890, 3800, 3720, 3630, 3530, 3480, 3440, 3400
//            100%   90%   80%   70%   60%   50%   40%   30%   20%   10%    0%

Changes from default:

• 20%: 3420 → 3480 mV
• 10%: 3300 → 3440 mV
• 0%: 3100 → 3400 mV

This provides ~100mV safety margin above the critical 3.3V threshold.

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Affected Devices

All nRF52840/RAK and other devices.

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How It Works

The firmware uses the OCV curve for two purposes:

1. Battery Percentage Estimation

// Power.cpp - interpolates between OCV points
for (int i = 0; i < NUM_OCV_POINTS; i++) {
    if (OCV[i] <= voltage) {
        // interpolate between OCV[i] and OCV[i-1]
        battery_SOC = ...;
        break;
    }
}

2. Low Voltage Shutdown

// Power.cpp - triggers deep sleep when voltage < OCV[10] (minimum)
if (batteryLevel->getBattVoltage() < OCV[NUM_OCV_POINTS - 1]) {
    low_voltage_counter++;
    if (low_voltage_counter > 10) {
        powerFSM.trigger(EVENT_LOW_BATTERY);  // deep sleep
    }
}

With minimum set to 3400mV, device will enter deep sleep while there's still enough voltage for safe operation and data preservation.

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Adding to Other Devices

To apply this fix to any nRF52840/RAK device, add to its variant.h:

#define OCV_ARRAY 4190, 4050, 3990, 3890, 3800, 3720, 3630, 3530, 3480, 3440, 3400

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Technical Background

LiPo/Li-ion Discharge Curve

Typical single-cell LiPo discharge characteristics:

• 4.2V: Fully charged
• 3.7V: Nominal voltage (50% capacity)
• 3.4V: ~10-15% remaining
• 3.3V: ~5% remaining (critical for 3.3V systems)
• 3.0V: Cutoff voltage (0%, risk of damage)
• 2.5V: Danger zone (permanent damage)

nRF52840 Specifications

• Operating voltage: 1.7V to 5.5V (MCU itself)
• Internal DC-DC: Can operate down to 1.8V
• But: External components (regulators, flash, display) typically need 3.3V

Fail-safe approach

Setting 0% at 3.4V means:

• Device shows 0% battery with ~5-10% actual capacity remaining
• This capacity is reserved for safe shutdown
• Better to show 0% early than to crash at 3%