Analyzing Battery Discharge Characteristics and Voltage Profiles

In this lesson, you will learn how to map battery discharge curves to your system’s power profile to accurately estimate runtime and anticipate voltage sag during high-load events.

Understanding the Discharge Curve

Battery chemistry dictates how voltage declines over time as capacity is depleted. Unlike an ideal voltage source, real-world batteries (like Li-Po or Li-ion) exhibit a non-linear discharge profile: an initial voltage drop under load, a long "plateau" phase, and a rapid "knee" toward the end of their life.

As an embedded engineer, you must distinguish between the Open Circuit Voltage (OCV) and the Loaded Voltage. The difference between these is primarily due to the battery's internal resistance ($R_{int}$). When your system executes a power-intensive task—such as triggering a radio transmission or powering an actuator—the current draw ($I$) causes a temporary voltage sag defined by the formula: V_loaded = V_ocv - (I * R_int)

Predictive Modeling in Firmware

If your system relies on an ADC to monitor battery health, relying solely on OCV is dangerous. A battery that looks healthy at idle might sag below your microcontroller's brown-out detection (BOD) threshold the moment a peripheral consumes peak current.

To handle this, you should implement an adaptive estimation routine. Instead of assuming a static capacity, use your ADC readings to track the voltage drop during known high-load spikes:

// Simplified logic to detect voltage sag depth
float get_sag_depth(float v_idle, float v_active, float current_ma) {
    // V = IR calculation to estimate effective internal resistance
    return (v_idle - v_active) / current_ma; 
}

void monitor_battery(void) {
    float v_load = read_adc_voltage();
    // If v_load approaches threshold, trigger power-saving mode
    if (v_load < SAFE_OPERATING_LIMIT) {
        enter_critical_power_mode(); 
    }
}

By characterizing your battery’s $R_{int}$ at different temperatures and charge states, you can effectively "predict" the knee of the curve. This allows you to gracefully disable high-drain features before a catastrophic voltage sag causes a system reset, ensuring your device remains operational for the longest possible duration.

Key Takeaway: Always account for internal resistance and transient load spikes when interpreting voltage profiles, as idle voltage is a poor predictor of usable capacity during high-current operations.