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Internal Resistance
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Load Current
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Voltage Drop
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Power Dissipated in Load
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To calculate internal resistance, measure the battery's no-load voltage and voltage under a known load. Use the formula: r = (V_no_load - V_loaded) × R_load / V_loaded, where r is internal resistance, R_load is the test load resistance.
Internal resistance is the resistance offered by a battery's electrolytes and electrodes to current flow. It causes voltage drop under load and affects the battery's ability to deliver current effectively.
An ideal voltage source has zero internal resistance, meaning it can deliver unlimited current without any voltage drop. Real batteries always have some internal resistance.
Lower internal resistance allows batteries to deliver higher currents with less voltage drop. This is why truck batteries can deliver more power than motorcycle batteries despite having the same voltage.
As batteries age, their internal resistance typically increases due to chemical degradation, corrosion, and electrolyte changes. Higher internal resistance reduces battery capacity and performance.
Internal resistance is affected by battery chemistry, temperature, age, state of charge, and construction quality. Cold temperatures and low charge levels generally increase internal resistance.
The voltage drop method provides a good approximation of internal resistance under DC conditions. For more precise measurements, AC impedance testing or pulse load testing may be used.
Choose a load resistance that draws a reasonable current (typically 10-20% of the battery's capacity) without overloading the battery. The load should be stable and accurately known.