A discharged storage battery of 3 cells has an open circuit voltage of 1.8 V per cell and an internal resistance of 0.1 ohm per cell. What voltage is needed to give an initial charging rate of 10 A?
• Ohm's Law for the whole circuit (external source, internal resistance, and battery EMF) • Total internal resistance of a multi-cell battery (how resistances in series add) • Total battery EMF/voltage for cells in series (how cell voltages add up)
• First, find the total internal resistance of the battery using the given resistance per cell and number of cells. How do resistances in series combine? • Next, find the total battery EMF (open circuit voltage) from the voltage per cell and the number of cells. How do voltages in series add? • Then, apply Ohm’s Law to relate the charging current, the total internal resistance, the source voltage you apply, and the battery’s existing EMF. Which direction does current have to flow for charging?
• Compute total internal resistance correctly from the 0.1 ohm per cell value • Compute total open-circuit voltage for all 3 cells together before using Ohm’s Law • When you write the equation for the charging circuit, make sure the applied voltage is greater than the battery’s EMF so that current flows into (charges) the battery. Check that your chosen answer satisfies this.
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