alleryStart learning →A charger is used to push current backwards through a battery. What must be true of the charger?
AIts emf must be greater than the battery emf
BIts emf must be smaller than the battery emf
CIts emf must match the battery emf exactly
DIts internal resistance must exceed the load
Answer & Solution
Correct answer: A. Its emf must be greater than the battery emf
1. Recharging means driving current into the battery, opposite to the direction it normally supplies.
2. Only a larger opposing emf can reverse that current, so the charger has to sit above the battery emf.
3. With the current reversed, I is negative in V = emf minus Ir, so the terminal voltage rises above the emf.
4. A charger matched exactly to the battery emf would drive no current at all and charge nothing.
_Source: OpenStax College Physics (CC BY 4.0), Ch 21 "Circuits and DC Instruments", section 21.2 Electromotive Force: Terminal Voltage_
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