For a battery of emf ε and internal resistance r driving current I through external resistance R, the terminal voltage V equals

1. The battery emf drops across both external R and internal r. 2. Terminal voltage V is the potential difference across the external resistance. 3. V = ε − I r, where I r is the internal potential drop. 4. When the battery is being discharged, V < ε; on open circuit V = ε. _Source: syllabus Physics, Ch 3 "Current Electricity", §3 EMF and internal resistance_