Class 4 (4th Engineer) MET 📅 Jan 2026

Exam Question

(a) Compare constant current method and constant voltage method of charging batteries. (6)

(b) A 24V emergency battery is to be charged from the 110V ship's mains when the e.m.f per cell has fallen to a minimum value of 1.8V. The battery consists of 12 cells in series, has a capacity of 100 Ahr at a 10 hr rate and the internal resistance is 0.03Ω/cell. If charging continues until the voltage per cell rises to 2.2V, find the value of the variable resistor needed to control the charging. The charging current can be assumed to be equal to the maximum allowable discharge current. (10)

Reference Answer

### (a) Comparison of Constant Current and Constant Voltage Charging Methods
A comprehensive comparison between the two primary methods of battery charging is as follows:
#### 1. Constant Current (CC) Method
* **Principle:** The charging current is maintained at a constant, predetermined level throughout the charging process, irrespective of the battery's state of charge or terminal voltage. This requires a variable voltage source to overcome the battery's increasing back EMF.

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