General: NiCd Battery Resistance

This week’s General Class exam question is from sub-element 6 (Resistors; capacitors; inductors) section B (batteries)… [G6B13]

What is an advantage of the low internal resistance of nickel-cadmium batteries?

A. Long life
B. High discharge current
C. High voltage
D. Rapid recharge

Nickel Cadmium, or NiCd, rechargeable batteries aren’t as popular as they once were, due to the current popularity of Lithium Ion (LI) and Nickel Metal Hydride (NiMH) batteries.  They do still have their place, though.

Let’s examine the incorrect answers first.  A) Long life?  A low resistance would seem to indicate that there’s nothing to keep the charge in the battery, right? So that one is probably not correct.  C) High voltage? NiCd batteries usually only have a 1.2V capacity, instead of the typical 1.5V.   D) Rapid recharge? This is not a normal quality of most rechargeable cells, as the potential for overcharging (and explosion) exists.  Slow or “trickle” charging is best.

The correct answer is B) High discharge current.  “High” is of course a relative term, but in order for a battery to do any good, it has to have a high enough current to generate suitable power, whether its in a flashlight, radio, or electric motor.  A typical 1.5V AA NiCd cell can supply up to a maximum 1.8A! That’s quite a bit for such a small package.  This answer also negates answer A) Long life, as a high discharge rate will unfortunately drain a battery quickly.

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