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Power Products Guide
Section 2 Batteries
2.2.2 Charging Temperature
2.2.3 Avoiding
Self-Discharge
2-6
confinement from the cells or enclosure. The gases may be
explosive and they can also be irritating to smell.
All rechargeable batteries, whether nickel-cadmium or
lead-acid, have one-way safety vents that open to relieve inter-
nal pressure generated by overcharging or very heavy dis-
charge. Their purpose is to keep the battery from bursting,
catching fire, or even exploding.
These vents do not open during normal operation. If they do
open, it indicates battery abuse. Venting shortens battery life
by depleting irreplaceable electrolyte. Avoid causing any of the
conditions that can make this happen.
One manufacturer of nickel-cadmium cells recommends charging
them within a temperature range of 32° F (0° C) to 110° F (45°
C). Below 32° F, charging increases the gas pressure within the
cell, and that may force the safety vent to open. Above 110° F,
charging efficiency decreases, and the high temperature accel-
erates deterioration of the plate separator material, hastening
end-of-life.
Measuring nickel-cadmium batteries shows an almost-constant
output voltage from charged to discharged. This is because the
electrolyte, water and potassium hydroxide (KOH), is not
depleted of ions (which affects conductivity) during discharge, as
is the electrolyte in lead-acid batteries. As a result, you cannot
measure the output voltage to calculate the remaining charge in
the battery. You can achieve the recommended 15-hour charging
interval by putting the battery on charge before leaving work in
the evening and then taking it off when you come in the next
morning (5
to 8
PM
AM
accepting the charge, it will stay cool to the touch. When fully
charged, the battery will feel warm, as it dissipates the charging
current as heat. Discontinue charging when the battery feels
warm.
As mentioned, nickel-cadmium batteries have a higher rate of
self-discharge than other types of batteries. After charging,
always try to return the battery to service within a reasonable
period of time (one to two weeks at most), to ensure the avail-
ability of most of the battery's capacity. Self-discharge does not
affect the life or usability of nickel-cadmium batteries, and it
does not mean the batteries are bad. It just means that the bat-
teries will not deliver full-rated capacity if you do not return
them to service promptly after charging.
Do not store the batteries in a hot environment, such as a room
with a furnace, boiler, or heater, or in direct sunlight, if at all pos-
sible. The self-discharge characteristic seems to increase with
age, probably due to the deterioration of the plate separator
material and the growth of crystalline "whiskers" between the
plates.
is 15 hours). As long as the battery is
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