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Power Products Guide
Section 2 Batteries
2.1.5 Self-Discharge of
Batteries
2.2 Model 934
Nickel-Cadmium
Battery
2-4
current through the cell, in effect, charging it backwards. This
amounts to the electrical reversal of the two poles of the cell, and
can ruin either type of battery.
In nickel-cadmium batteries, cell reversal causes gas generation,
and that may force the vents to open. In lead-acid batteries, the
reversed cell presents a high resistance to the rest of the circuit,
making recharging difficult. Finally, while the battery may show
nearly full terminal voltage, there is, in fact, very little current
capacity left in it.
Self-discharge is the characteristic of all batteries that makes
them run down completely over time, even though there is no
load attached. Self-discharge is the result of inevitable chemical
reactions occurring inside the cell. This characteristic is more
serious in some types of batteries than others.
Nickel-cadmium and the newer nickel-hydride batteries have
poor charge retention. Loss of as much as 1% per day of
remaining charge has been reported. On the positive side, com-
plete self-discharge of nickel-cadmium batteries does not perma-
nently harm them. Alkaline (primary) cells, (not rechargeable)
have very good charge retention, keeping most of their charge
for several years at moderate (room) temperatures. Lithium bat-
teries, also nonrechargeable and often used for memory backup,
have excellent charge retention, as long as ten years.
Lead-acid batteries have good charge retention, retaining
about 50% of capacity after one year when stored at room tem-
perature. However, you should never allow a lead-acid battery to
self-discharge completely. Unlike the nickel-cadmium battery,
complete self-discharge will generally ruin a lead-acid battery.
Because chemical reactions cause self-discharge, temperature
has an effect. Avoid storing batteries in hot environments. The
rate of chemical reaction doubles for every 10 C increase in
temperature. Finally, note that the rate of self-discharge tends to
increase as the batteries age, especially for nickel-cadmium
types, while their ampere-hour capacity usually diminishes.
The Isco Model 934 Nickel-Cadmium Battery is the most
popular battery for Isco equipment. This battery offers advan-
tages over other types of batteries, particularly lead-acid types.
The number of charge/discharge cycles is quite high, as many as
500 or more, according to one manufacturer. A nickel-cadmium
battery can stay discharged indefinitely, even at very low temper-
atures, without sustaining damage. This condition would ruin a
lead-acid battery.
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