NiCd Battery Basics
Agenda
• Electrochemical reaction
• NiCd battery characteristics
• Typical failure modes
• Different technologies
• Main advantages/disadvantages
Electrochemical reaction
• Positive electrode (the nickel-hydroxide electrode):
During discharge the Ni3+ ions are reduced to Ni2+
according to the (simplified) equation
– NiOOH + H2O + e- => Ni(OH)2 + OH-
• Negative electrode: This reacts via solution similar to
the lead electrode, during discharge follows:
– Cd + 2 OH-
=>
Cd(OH)2 + 2 e-
Discharge
2 Ni(OOH) + 2 H2O + Cd
2 Ni(OH)2 + Cd(OH)2
Recharge
NiCd characteristics
• Electrolyte
– The electrolyte is alkaline – a solution of
potassium hydroxide (KOH)
– The electrolyte doesn’t change during charge &
discharge – it retains the ability to transfer ions
between cell plates
– The specific gravity (nominal 1.20 g/cm3 at 20°C)
remains the same regardless of the state of
charge
– Stratification does not occur
NiCd characteristics
• Plates – structural components are made of
steel
– Positive active material: Nickel hydroxide
– Negative active material: Cadmium hydroxide
• Alkaline electrolyte doesn’t react with steel - the
plate integrity is maintained throughout the life of
battery
Typical “pocket plate” NiCd assembly
NiCd characteristics
• Voltage
– The open circuit voltage of the NiCd system is 1.2 to
1.3 V (recognized nominal voltage is 1.2 V)
– Requires more cells than lead acid batteries
– Ex) 92 cells needed for substation battery string
floating at 133V
• Float current
– Typical float current for pocket type NiCd cells is
100mA / 100 Ahr
• Factors to consider when determining # of cells /
string
– Min/Max string voltages the system can handle
– Must consider the float charge voltages
NiCd characteristics
• The equilibrium potential of the nickel-hydroxide
electrode is slightly above that of water decomposition,
so as in lead-acid batteries, oxygen evolution cannot be
avoided at the positive electrode; however:
– Corrosion of the nickel substrate in the + electrode is
negligible
– The reduction by oxygen at the nickel substrate of the
negative cadmium electrode is faster than in lead-acid
batteries - higher rate of self discharge compared to Lead
Acid
– The difference is self discharged NiCd cells can be
recharged without loss in capacity – this is one of the
major difference between NiCd and lead acid batteries
Thermal characteristics
• The reversible heat effect:
– During discharge, the heat is generated – about 10.5 %
of the discharged energy
– During the charging process, the reversible heat effect
causes cooling
Thermal characteristics
• Charge acceptance
– Charge acceptance is significantly reduced at
elevated temperatures, for example at the end of
discharge, or after electrolyte filling at
commissioning
– Thus, it is necessary or recommended that batteries
are allowed to cool before recharging (specially
important for capacity witness testing)
– Low temperatures also affect charging –
compensate by raising the float voltage
Failure modes for NiCd Cells
• Main mechanism is Cadmium dendrite
penetration through the separator, creating “soft
shorts”, leading to full short circuits in time
• Secondary mechanism is poisoning of the
Positive active material by Iron (Fe) coming from
the steel structure of the electrodes (mostly
Nickel plated, but some small exposed areas)
• Contamination of electrolyte from refilling water chlorine and carbonate
Range of Plate technologies
• There are 5 main technology types of NiCd
plate available in the market today
–
–
–
–
–
Pocket Plate
fibre (FNC)
foam
Sintered
Sintered / plastic bonded
Comparison of plates
Performance
classification
Thickness
(mm)
Substrate /
carrier
Porosity of
substrate /
carrier (%)
Capacity
(Ah/cm3)
Pocket
Fibre
Sintered
L,M,H
L-X
1.9 – 4.3
0.7 – 2.0
2 layer
Ni-plated
steeltape,
perforated
Ni-plated
PP-felt
18 - 21
0.2
H-X
Plasticbonded
M, H, X
Foam
M, H, X
0.5 – 1.0
0.5 – 0.9
0.3 – 2.0
Single
Single
Ni-plated
layer
layer
Polyurethane
Nickel
Nickel
foam
plated
plated
punched punched
steel plate
steel
+ sintered
plate
nickel
> 90
80
95
> 90
NPPS:
50
0.25 – 0.4
0.4
0.5 – 0.6
0.2 - 0.6
NiCd sizing
• NiCd sizing follows IEEE-1115 (latest revision)
• Must have the following parameters
–
–
–
–
–
–
Operating voltage window (min / max)
Operating temperature
End of discharge voltage
Aging factor
Design margin
Load profile (# of steps, duration, and amount of
current for each step)
Configuration Options
Mounting as compact blocks
Assembling in stainless steel crates or battery trays
Assembling in plastic crates
Main advantages of NiCd
•
•
•
•
•
•
Very good high power rating
Reduced loss of capacity at low temperature
No freezing at temperatures below 32°F (0°C)
Robust against deep discharges
Long shelf life
No passivation in case of prolonged storage in
partial discharge conditions
• Robust against abusive environment & optimized
for harsh operating conditions
• No electrolyte stratification
Main disadvantages of NiCd
• Contains cadmium
• Cost
• More cells needed to reach
operating voltage
• Possible memory effect
• High self discharge
Summary
• NiCd batteries have unique operating
characteristics
• NiCd batteries have distinct advantages and
disadvantages
• Choose the right battery for the application