Marantz 5220 User Manual download pdf (Page 37)

long

as the charge rate is kept

below

0.1C, which means

that

current

at the

rate

of 0.1C or

lower

can

be supplied

indefinitely

to the

cell.

Higher

current

rates

-up to 20C

in special appli-

cations -can

be accommodated as

long

the

positive

plate

of the sealed

cell

is not

overcharged.

It is

difficult

(but

not impossible)

to tell just

when

overcharging

sets

in. (Fast

-rate

char-

gers are complicated and

expensive to

build,

which

precludes

them

from this

'discussion.)

The

amount of oxygen

generated

before the

cell is fully

charged is small,

but

does compete

with the desired

oxidation

of the

nickel hydroxide.

It is

this

reaction

that

defines

the

minimum

charge current that will

effectively

charge

a cell.

charge

rate lower

than

0.01C

results

current

being

used

to generate oxygen than is used

to convert

the active

material. Hence,

currents smaller than 0.01C produce

little increase

in the charge contained

in the cell.

Most chargers

supply current at the

0.1C

rate.

This

represents

the

rate

that

will

recharge

the ordinary cell

the

least

possible time without

endanger-

ing

the life of the

battery

accidentally

left

connected for a long time.

important

note

that, while current

the

1C rate

will discharge the cell in 1

hour,

than

10 hours

are

required

to charge it at the 0.1C

rate.

The

oxy-

gen

generated

and

losses in

the

cell's

internal resistance

are two

reasons. In

general,

at the

0.1C rate, one must put

about

140% of the

energy

that the

cell can store

before a

completely dis-

charged

cell can

considered

fully

charged.

There are

several other

facts

about

charging nickel-

cadmium cells that

are useful to

know. If you

charge at a

rate,

only about 120% of

the

cell's

capacity

can be supplied before over-

charging commences. If

a0.05C

rate is

used,

the cell

will

be difficult to

charge

above

75%

its capacity. Allowing

the temperature of

the

cell

reach

about 50°

will cause

difficulties

when

attempting

charge above

75%

capacity, even with a charge

rate

0.1C. Full charge is

assured at

25°C. At

very low temperatures, like

5°C, some

hydrogen

is generated

the negative

plate

of the cell during charging.

There is no

rapid

recombination reac-

tion

rid

the cell of this gas, so it

tends to

increase

the

pressure

inside

the cell. If

the

cell

must

recharged

low

temperatures, the

only way

overcome this problem

derate

the

MARCH

1976

maximum

permissible

overcharge

rate

0.02C

at 20° C.

Failure

Modes.

Because nickel -

cadmium

cells use

active

materials

that

are

highly

insoluble in

their al-

kaline

electrolyte, failure

modes

ar6

few. Most

sealed cells are guaranteed

for

500 to 1000 charge /discharge cy-

cles. This might

appear to

be a

limited

number,

but

when

you

consider 1000

cycles

at a

rate

two

cycles

per week,

these

cells

will

last 10 years.

the case

of sealed cells,

the qual-

ity

materials

used

making

them

has

marked

effect on their useful life.

Although

failures

are

rare,

they

occur

(catastrophically)

for

two

major

reasons:

internal

shorts and loss

electrolyte.

Internal

shorts develop when

time

and temperature

cause

decomposi-

tion of the

materials

that

separate the

positive and negative

plates

of the cell.

Shorts are generally

low- charge

phenomenon.

Loss

of electrolyte

reduces

the

capacity

the

cell and increases its

internal resistance. The

electrolyte

usually lost

some

combination

two

ways. Even

the best of

hermetic

seals will

allow some

hydrogen

and

oxygen to

escape.

the case of

high -quality

seals, 10 years

will

elapse before an

appreciable

amount of

electrolyte

lost.

the cell

abused by

excessive overcharging

reverse

charging,

excessive gas

the cell will

cause

the

safety

valve

vent

the

excess

pressure

into the

at-

mosphere.

Needless

to say,

the

hermetic

seal is now

broken and

evaporation of the

electrolyte will

much

faster.

Even if the safety valve is

resealable

(quite

common),

signifi-

cant

amount of vapor will

escape with

the

excessive pressure

and eventually

cause

the

cell to dry

out with

con-

tinued venting.

There

are also

non

-catastrophic

failures

common to nickel- cadmium

cells. These,

however,

are

reversible

so that the cell

can be

restored

full

capacity.

One

reversible failure mode

is due

to long and continued

overcharging

(as when

standby

power supply

kept

float

charge

for

month

without discharging it). The

ef-

fect

accentuated by

high

tempera-

tures.

The

second reversible failure

mode

appears in cells used in a

regu-

lar

cycle.

a group

cells

is regularly

called upon to deliver,

say,

25%

their full capacity

and

then

recharged,

they will

eventually

"memorize" that

only 25% of capacity

will be

required

them and

become

incapable of

supplying

the

remaining 75%

capacity.

This phenomenon is

most

likely

to occur

a cell is

rarely

over-

charged,

the

rate

of discharge

great,

and

/or

the temperature

high.

Non -catastrophic failures

can be re-

versed

completely discharging

the

cell at

low

discharge rate

and then

recharging

at a 0.1C

rate

for

20 hours

25°

C (80° F).

One or

two recondi-

tioning cycles like

this

are

generally

all

that

is needed

restore

cell

its

full

capacity.

Storage

Characteristics. Sealed

nickel-

cadmium cells

readily

lend

themselves

to prolonged storage,

whether in

a partially or

fully

charged

state

or completely discharged. If

stored in a charged

state, the cell will

self- discharge,

at a

rate that

depends

on cell design

and

storage

conditions.

In general,

cell will lose

about 1% of

its

charge per day so that

at the end of

about three

months

an initially fully

charged

cell will

completely dis-

charged.

stored at

high tempera-

tures (50°C or

higher),

the cell

will

lose

up to

5% of its charge per day, with

the

charge

lasting less than

month.

The lack of a charge in

cell

when

put

into

storage

has no

effect

the

cell's life. The cell can be

put back

into

service after one or two charge/

discharge cycles. Over

a wide

range

temperatures ( -50°

C to 50° C),

nickel-

cadmium cells can be stored

for years

with no significant degrada-

tion

in performance.

Closing

Comment.

Sealed

nickel

cadmium cells have

number

of out-

standing characteristics

that

make

them

good

first

choices for

everyday

use.

They are

reusable,

permitting

to 1000

charge

/discharge

cycles.

Their terminal

voltage during

dis-

charge holds

relatively

constant. And

they

require

special care.

There

are, of course,

some

minor

disadvantages. High initial

cost is one,

although it is

counterbalanced

the

fact

that

the cells

are

reusable.

Another

is that

the typical nickel -

cadmium

cell, when

compared with

the same -size

carbon

-zinc

cell, has

lower

capacity

and

lower terminal

voltage (1.2 V

as opposed

1.5 V for

the

carbon -zinc

cell). The

balance,

however,

is in the nickel-

cadmium

cell's favor

when it comes to long

life,

convenience

use,

and

reliability.

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