Marantz 5220 Manuel d'utilisateur télécharger pdf (Page 36)

TABLE

I- TYPICAL

CHARACTERISTICS

Size Capacity

(AH)

Internal

Resistance

(milliohms)

Max.

Charge

Rate

(mA)

1C Rate

(A)

0.5

35 50 0.5

C 1.0

10 100

D 1.2 7

100 1.2

D* 4.0

350 4

*High

power.

able of sustaining

overcharge

be-

cause there is no

convenient

way of

determining

when

they are

fully

charged (and

also

because

people

who

use

rechargable

cells

tend to

forget

turn

off

the

chargers).

sealed

cells,

the

gas

problem

solved by

making the negative

plate's

capacity

higher than that

of the

posi-

tive

plate.

When the

positive plate

fully charged

and

releasing

oxygen,

the

negative

plate

has

not yet come up

full

charge. The oxygen is permitted

to migrate

over

to the negative

plate,

where it combines with that plate and

prevents

it from further

charging.

Thus,

hydrogen

gas is never released,

and the

oxygen is

completely

used up.

This procedure can continue indefi-

nitely

long

as it proceeds slowly

enough to

allow

the

oxygen

time

to get

to the

negative

plate.

Most

sealed

cells

have

an emergency

high -pressure re-

lief valve to prevent

heavily over-

charged

cell

from

bursting.

For the

purposes

of this article, our

discussion

will

be limited

to the sealed

type

nickel-

cadmium cell.

It is

this

type of cell that is in

most

use in elec-

tronics.

Sizes and Capacities. There are

several varieties of sealed cells. Some

are

designed to

operate

over wider

temperature

ranges

than others,

some

have

larger

capacities, and others

permit

faster

charging

rates. However,

all

nickel-

cadmium cells

of the sealed

variety

are very similar

the

general

details of

their

care and

use.

The

most

popular sizes

nickel -

cadmium cells

and some

their

characteristics are

listed

in Table

The

information given here is

very

use-

ful,

but

it does

require

some clarifica-

tion. While the

AH (ampere

-hour)

fig-

ures listed under "Capacity" might

imply that

any

product of current in

amperes and

time in hours

will

yield

the

correct

figure

for a given cell,

this is

not

strictly the case.

A number

variables

(like

temperature,

end

volt-

age, current, and

duty

cycle)

have an

effect on the

number used to repre-

Sent

the capacity

of a

cell.

Fortunately,

these

effects

are usually

very

small

and can be ignored.

A procedure

often used for measur-

ing

capacity is

to select 0.5 V

as the

potential at which the cell

is declared

fully

discharged and

then

select

cur-

rent

that will discharge

the cell in

one

hour.

This is

termed the 1 -hour

rate

and that current is

called the 1C rate,

which is the rate

to which

all other

rates

are

referred.

The AA cell

Table

I has

a 0.5 -AH

capacity,

which

means

that

the termi-

nal potential

will be 0.5 V

after 1 -hour

drain

the

rate

of 500 mA. The

rate

this particular

case

500

mA.

Ideal-

ly,

this figure

would

mean

that

(for

example)

you

could get 1A from

the

cell for

half

hour

before the

potential

drops

to 0.5

V. But,

as we shall see, this

is not quite

correct.

you

selected 1 V

the

cutoff

po-

tential (the voltage

at which the cell

considered

completely

dis-

charged),

you would

expect to obtain

less

energy

from

the cell

than if cutoff

was

at 0.5

Furthermore, 1 V

appears

to be

practical cutoff point

than

0.5 V. So, why not

use 1 V? The

answer is

that the

voltage

characteris-

tics

of the

cells

are such that

the 0.5 -V

figure

produces

more reliable

number

than

does a

higher

cutoff

voltage.

Information

how

higher

dis-

charge rate

and

higher

cutoff voltage

affect

the

capacity of

typical nickel -

cadmium

cell is given in Table II. As

example of

how to use

this

table, con-

sider

cell

that is

to supply

until its terminal potential

reduced

to 1 V. From Table I,

the discharge

rate

for

AA cell

is 500

mA. Therefore,

our

rate

is 10C. Moving along the 10C

row

Table

II until

we get to the

1.0

-V

column, we

find

that, at

10C

dis-

charge rate to 1.0 V, 10%

of the cell's

capacity is

not

available. We can

ex-

pect to get only 0.45

AH (about

min-

utes of energy

this

high rate)

from

the

cell

under these conditions. Be-

cause Table II is given in terms

mul-

tiples of the 1C

rate, it

can be

used

with

all sizes

of sealed nickel-

cadmium

cells. The

table clearly

shows that

you

can

use

nickel-

cadmium

cells

at a

100

rate

an end

potential

of 1.0 V

with

very little reduction

capacity.

Discharge

Characteristics. One

of the

welcome

characteristics of

nickel-

cadmium

cells is

their

excellent

discharge

characteristic.

Their

termi-

nal

potential remains

a fairly steady

1.2

V until

the cell is

almost

completely

discharged,

after

which it drops

off

rapidly.

The

details of the

discharge

characteristics

for

an AA

cell are

shown in the

diagram (previous

page),

which

displays the voltage

versus AH

delivered

at various

discharge rates.

While the

plots are for

a typical AA cell,

they

also give the main

characteristics

of the

discharge

curves

for

any sealed

nickel-

cadmium cell.

Note

from

the

graph

that

the termi-

nal

potential

reduces

to 0.5 V when

the

cell has

delivered

0.5

the 10

rate.

(Because

the 1C

rate

is 500

mA for

the

cell, this

will take 1

hour).

the

0.1C

rate,

which is

50 mA for

cell, you

will

obtain

about

0.525 AH,

somewhat more

than 10 hours

use

because

the cell

will deliver

power

at that

slower

rate.

Similar

cal-

culations

can

made

from the curves

for

the

other cells

listed

in Table I. The

main

feature

illustrated

the curves

that the terminal voltage

any

cell

about

1.2

volts

for most

the time

is supplying (a

wide

range

of) current.

TABLE II- INACCESSIBLE

CELL CAPACITY

AS A

PERCENTAGE OF

TOTAL

CAPACITY

TO 0.5

VOLT

Discharge

Rate

Cutoff

Volta

0.1C

10C

0.5

1.0 1.1

0 3

4 7

0 5 9

0 10 30 -40

Charging Characteristics.

Sealed

nickel-

cadmium

cells

can be charged

under

a wide

variety

conditions,

but

the chemical processes do place

some limitations on the charging

pro-

cess. A little oxygen is generated at

the

positive electrode

during charge and a

lot

during overcharge.

This oxygen

puts

both an

upper and a

lower limit

on the charge

rate.

Sealed cells are designed to get

rid

the

oxygen

generated

during

over-

charge as quickly as

generated, as

POPULAR

ELECTRONICS

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