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55

lead-acid (VRLA), vented nickel

cadmium (Ni-Cd), and recombinant

nickel cadmium (RNi-Cd). The testing

regimes have to be appropriate to

battery capacity characteristics.

The lead-acid VLA and VRLA require

frequent testing by the time 80% of

capacity is reached. Thereafter the

testing frequency should be increased

to yearly intervals.

With the Ni-Cd batteries, the test

frequency should be at five-year

intervals – and when at 90% of rated

capacity it should be annual. The thing

to realise is that rated life is a bit like

how long a piece of string is. Figure 2

makes that plain

Regardless of the amp-hour capacity

of a particular battery, the more

discharge cycles it is subject to the

shorter its lifespan will be. Battery

tests, if they include a 100% depth

of discharge (DOD) will, as the graph

illustrates, shorten life the most.

Battery maintenance practice should

include amp-hour data collection.

(This is not a DOD statistic, since float

charging takes place – or usually can be

expected between discharge events.)

The VRLA battery is the most

popular type in substations. Its

one-way valve can, in cases of over-

charging, vent gases (hydrogen and

oxygen) but that’s not what should

usually take place.

In VLA (flooded cell) batteries the

creation of hydrogen gas, H2 and oxygen

radicals (1/2-O2) causes water loss in

the electrolyte, and a reduction in the

electrolyte specific gravity resulting a

decrease in cell open-circuit voltage.

In VRLA batteries the electrolyte

is immobilised by means of a porous

separator between the positive plate

(lead oxide) and the negative plate

(lead, in sponge form). The separator is

an absorbent glass mat (AGM) trapping

the oxygen – or, better put, making the

gas diffuse slowly to the negative plate

where lead oxide is slowly formed

during the battery discharge.

Despite the inherent ruggedness

of VRLA batteries, over-charging will

lead to oxygen generation in excess of

the AGM’s ability to trap the gas, and

out-gassing will occur via the one-way

valve. If the over-charging continues,

a premature drying out will occur

with a permanent loss in capacity.

The recharging process is therefore

very important.

Given a battery’s capacity, C, in amp-

hours, the total charge that needs to

be replenished in order to restore its

capacity is of the order of 1.10C. There’s

no such thing as a free lunch, and more

energy needs to be put in than will be

extracted during the discharge.

The energy loss in a battery during

charging is I

2

r

b

where I is charging

current and r

b

, the internal resistance.

The specifics of battery charger

operation are crucial for substations.

They form part of the general DC

power requirements during normal

operation, thus requiring some special

circuit considerations.

Under normal operation only the

charge loss due to internal charge

leakage needs to be made up. The

process is variously referred to as

trickle charging or float charging.

Irrespective of the semantics, making

up for more than the charge lost will

Charging

Voltage Degrees C

Time in Hours

AH% Restored

Temperature

Voltage

AH%

20

55

50

45

40

35

30

25

20

15

10

20

20

20

20

100

120

140

5

0

0 1 2 3 4 5 6 7 8 9 10

0

18

16

14

12

10

Battery

Bank A

Auxiliary DC

Supply Panel A

Main

Tie

A1

A2

A3

A4

An

An

N.O.

Tie

Main

B1

B2

B3

B4

Bn

Bn

N.O.

Auxiliary DC

Supply Panel B

Load Bank

Connection

Battery

Bank B

Load Bank

Connection

Battery

Charger A

Battery

Charger B

Figure 3: Constant current charging of a VRLA battery.

Figure 4: Typical redundant layout for a substation.

BY

PHIL

KREVELD