Page 116 - Applied Photovoltaics

6.4.6 Depth-of-discharge
Depth-of-discharge is the percentage of the rated capacity withdrawn from the
battery. Shallow cycling batteries should not be discharged more than 25% of rated
capacity, while up to 80% of the capacity of deep cycling batteries may be discharged
(Ball & Risser, 1988). Since battery life is a function of the average state of charge of
the battery, a compromise must be made when designing a system between cycling
depth and size of the battery.

6.5 LEAD-ACID BATTERIES

6.5.1 Types
Lead-acid (Pb-acid) batteries are the most commonly used in present stand-alone
power systems. They come in a variety of types—deep or shallow cycling, gelled
batteries, batteries with captive or liquid electrolyte, sealed or open batteries (Ball &
Risser, 1988; Sauer, 2003).

Valve-regulated lead acid (VRLA) or ‘sealed’ batteries allow for evolution of excess
hydrogen gas. Catalytic converters are used to convert as much evolved hydrogen and
oxygen back to water as possible and gas is vented only in the case of excessive
pressure in the battery. They are called ‘sealed’ because electrolyte cannot be added.
They require stringent charging controls, but less maintenance than open batteries.

‘Open’ or ‘flooded electrolyte’ batteries contain an excess of electrolyte and gassing
is used to reduce electrolyte stratification. The charging regime need not be stringent.
However, electrolyte must be replenished frequently and the battery housing must be
well ventilated, in accordance with local standards, to prevent the build-up of
hydrogen gas.
6.5.2 Plate material

Lead-acid batteries are produced with a variety of plate types.
1. Pure lead plates have to be handled extremely carefully since the lead is soft
and easily damaged. However, they provide low self-discharge rates and long
life expectancy.
2. Calcium can be added to the plates (giving lead-calcium plates) to provide
strength. Their initial cost is less than that of pure lead batteries, but they are
not suitable for repeated deep discharging and have slightly shorter lifetimes.
Lead-calcium plates are used in VRLA batteries for low gassing rates.
3. Antimony is also often added to lead plates for strength and low contact
resistance. Lead-antimony batteries are common in automotive applications.
They are substantially cheaper than pure lead or lead-calcium batteries but
have shorter lives and much higher self-discharge rates. In addition, they
degrade rapidly when deep cycled and need to be kept almost fully charged at
all times. They are consequently not ideal for use in stand-alone PV
applications. Lead-antimony batteries are usually only available as open
batteries, owing to the high rate of electrolyte use and consequent need for
topping-up regularly.

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