Page 117 - Applied Photovoltaics

6.5.3 Charging regimes
Photovoltaic system batteries are usually operated in either the constant potential
(float) or cycling mode. Many PV powered batteries spend long periods at low states
of charge, typically over winter, which can cause problems. For instance, crystals of
lead sulphate grow on the battery plates during periods of low state-of-charge,
reducing the battery efficiency and accessible capacity. This is known as sulphation.

Limiting discharge levels to a maximum of 50% can minimise this effect and keep
sulphuric acid concentration high. There is also less chance of the battery freezing if
acid concentrations are high. Another means of minimising winter problems is tilting
of the array to a steeper angle to fully utilise the winter sun, at some expense to the
collection of summer radiation.

Overcharging also has its problems, although it is good for short periods as a means
of charge equalisation. It causes gassing, which agitates the electrolyte, hence
preventing the more concentrated material from settling in lower regions of the
battery. However, over prolonged periods the gassing leads to loss of electrolyte and
shedding of active material from the plates.

To prevent overcharging, the voltage of each cell is usually restricted to 2.35 V by
using a voltage regulator. This limits the battery voltage to a maximum of about 14 V.
The charging characteristics of a typical lead-acid battery used with PV systems is
shown in Fig. 6.3, and the discharge characteristics in Fig. 6.4.

The most common method of regulation and control of lead-acid batteries is based on
the approximate state of charge, measured via battery voltage. Charging is halted at a
specific high voltage disconnect (HVD) point, chosen to allow a limited amount of
gassing, charge equalisation and electrolyte agitation, without excessive loss of
electrolyte. Similarly, discharge is halted at a specified low voltage disconnect (LVD)
point, chosen to maintain a reasonable battery life.

Figure 6.3. Constant-current charging characteristics at 25°C of a 500 Ah lead-
acid battery suitable for use in a PV system (Mack, 1979; reprinted with
permission of the Telecommunication Society of Australia).

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