Page 120 - Applied Photovoltaics

6.6.3 Rechargeable alkaline manganese (RAM) batteries
Rechargeable alkaline manganese batteries have been developed from the primary
batteries that have been available for decades. They have a nominal cell voltage of
1.5 V and are sealed. They contain no heavy metals and so may be less of an
environmental concern than most other battery types, but they are so far available
only as small batteries.

Unfortunately, they have a high internal resistance and short cycle life for deep
cycling. Limiting the depth-of-discharge to just a few percent is required to achieve
long cycle life. These batteries may have applications in photovoltaic systems in
special cases such as emergency lighting or where shallow cycling is acceptable
(Sauer, 2003).

6.6.4 Lithium-ion and lithium-polymer batteries

Lithium batteries are in common use in portable appliances such as computers,
cameras, personal organisers and mobile phones. Since they have a nominal cell
voltage (3.6 V) above that required to electrolyse water, water-based electrolytes are
forbidden and they use lithium salts in an organic solvent. They are sealed but have a
safety vent. The inclusion of highly reactive metallic lithium poses a safety risk of
explosion or fire and care needs to be taken to protect against over-charge, over-
discharge, over-current, short circuit and high temperatures (Sauer, 2003).

6.6.5 Redox-flow batteries
Redox batteries use reversible reactions to store and discharge electricity in liquid
electrolytes. An ion-selective membrane is used to control ion transfer between
separate solutions of the active materials as they are pumped through the battery. The
electrolytes are stored externally in tanks, where there is no self-discharge. In
Australia, both vanadium and zinc-bromine redox batteries are under active
development. Compared to lead-acid batteries, they can have high cycle life, high
energy density and can be fully discharged.
The capacity of a redox battery is a function of the quantity of active material stored,
and the power rating is a function of the battery chamber size. Hence, the capacity
and power rating can be independently selected for each application. The efficiencies
of redox batteries can be high, as illustrated by the vanadium redox battery (Largent
et al., 1993):
x coulombic efficiency—97%
x voltage efficiency—92%
x energy efficiency—87% (including pumping losses).

Problems that may arise with the use of redox batteries in PV applications include the
need for maintenance and the contamination of active materials in harsh
environments. Cross-contamination problems, such as experienced in iron-chromium
redox batteries, are eliminated in the all-vanadium battery.

6.6.6 Super capacitors
In contrast to normal capacitors, these electrostatic storage devices use an ion-
conducting membrane between the electrodes, rather than a dielectric. Three of their

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