Information on lead acid batteries
Lead acid batteries are the common means of energy storage used in renewable
based rural energy systems. The following section provides information on some
design features of lead-acid batteries.
Basic battery information:
Batteries produce direct current and are measured by a capacity, specified as
the ability to supply a specific current for a specified length of time
(Amp-hours or Ah) and a direct current voltage (Volts DC, or VDC).
A specific battery is made up of individual cells that can be combined in
series arrays to achieve a desired voltage. A cell voltage will depend on the
battery type, generally around 2.0 VDC for lead acid technology and 1.2 VDC for
NiCd technology. For example, a 12 VDC lead-acid battery will be made up of six
individual cells in series while a 6 VDC battery will have three cells. The
term volts per cell (V/Cell or V/C) is used to specify a specific voltage for
each cell of a battery in place of specifying different voltages for different
sized batteries; 6, 12, and 24 VDC batteries for example.
As with battery cells, batteries can be connected in series to increase the
battery bank voltage. Two 12 VDC, 500 Ah batteries placed in series will
provide a battery bank providing a nominal 24 VDC, 500 Ah supply.
Batteries can also be connected in parallel to increase the capacity of the
array. Placing two 12, VDC, 500 Ah batteries in parallel will provide a nominal
12 VDC, 1000 Ah supply. If a larger capacity battery bank is required, this can
be achieved by increasing the capacity of the specific battery or by increasing
the number of batteries in the battery bank. Note that the energy storage
capacity of the battery is the same in both of the last cases, 24 VDC time 500
Ah provides the same number of watt-hours as a 12 VDC battery with 1000 Ah
General types and battery casing:
Vented (flooded) battery: the cells of the battery are filled with a liquid
electrolyte and have caps that can be opened to replenish the electrolyte with
Valve-regulated lead-acid battery (VRLA): pressure-release valves seal the
cells and loss of water is prevented by internal recombination of hydrogen and
Batteries are provided in hard plastic (PVC) or metal casings. Additionally PVC
casings can either be opaque or translucent (clear) that allows one to view the
battery plates and electrolyte level.
Vented / flooded: the electrolyte is free to move between the plates.
VRLA / AGM: the electrolyte is immobilised in Absorptive Glass Mats between the
VRLA / GEL: the electrolyte is immobilised in gel between the plates.
A battery cell contains positive plates connected in parallel and negative
plates connected in parallel. Among other parameters, the surface area of the
plates determines the maximum current that the battery can deliver.
Pasted plate: the plates are made of a grid with a lattice structure on which
the active mass, a metallic paste, is spread. As a battery undergoes deep
charge and discharge cycles the plate structure degrades, leading to battery
failure. Starter batteries are designed to provide high current in a limited
space, so the plates are typically very thin and spaced close together to
provide a very large surface area. Deep cycle batteries, typically used for
renewable energy systems, are designed with thicker plates that are widely
spaced to provide higher capacity and longer cycle life. Heavy-duty starter
batteries for lorries and other large vehicles usually have thicker plates than
starter batteries for cars.
Tubular plate: the grids of the positive plates are made of spines around which
the active mass is retained by polyester or glass-fibre sleeves. The negative
plates are commonly pasted plates.
The grid is made of a lead-alloy that can contain materials such as antimony
for flooded batteries, calcium for VRLA batteries, tin and other metals. The
choice of the alloy is a compromise between resistance to corrosion, mechanical
strength and the rate of electrolysis (hydrogen/oxygen production; gassing). An
alloy without antimony reduces the rate of electrolysis with respect to an
alloy with (low) antimony content. However, antimony generally improves the
cycling behaviour of batteries.
active mass on the plates (lead and lead oxide with additives called
a paste) must have a spongy structure with a certain porosity. A high porosity
structure has more surface which enables high currents (for starting engines).
A less porous, more compact structure however is better for deep cycle purposes
such as for renewable energy applications.
The nominal capacity CN
is the amount of charge that a fully charged battery can deliver at standard
conditions and at a constant rate in which the complete capacity is discharged
over N hours. The standard conditions depend on the application and are
generally defined by the manufacturer. For stationary applications the standard
conditions are 25°C and a 10h discharge rate. In some cases the nominal
capacity for solar batteries is based on the 100h or the 120h discharge rate.
The end of discharge is always defined by a voltage limit, such as 1.8 V/Cell.
The available capacity depends on the discharge rate. Batteries are typically
rated by the manufacture for a specific capacity at a specific current. If the
discharge current is higher (more current) than the rated current, the capacity
will be reduced. If a lower current is being used, a higher capacity can be
expected. To estimate the capacity at other currents than the rated current
Peukert's law can be used: tx / tN = (IN
/ Ix)n. Here tx is the discharge time
at a current of Ix A. The value of n is in the order of 1.15 for
small currents. Using this estimate the C20 rate (the current
that it takes to discharge a battery over a 20 hour period) of a battery would
be 11% higher than its C10
rate (the current to discharge the battery over 10 hours).
The capacity depends on the battery temperature: at low temperature the
capacity is reduced. Below 25 °C the capacity drops with about 0.6% /°C.
However at lower temperatures the dependence is much stronger. Although this
dependency differs per battery type some figures are given here as an
indication: around 0°C the capacity is about 75% and at -25 °C about 50% of the
capacity at 25°C.
A new battery may have a considerably higher capacity than the nominal
capacity, sometimes more than 50 % higher. Battery manufacturers sometimes try
to achieve lifetime guarantees by oversizing the batteries. Additionally, due
to the variability’s in the battery manufacturing process, even batteries of
the same model can have very different capacities. Finally, it is common that
the capacity of a battery will increase during the initial charge and discharge
cycles before settling out to a constant value.