Page 221 - Battery Reference Book

Maintenance-free lead-acid batteries 1815
of 400 Ah at the 2 h rate of discharge. This is because. form lead sulphate and water. The oxygen is returned
at higher rates, the voltage drop IS more rapid and the to the electrolyte in the form of water and the constant
final voltage is reached more quickly. applied current is used up electrochemically to convert
Motive power battery capacities are normally given the lead sulphate back to spongy lead. This cycle can
at the 5 or 6h rate of discharge. This hypothetical, be repeated many times without losses to the system.
continuous rate of discharge relates most closely to the The act of absolute recombination can present a
actual performance of an industrial truck; stop-start problem. It has been demonstrated that a sealed
operation over an 8 h shift. Ambient temperatures also lead-acid battery can develop thermal runaway in a
affect the capacity of a battery. At low temperatures manner similar to sealed nickel-cadmium cells. This
the capacity is considerably reduced. is a condition whereby a battery on charge under volt-
age regulation begins to accept progressively higher
currents resulting in the build-up of excessive internal
18.2 Maintenance-free Bead-acid heat. Both recombination and temperature decrease the
batteries back e.m.f. of the battery, allowing higher and higher
currents to be accepted. The result is an uncontrollable
Any battery. when overcharged, will liberate hydrogen system which can become hazardous.
and oxygen gam as water is decomposed. For a bat- Low-pressure vented maintenance-free batteries
tery to be maintenance free, it is necessary to retard gas have the ability to recombine these gases. However,
liberation, otherwise the electrolyte would be depleted the degree of recombination is not complete since,
prematurely and, catastrophic failure would result. from the hazard point of view. it is not considered
There are two principal types of maintenance-free advisable to operate batteries under excessively high
lead-acid battery: gas pressures.
1. The type featuring calcium-lead alloys and im- In practical designs of low-pressure maintenance-
mobilized sulphuric acid electrolyte, which reduces free batteries, reduction or elimination of gassing
but does not completely eliminate gassing, i.e. there towards the end of charge and self-discharge is handled
is electrolyte volume reduction. Such cells are usu- by battery design features such as special grid alloys
ally manufactured containing a reserve of electrolyte and immobilized electrolytes, while gassing on over-
so that topping up is not required during battery life. charge is avoided by attention to charging method and
2. The type in which complete recombination of elec- charging control.
trolysis gases occurs, Le. virtually no electrolyte
loss occurs. 18.2.2 Use of calcium alloys to control
gassing on overcharge or during
18.2.1 Calcium-lead alloy batteries self-discharge
This type of lead-acid battery is typified by With lead-acid batteries, electrolyte decomposition
batteries in the Sonnenschein Dryfit range. Under into hydrogen and oxygen is accompanied by reduction
constant-potential charging, the maintenance-free of electrolyte volume, thereby increasing the concen-
lead-calcium-acid battery will accept relatively high tration of the electrolyte. Damage is then caused by
currents initially when the system is most efficient. increased corrosion of the positive plate due to the
This stage is followed by a second period when the highly oxidizing effect of the atomic oxygen being
back e.m.f. of the battery begins progressively to liberated. A further reason for the evolution of oxy-
increase to control the amount of current accepted. At gen and for corrosion is anodic oxidation due to anode
full charge and under proper voltage regulation, the potential. At the same time there is a risk of explosion
current accepted will be reduced to a few rnilliamps since the oxygen-hydrogen mixture evolved can be
input, thus restricting the degree of overcharge which ignited at relatively low concentrations and tempera-
protects the unit from excessive electrolyte loss. tures. In partially recombining sealed lead-acid batter-
Various designs of these batteries further support ies, liberation of gas is reduced to minimum values by
this function by developing a very high hydrogen and using a lead-calcium alloy containing only 0.08% cal-
oxygen overvoltage, which enhances the efficiency of cium rather than the more usual lead-antimony alloy
conversion and ensures a sharp and reproducible rise in which contains 4-6% antimony. It is thus possible to
on-charge voltage to regulate current acceptance best. reduce the self-discharge and the associated liberation
Absolute recombination in a battery of hydrogen of gas by a factor of about 5. In addition, other factors
and oxygen produced towards the end of charge or affect the self-discharge rate of conventional lead-acid
on overcharge is achieved only in a closed container in batteries when not in use. One example is the virtually
which the gases are held under relatively high pressure. unhindered passage of atmospheric oxygen into the cell
In such a system equilibrium occurs where the amount interior and then to the negative plate. This effect is
of oxygen electrochemically generated at the positive hindered in many battery designs by the provision of
plates is chemically reacted with spongy lead to form one-way self-sealing vents. It is for these reasons that
lead oxide, which then reacts with sulphuric acid to although the capacity of a conventional 5% antimonial