Page 223 - Battery Reference Book

Maintenance-free lead-acid batteries 1
i
In a flooded lead-acid cell, this diffusion of gases 18.2.5 Gas recombination tec~nolo~~
is a slow process and virtually all of the hydrogen and automotive and commercial! vehicle lead-acid
oxygen escapes from the cell rather than recombines. batteries
In the Gates cell, the closely spaced plates are
separated by a glass mat separator, which is composed Several principles have been used by various manufac-
of fine glass strands in a porous structure. The cell turers in the production of low-maintenance batteries,
is filled with only enough electrolyte to coat the Le. batteries which do not require topping up of the
surfaces of the plates and the individual glass strands electrolyte during service. Catalytic recornbination of
in the separator, thus creating the ‘starved electrolyte’ electrolysis gases is not used for this type of battery.
In general, for automotive, vehicle and traction bat-
condition. This condition allows for homogeneous gas teries the antimony-free grid technology such as used,
transfer between the plates, which is necessary to for example, by Sonnenschein in their sealed batter-
promote the recombination reactions. ies for power tools is not now used. A few years ago
The pressure release valve maintains an internal automotive SLI battery production fell into two cate-
pressure of 40-60 psi. This condition aids recomb- gories based on grid alloy composition. It was either an
ination by keeping the gases within the cell long antimonial lead or a calcium-lead battery. Within the
enough for diffusion to take place. The net result is last few years, however, this distinction has become
that water, rather than being released from the cell, is less clear as many manufacturers have gone into what
electrochemically cycled to take up the excess over- could be termed hybrid constmction, which uses a
charge current beyond what is used for conversion of low-antimony positive grid and a lead-calcium alloy
active material. ‘Thus the cell can be overcharged suf- negative grid. In taking this approach, many battery
ficiently to convert virtually all of the active material manufacturers have been able to minimize some of
without loss of water, particularly at recommended the potential shortcomings associated with the purely
recharge rates. lead-calcium alloy batteries. At the same time, they
At continuous high overcharge rates (e.g. Cl3 and have also minimized many of the undesirable traits of
above), gas build-up becomes so rapid that the recomb- the conventional antimonial alloy battery.
ination process is not as highly efficient and oxygen The progression from a lead-calcium alloy battery
as well as hydrogen gas is released from the cell. to a hybrid construction employing a low-antimony
The D-cell was the first cell available within this positive grid and a lead-calcium alloy negative grid
range. It is rated at 2.5 Ah at the 10h rate. It is 61 rnm has reduced the necessity for a microporous separator
in height and Mmm in diameter. The construction in all battery designs. Enveloped polyethylene micro-
of this cell is similar to that of standard cylindrical porous separators are still widely used in batteries of
the lead-calcium type, and are recommended at vir-
nickel-cadmium cells. It is sealed, has a safety vent, tudy all plate spaces. However, as one moves to the
makes use of a cylindrical spiral-wound plate design hybrid construction, the necessity for a microporous
for high energy density and low internal impedance, separator in all battery designs has been reduced. When
and can be charged and discharged in any orientation. plate spacings are less than about I mm, enveloped
The starved system, porous separator and oxy- microporous separators are stili recommended. This
gen recombination allow efficient space utilization for separator construction precludes side and bottom shorts
active material in the spiral-wound sealed lead-acid due to mossing or dendrite growth. At the closer
cell, resulting in a 15-50% increase in volumetric plate spacing, shorting through the separator can be
energy density over gelled-electrolyte systems. a problem and the use of a microporous polyethylene
Low impedance is derived from the tightly wound separator virtually eliminates this failure mode. Low
design with multiple interconnecting tabs. In addition, electrical resistance leaf-type seprators can be substi-
the spiral-wound design maintains place spacing bet- tuted for enveloped microporous separators when piate
ter than flat-plate designs; therefore the impedance spacings are greater than 1 mm since the propensity for
remains more constant over life (as low as l0mQ shorting at the greater plate spacing is reduced.
compared 23.5 mQ for the closest gelled-electrolyte
battery). 18.2.6 Highly porous separator, low free aci
The spiral-wound plate roll is mechanically struc- volume technology
tured to prevent interplate movement found in flat- In a lead-acid battery, when the positive and negative
plate design. plates become fully charged they start gassing. The
The sealed design, accomplished with a welded cell positive plates give off oxygen and the negative plates
container, self-resealing pressure safety vent and oxy- give off hydrogen (oxygen is usually given off from the
gen recombinat:ion, obviates the need to add water. positive a little before hydrogen evolution starts at the
Oxygen recombination prevents electrolysis of the negative), reducing the water content of the electrolyte
water in the electrolyte during overcharge. This design and therefore the water level in the battery. Unless
does not vent hydrogen and oxygen from the elec- this water is replaced by topping up, the battery will
trolyte during normal operation. eventually cease to operate.