Page 347 - Handbook of Battery Materials

11.2 Separators for Lead–Acid Storage Batteries 317

into cells, with electrodes weighing several kilograms, without damage. Modern
polyethylene traction battery separators have backwebs of about 0.50–0.65 mm,
that is, about three times the backweb thickness of starter battery separators with
the respective effect on stiffness, electrical resistance, and also production process
line speed. The larger backweb thickness combined with a higher oil content
(∼15–20%) gives the separator the required oxidative stability, which to a first
approximation is proportional to the product of backweb thickness and its oil
percentage. A somewhat lower porosity and thus lower acid availability are the
consequences.
The microporosity is also important for this application, in order not to allow
shorts through the backweb during battery life. Bottom shorts are avoided by a
mud room of sufficient dimensions, and side shorts by plastic edge protectors
on the frames of the negative electrode. Some manufacturers have switched to
using sleeves of polyethylene separator material, rendering an edge protection
superfluous. The use of three-side-sealed separator pocket in traction batteries
should be avoided, because experience has shown this can lead to increased acid
stratification, subsequent sulfation, and thus capacity loss.
The choice of a suitable oil has special importance. Besides beneficial effects
of the oil on the oxidative stability of the separator, other consequences have to
be considered. From the chemical mixture of which an oil naturally consists,
polar substances may migrate into the electrolyte. Being of lower density than the
electrolyte, they accumulate on its surface and may interfere, for instance, with the
proper float function of automatic water refilling systems. Some oils which fully
meet both of the above requirements have been identified, that is, they provide
sufficient oxidation stability without generating black deposits [53].
An effect similar to the water loss in starter batteries is characterized as
top-of-charge performance in traction batteries. Antimony is dissolved from the
alloy of the positive electrode, migrates through the electrolyte, and is deposited
on the negative electrode, where – because of its far lower hydrogen overvoltage
than lead – it catalyzes hydrogen evolution, thus reducing the charging voltage at
constant current during the overcharge period [76]. From long experience it is
known that some separators are able to influence this behavior [77–80]. Many
hypotheses have been proposed, examined, and discarded again; for the current
status of the discussion reference should be made to the literature [69, 70]. Suitable
additives, such as uncrosslinked natural rubber [81] or Voltage Control Additive
(VCA [82]) allow significant improvement of the top-of-charge performance of
batteries, helping polyethylene separators to gain acceptance in the great majority
of applications.
Traction batteries are assembled either with pasted and glass mat-wrapped
positive electrodes, as is the case predominantly in the USA, or with tubular
positive plates, which prevail in Europe. The former electrodes place no particular
requirement on the separator profile; vertical ribs on the positive side are stan-
dard. The construction with tubular positive electrodes preferably uses a diagonal
(Figure 11.21) or sinusoidal (Figure 11.22) rib pattern. Insufficiently narrowly
spaced supporting contact points between tube, rib, and separator backweb have