Page 351 - Handbook of Battery Materials

11.2 Separators for Lead–Acid Storage Batteries 321

separator material. The solvent is extracted by hot water, which is evaporated in
an oven, and a semiflexible, microporous sheet of very high porosity (∼70%) is
formed [19]. Further developments up to 75% porosity have been reported [84, 85],
but these materials suffer increasingly from brittleness. The high porosity results
in excellent values for acid displacement and electrical resistance. For profiles, the
usual vertical or diagonal ribs on the positive side and, as an option, low ribs on
the negative side, are available [85].

11.2.3.1.5 Comparative Evaluation of the Traction Battery Separators Which
separator properties are important for use in traction batteries? For this aspect
primarily the highly predominant application, namely forklift traction batteries, is to
be considered: chemical resistance against attacks by acid and oxidation, mechanical
stability for problem-free assembly, stiffness to counteract overexpansion of the
negative active material, and low acid displacement are particularly desirable.
Delay in antimony poisoning, absence or near-absence of oily deposits in the
cells, and – last but not least – a low electrical resistance complete the requirement
profile.
In Table 11.11 an attempt is made to include the above criteria in the form of
quantitative data or qualitative evaluations.
Polyethylene separators offer the best balanced property spectrum: excellent
mechanical and chemical stability as well as good values for acid availability
and electrical resistance have established their breakthrough to be the leading
traction battery separator. Rubber separators, phenolic resin–resorcinol separators,
and microporous PVC separators are more difficult to handle than polyethylene
separators; their lack of flexibility does not allow folding into sleeves or use in a
meandering assembly; in addition they are more expensive.
Special applications are often governed by different priorities: as already discussed
in relation to golf carts, the low water loss and the delay in antimony poisoning
in heavy-duty service of a forklift are of eminent importance, with the result that
rubber separators remain the preferred product there. Submarine batteries offer
a different picture: the number of cycles to be reached is far lower (∼500) and,
due to the slow (∼100 h) but very deep discharge, the acid availability becomes
the decisive criterion, which favors, for example, the phenolic resin–resorcinol
separator. Such requirements are already similar to the application in open
stationary cells.

11.2.3.2 Separators for Open Stationary Batteries
Stationary batteries serve predominantly as an emergency power supply, that is,
they are on continuous standby in order to be discharged for brief periods and
sometimes deeply, up to 100% of nominal capacity, in the rare case of need.
The following profile of requirements for the separator thus arises: very low
electrical resistance, low acid displacement, no leaching of substances harmful to
float-service, as well as an excellent mechanical and chemical stability, especially