Page 335 - Handbook of Battery Materials

11.2 Separators for Lead–Acid Storage Batteries 305

Figure 11.14 Polyethylene separator: cross-ribs in the margin area.
process, and apart from this they protect the particularly exposed edges of the
pockets during the life of the battery.One noteworthyversionof aprofile has recently
been presented by providing cross-ribs within the margin area (Figure 11.14) to
keep the backweb in this area always at a safe distance from the grid edge of
the positive electrode; the oxidizing substances originating there will thus do less
damage [55, 56]. A similar protection is offered by profiles with a continuous rib
pattern, that is, extending also into the welding zone, be it as narrow vertical ribs
or especially as sinusoidal ribs [57].
The narrow tolerances to be maintained for the total separator thickness are
tightened even further by the trend toward high-performance batteries with many
thin electrodes and therefore many separators also. One can easily calculate that for,
say, 10 or more electrodes and an equal number of separators per cell, the permitted
tolerances become very small for fitting the electrodes/separators stack into the
cell container. With electrodes and separators being produced continuously, that
is, the thickness of consecutive individual pieces all having the same tendency, this
means that if they are too thick, the stack does not fit into the cell without great
pressure; if they are too thin, there is the danger of the electrode stack suffering
in service due to vibration. As one solution, compressible ribs have been proposed
[58] with groups of three ribs of which the middle one which is not back-to-back, on
the opposite side of the backweb, generates a spring effect, balancing the tolerances
and fixing the electrode stack within the container by its resilience (Figure 11.15).
The desire for cost savings starts with utilization of material. Is the continuous
vertical rib necessary? Interrupted rib versions [56] or so-called dimples [47]
have been proposed repeatedly, but they have not succeeded because production or

Figure 11.15 Polyethylene separators: compressible rib design.