Page 346 - Handbook of Battery Materials

316 11 Separators

in special cases. Also under these conditions of use the microporous polyethylene
pocket offers the preferred solution [40]. Lower electrical properties at higher
temperatures, especially decreased cold crank duration, are battery-related; the
choice of suitable alloys and expanders gains increased importance.
However, it has to be conceded that after battery life cycle tests at such tempera-
tures polyethylene separators also reach their limits (although this fact does not yet
reflect in failure-mode studies [49]) even in locations with extreme ambient tem-
peratures. The tendency toward using ever-thinner backwebs cannot be continued,
however, without seeking protective measures. Suitable provisions have to be made,
especially with respect to the separator’s oxidative stability at elevated temperature.
The leading producers of polyethylene separators have recently presented solutions
[41, 47] which, even at 150 µm backweb, provide for oxidative stability and puncture
strength in excess of that for the standard product at 250 µm backweb [41].
Without any doubt, the microporous polyethylene pocket will meet all
requirements of modern starter batteries for the foreseeable future. Whether
and to what extent other constructions, such as valve-regulated lead–acid (VRLA)
batteries, other battery systems, or even supercapacitors, will find acceptance,
depends – besides the technical aspects – on the emphasis which is placed on the
ecological or economical factors.

11.2.3
Separators for Industrial Batteries
11.2.3.1 Separators for Traction Batteries
Traction batteries are the workhorses among batteries; day in, day out they have
to perform reliably, that is, for years. They are discharged to about 80% by their
nominal capacity, typically during an 8 h shift of a forklift, and are recharged during
the remaining hours of the day. A life of 1500 cycles or five years is taken for
granted, with concession regarding the life expectancy only made under extreme
conditions.
It can be stated generally that requirements for traction battery separators in
respect to mechanical properties and chemical stability are considerably higher
than those for starter battery separators. This is due to the fact that a forklift
battery is typically operated for about 40 000–50 000 h in charge–discharge service,
whereas a starter battery for only about 2000 h. The requirements for electrical
resistance are lower because of the typically lower current densities for traction
batteries. These differences are of course reflected in the design of modern traction
battery separator material.
11.2.3.1.1 Polyethylene Separators A detailed description of the production
process and the properties of polyethylene separators can be found in
Section 11.2.2.1, so only the modifications which are important for traction battery
separators are covered here.
Industrial battery separators are often supplied in cut-piece form, that is, they
have to have a certain stiffness and robustness in order to withstand the assembly