VRLA battery capacity cycling: influences of physical design, materials, and methods to evaluate their effect

The float and cycle life of VRLA batteries are closely related and depend on, among other parameters, the acid electrolyte distribution. In this work our concern was directed toward the electrolyte and its distribution within the fiberglass matrix of the AGM separator. To decrease the mobility of the acid and at times excessive height to which it must wick, a somewhat unique physical design was suggested. The proposal included positioning the battery so that the plates were oriented as in a “pancake” stack or horizontal to the shelf the cell rested upon. Charge-discharge cycle testing was employed to determine the influence of plate orientation on the movement of the electrolyte. To closely monitor this movement the voltage drop in the acid path (IR) was measured during charge and discharge cycling. The measurements made during discharge found progressively larger voltage losses in the electrolyte with the cell plate stack discharging in the vertical position. It was concluded that this was the result of an insufficient volume of acid at the top of the plate stack. As a result of this work it has been demonstrated that: increasing the surface area of the separator glass fibers and decreasing the void volume average pore diameter can significantly improve cycle life; with the plate stack in a vertical orientation, cycle life capacity was limited by insufficient sulfate ion at the top of the stack; and with horizontal plate orientation, premature capacity loss was prevented because the SO ₄^-2 ion remained uniformly distributed along all of the plate surfaces