Abstract
Keyword - battery management, acid stratification, aging detector, service life, shallow cycling
In more and more applications, lead-acid batteries are subjected to shallow cycling. In recent years, the effects of shallow cycling on battery life have often been underestimated. Shallow cycling reduces the service life oft the battery because of stratification effects.
How can the service life under shallow cycling conditions be increased?
Another important question is, how can a battery failure be detected?
These two topics were the motivation for the work reported here.
We compared conventional (SLI) batteries against valve regulated (VRLA) batteries. The first had pocket-type separators, the second absorbent glass mat separators. Two 12V-batteries of the same type were connected in series. One of these batteries was subjected to an external mechanical pressure, and the other was not. Both batteries were equipped with a center tap, which permitted to compare the sum of voltages of first three cells with the sum of voltages of the other three cells.
We already knew from tests in the past, that mechanical pressure is a simple and efficient means to increase the cycle life. As shown in this paper, this is not always the case for shallow cycling conditions.
The center-tap connection is a useful means to detect defective cells. It permits the comparison of the voltages of the two battery half´s. If the voltage difference between the two half´s exceeds, for example, 2 volts, one can conclude that at least one cell is defective and is approaching its end of life. This technique is much more effective than the measurement of the total battery voltage, because the latter is the sum of six individual cell voltages, which tends to mask the effect of a single defective cell. Moreover, the total cell voltage depends on factors such as temperatures. In comparing the voltages of the two batteries half´s, such factors are automatically compensated for. A disadvantage of this type of diagnostic system is however; the battery needs a center tap.
First results indicated, under shallow cycling conditions, the life-limiting factor of the conventional automotive (SLI) battery (with pocket-type separators) is not anodic corrosion of the grid, but acid stratification. This battery type achieved only a limited number of cycles. Valve regulated lead acid (VRLA) batteries with absorbent glass mat separators showed a much better result. Furthermore, with this type of battery, also mechanical pressure had a positive effect on battery cycle life.