Catherino, H. and Feres, F., "Consequences of Deep Cycling 24 Volt Battery Strings," SAE Int. J. Alt. Power. 4(2):378-387, 2015, doi:10.4271/2015-01-9142.
Deep charge and discharge cycling of 24 Volt battery strings composed of two 12 Volt VRLA batteries wired in series affects reliability and life expectancy. This is a matter of interest in vehicle power source applications. These cycles include those specific operational cases requiring the delivery of the full storage capacity during discharge. The concern here is related to applications where batteries serve as a primary power source and the energy content is an issue. It is a common practice for deep cycling a 24 volt battery string to simply add the specified limit voltages during charge and discharge for the individual 12 Volt batteries. In reality, the 12 Volt batteries have an inherent capacity variability and are not identical in their performance characteristics. The actual voltages of the individual 12 Volt batteries are not identical. The experimental measurements reveal that one of the two individual batteries in the series configuration is driven to voltages exceeding the acceptable operational limits for a single 12 Volt battery. This condition results in a loss of battery capacity during cycling and results in early string failure because of cell reversals. This condition appears whenever the battery polarization is determined by a single cell rather than being shared by a number of cells that are presumed to be matched.