An interesting description of this hybrid Lithium Ion-Lead Acid Battery System is provided by BOS AG:
"Longer Battery Lifetime. At Low Costs
Like in all BOS AG Hybrid battery systems, the lithium battery takes most of the charging cycles while the lead acid battery provides inexpensive backup capacity. The lead acid battery is charged with higher priority, while the lithium battery takes all surplus energy. This process helps to increase the lead acid battery lifetime"
