This battery charger circuit use a flyback converter topology, and implements a current-limited power supply to charge lead-acid batteries.  Here is the schematic diagram of the charger circuit:

lead acid battery charger circuit circuit schematic

The flyback transformer provide isolation and voltage input range flexibility, event at supply voltage lower that the battery voltage. Monitoring the charging current is done by sensing the output using MAX471 current sense amplifier. The result of the output current monitoring is then used to give a feedback to a threshold detector, to detect if the value falls below the predetermined threshold. This detection is used to switch the charger into trickle mode, when a lower voltage is applied for lower charging current. [Circuit schematic source: Maxim Integrated Products Application Notes]