The reduction of the battery voltage will be make voltage at pin 2 of IC1 is lower than at pin 3, This makes style voltage at pin 6 of IC1 to positive and if it is higher than 3.3V, it will result in the base current (Ib of Q1) enter biased to Q1 doing and the relay also work, to contact of relay will connected the ACV to the transformer T1 so has the secondary coil ACV to a bridge circuit BD1 to DCV (15Vdc) and across drop at C1-100uF for filter signal full wave rectifier form, to the charging battery system again.
Until the voltage battery full will makes pin 2 of IC1 is positive when compare with voltage at pin 3, This circuit will cut off the current that charge battery at once, and begin renew charge again when voltage lower than you setup.