When using a modern battery charger to recharge a basic car battery that is in good condition, but is run down to the level that it is unable to turn an engine over, under what circumstances should you use the trickle charge setting as opposed to the fast charge setting, and vice-versa.
A true trickle charger can be left connected to the battery indefinitely and should not cause any damage to the battery from over charging. It will take longer for the battery to charge.
A fast charger will have a higher output voltage which will therefore pass a higher current through the battery. Care must be taken not to leave the charger connected for too long as this can cause damage to the battery. The battery can overheat and the electrolyte can boil away.
A modern charger may have a built in timer or sensing circuitry that will switch from fast charging to trickle charging when the battery is full.