Hi,
If you're making a battery charger, there are many ICs that include that kind of feature, or you could browse
battery management/monitor ICs made by various manufacturers to see if that's what you're looking for.
Do you want do see the voltage on a display, or have the 13.4V trigger an event? A comparator could be used to sense the 13.4V, but without knowing what for it may be not a solution...you'd probably want to explain the full circuit for other members to offer viable solutions.