Battery powered emergency lights (and/or exit signs) are very simple machines.
When AC is available, the battery is on a charger.
When AC disappears, a contact changes and the lights come on.
The question that I can't answer is- What manufacturer has the best battery charger with the best battery for longest battery life?
Not relevant to the OP's needs, but, emergency lights only need to last for 90 minutes per NFPA. They're not designed for "long" use. Just to provide enough time for anyone in the space to find their way out of the building.