As exactly as it says, what if all capitals and cruisers could slowly self repair to a % threshold while in battle? Possibly 30% and 20% respectively, it doesn't have to be these exact numbers.
This would really distinguish capitals and cruisers from destroyers and frigates.
Generally don't like the idea. There's already an elite officer skill that gives self-repair up to 50% hull anyway.
Cruisers and Capitals are already very powerful, and with the way fleet sizes work, they're favoured. A well-equipped one has plenty of means to avoid taking hits in the first place - large weapons with longer range & DTC/ITU on top, plenty of small mounts for point defences, large flux pools and dissipation.
If any class of ship were to get such an ability, it should be Frigates (and
maybe Destroyers), to help offset their comparative lack of power and survivability - but I reckon it should require retreating off the map first (they'd be allowed to re-deploy to the same battle after X time has passed). Remaining CR is preserved - bumped back up to yellow if it had fallen below - and PPT is reset.