I have noticed that SB start losing HP after they have been around for awhile, (not at war with anyone). Is this due to lack of repair research or lack of defense modules or both? Also, do culture modules have any value if they are not near rival systems? I know they will effect nearby sectors but I don't know what the range is.
Are you sure they're loosing HP, or are they just showing the damaged effect? If the latter, then it's probably that you've developed a tech/trade good that increased HP, and they're all "repairing" themselves to the new, increased level of HP.
You will see this on ships as well. Once you research a tech that adds bonus HP's, all your ships will get it, and they will start "building up" to their new maximum.
If the diffence between current and full HP's is great enough (20% methinks), they will sport the new, classy "Lightning Look" (tm).