I just ran a hardware test and got the message: Battery may not be working properly, unplug adapter and plug in again and re-run test (which I did). The results were no problems found.
You might consider the possibility that bits of dust or debris are partially clogging the mating surfaces of the charger plug and MacBook port. I've experienced this with an iPhone, and the symptoms included some wonky behavior, i.e. charging sometimes happened, but sporadically.
