I think you misunderstood what Sin suggested. Let me try to paraphrase:
(1) Start the car and let it warm up for a few minutes (to recharge the battery).
(2) Idle, in Neutral or Park; lights ON, all other accessories (like radio, A/C, rear defrosters, seat heaters) OFF.
(3) Now, rev the engine to 1500 RPM (approximately).
(4) Do the healights become noticeably brighter (compared to the brightness at idle)?

If yes, I agree with Sin's assessment that either the battery is failing, or, one or both of the battery cables are making poor connection. I would list the likely causes in the following order (of likelihood):
. Poor cable connections; check these first. If you can move the battery post clamp by without any tools, it is clearly not tight enough. They often loosen over time.
. Dry battery (is this the original factory battery? Has anyone ever checked or added water to the cells?) Even the "maintenance free" batteries sometimes need water added.
. High battery internal resistance at one or more cells (the beginning of the end of life; battery shops can measure this).
. Loose alternator belt (slips under eletrical load, resulting in battery undercharge)
. Failed rectifier diodes in the alternator (also causes battery undercharge)