How do I determine what is wrong with my battery charging system?

Yes the IMA light has been on for a while, here's what has me puzzled, when I start the car the red square battery light comes on, no matter how much you drive the car it will not assist or charge. According to my gauges. The car gets weak and loses power while driving. Once in a blue moon the red battery light will not come on, when that happens the car runs great, charges and assists. The car works fine and builds up battery until I turn the car off. When I turn the car back on, the red light comes back on and it will not charge or assist again. I do not know what to do?

Same issue here (2002 Honda Insight). Last year my IMA light came on. I assumed the main battery pack had bit the dust. I kept driving, as I have read it's perfectly fine to do. About a month later I got stranded when the car just died. I determined that the 12 V battery was dead (behavior after stall). Put a new one in, and EVERYTHING started working normally. IMA was fine, assist, all of it. Now the car has been sitting a few months and I recently pulled it out of retirement. I'm having the same issue, only, this time the battery is not testing bad... Something tells me it's not the IMA system causing this. I charged the 12 V battery, and again, EVERYTHING works fine until it ran for a few minutes (and I assume the 12 V lost some of its charge), then IMA shut down and red battery light on. Is the DC-DC charging system known to have faults?

Thanks for your response 'aurias' you said enough to save me from typing a lot more, thanks. YES THE IMA system DC-DC converter is responsible for charging the 12 volt battery , that is why it was my first suggestion for 'ballardconcrete1' was to replace it and see what happens.