In December the IMA light on my 2000 Insight came on along with the check engine light. Prior to that I had been experiencing what appeared to be a weaker IMA response. The dealer confirmed that I needed a new NiMH battery; the cost: $3000 plus tax and labor, $3600. I put it off. In January one Monday morning I got IMA assist ascending a hill and then nothing again after that: no assist, no charge, no autostop. The only difference I noticed was a 5 mpg drop in fuel economy and the need to downshift sooner ascending hills. I calculated that at $3.00/gal I would have to drive 600,000 miles to make the new battery pay for itself (1,000,000 miles in Oklahoma where gas prices are less.) I have 148,000 miles on it now. It didn't make economic sense, but the question was could I get along on only the 12v battery. The dealer found out from the IMA technician in CA that >3000 rpm would charge the battery. So for over a month I have been getting by primarily by plugging in the 12v battery to a charger a couple times a week. However recently this has not been effective and when I thought the battery was charged my battery warning light came on as I was on a trip. The >3000 rpm technique didn't work either except to add three bars to the IMA charge indicator. I arrive at the next town and the instrument panel went blank. I could only go about 2 miles per hour and the lights dimmed. Fortunately a friendly townfolk allowed me to charge the battery on his housecurrent for 30 minutes. I made it the rest of the way but had a similar problem on the return trip. My conclusion is, yes, the Insight will function without the IMA battery with only a minor loss in performance (5 mpg less and poorer acceleration compensated for by using lower gears), but the lack of an alternator is a serious drawback to trying to depend on the 12v battery. I am getting a new IMA battery installed whenever the dealer can get one.