Your problem is more than likely "driver error". The majority of Insight drivers rely too much on IMA assist. Especially if there are no hills to coast down to charge the battery. My rule of thumb is that my tachometer never goes below 2,000 rpm. When my battery level drops down below 1/2, especially on the long uphills around Southern Calif. I downshift to keep assist at a minimum and let the engine get into its power band. You can also recharge your battery if you get above 3,200 rpm and stay with traffic. I drive on the power band a lot, and my overall mileage for 67,000 miles is still at 59.9 mpg.
The battery level depends very much on the conditions. During my usual, 90% highway commute, my level stays near full all the time. But if I happen to venture into the city of Boston or Cambridge, the level quickly goes down to about half and stays there until I'm back on the open road. Lots of stop-and-go driving, frequent idle-stops and, of course, hills tend to drag the battery down.
It's funny, because I try to keep the revs as low as possible and rely much more on the IMA. I find that this has consistantly given me the best mileage and performance. However, I am always quite light on the pedal, which is not a large draw on the batteries. Yesterday, battling traffic resulted in a 3/4 discharge, which is the most I have ever drained (except when I did the top-speed run).
I have been successful in my 1 week ownership of the Insight to have between 70-100% SOC battery level, with mixed city and highway driving. I find that if I go easy on starts and let the "power band" do the work and have high rpms, its minimal assist and the mileage stays. Lifetime for the car is like 56Mpg, and Im averaging about the same. If I feather it and "baby" the throttle and upshift when it wants me to, Im finding that my SOC is 99-100% so that no charging occurs at that level!
I'm usually one light below Full, why is that, I almost never get a full 100% unless i'm goin down a really long strech o hill. usually i range around 70 to 95% but I'm a fuel mizer, I rarely drive over 65mph on the 57, 210 and 10 fwy in California. I usually tuck myself safely behind a Semi in one of the slow lanes if he's going over 55mph. I don't have to worrry about being cut off that way.