Just another note about efficiency:
When I lived in a solar electric house, the electrical equivalent of a car's gas guage had to be manually configured for battery efficiency. Apparently, it is impossible to actually measure amps directly. Instead, you put a low-resistance resistor in the line and measure the very slight voltage drop across that resistor and calculate what the amperage must be to cause that voltage drop. Measuring amp-hours is another layer of abstraction added to that.
The amp-hour guage that was supposed to predict when my solar electric system was fully charged or about to black out had to measure amp hours charging into the batteries or discharging from them, and when I told it my interpretation of what the battery efficiency was, it could then figure out its guess as to the battery's State Of Charge. This is basically a very scientifically derived wild guess.
I had to experiment to get this number right, noting when the batteries were bubbling (fully charged) whether the meter said they were fully charged or not and resetting the top-of-charge point on the meter and adjusting the efficiency setting to make the guess work better next time. My lead-acid batteries were about 90% efficient. In other words, if I put 10 amp hours of electricity into the batteries, I could get 9 amp hours back. The other amp hour was consumed by resistance in the wiring, heat conditions affecting battery chemistry, the inherant inefficiency of electric/chemical conversion, and elves, dragons and other mysteries.
Lead-Acid batteries are the most efficient batteries man can make. Cars, laptops and hand-held vaccuum cleaners would all use lead-acid batteries, except for one thing: They are heavy, so they are not all that good for devices you wish to carry or otherwise transport.
Ni-cads, Nickel-Metal Hydride and Lithium batteries are much lighter than lead-acid for the charge they can take, so they are preferred for portable settings. Ni-cads have really bad "chemical memories" meaning that unless you very loyally manage their state of charge, discharging them all the way to zero and then charging them all the way to the top, and don't let them sit at one charge level for too many days, the battery chemistry changes and the batteries stop working. The top and bottom of the charge get closer and closer to each other until there is no longer any way to store electricity in these batteries or get electricity out of them.
Lead-acid batteries can get a form of chemical memory at the bottom of their charge, but not the top. Sulfur from the sulfuric acid deposits itself on the lead plates as the battery is dischargeed, leaving water where there was sulfuric acid and sulfur deposits where there were clean lead plates. When you recharge a lead-acid battery, the sulfur dissolves back into the water, turning it back into sulfuric acid.
If you let a lead-acid battery sit too long at the bottom of its charge, the sulfur deposit hardens and will be quite resistant to dissolving back into the water. Overcharging lead-acid batteries can help because it causes the plates to heat up enough to boil water. The boiling action physically breaks the sulfur deposit and pushes it off the plates. Some of this sulfur will now dissolve into the water, while the rest drops into the dregs at the bottom of the battery.
Lead-acid batteries generally die because of one of three causes:
1. The plates heat up so much that they warp far enough that the positive and negative plates touch, shorting out the battery, killing all electricity in the cell. This is a classic "dead cell" in a car battery, for example. Car batteries are vulnerable to this because the lead plates are very thin, giving you maximum surface area in order to pull a lot of amps quickly to the task of starting an engine in cold weather.
Deep-cycle batteries used in solar homes or fork lifts or floor scrubbers use much thicker plates that are resistant to warping. They are often intentionally curved plates, specifically shaped to resist warping.
2. Some of the sulfur pieces bubbled off the plates fall and do not dissolve, settling into the bottom of the battery. They combine with other conductive contaminants in the dregs at the bottom of the battery until the deposit becomes deep enough to touch the bottoms of the plates and short them out. This is the OTHER way to get a dead cell.
3. Batteries can also die because they run dry. If you let them boil off their water until the lead plates are exposed, the deposits on the plates harden so much as to ruin the battery.
The chemistry is different for other batteries, but all of them have some form of chemical memory. The nickel-metal-hydride batteries chosen for the Insight are very light for the charge that they hold and very resistant to forming chemical memory. This is enhanced by the electronics in the car which will periodically discharge and recharge the batteries for the specific purpose of defeating any chemical memory that would otherwise form. This makes the batteries last a long time. That's why they offer a warranty for 8 years or 80,000 miles.
Still, I strongly suspect that these batteries offer less than 90% efficiency, even in good weather. If you really want to get the absolute best gas mileage you can, then try to not drive the Insight so that the boost light comes on.
Meanwhile, remember that the electric side of the Insight is not really there in order to give you the absolute best gas mileage possible. If all you care about is gas mileage, remove the battery and drive the gas engine only.
The electrical side is there because that 1-liter, 3 cylinder engine that gets GREAT gas mileage is extremely unfun to drive. Low end torque is disgraceful. Accelleration is laughable. Every time you pull out into an entrance ramp, you'd be chanting, "Come on, little Insight. You can do it. Come on...." and you'd have a lot of time to chant, since the 0 to 60mph time would be measured in minutes instead of seconds. You'd have to take every hill in 3rd gear. Forget passing anyone under any conditions.
The electric side gives you all the surge that the gas engine lacks. Every time you use the electric side, you lose gas mileage. Meanwhile, you don't lose MUCH gas mileage, and you gain a very fun ride. You get the accelleration that would be impossible for a gas engine that can yield anywhere near this gas mileage.
That's why it is a hybrid. Life is a balance. Balance your driving by enjoying the spunkiness of the electric boost every now and then, and the rest of the time, you sip very little gas cruising at extremely high efficiency.
Sorry for babbling. I like this stuff. It's fun to think about.