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I’ve recently been watching with some interest the Priusplus project, where they are installing additional batteries that are recharged at night to reduce fuel consumption in an otherwise standard ’04 Prius. Despite starting out with a lead-acid powered (ie heavy!) test mule, they’ve already managed to increase mileage from about 41 mpg to 65-74 mpg just from using the extra “assist” obtained from the grid-charged battery (see here for more numbers on how their experiments are going so far).

So it got me to thinking – how much could be achieved from adding an additional plug-in battery to the Insight? Here are the calculations I’ve managed to jot on the back of an envelope….

Assumption 1 – a really heavy battery would slow things down too much and would be detrimental to efficiency, so should be limited to, say, the weight of an extra passenger - maybe 80kg?

Assumption 2 – it ought to be cheap enough for an enthusiast to try, so deep cycle lead acid at around $400/kWh and 35 Wh/kg energy density is the ball park for a real world conversion.

Assumption 3 – no point trying to fiddle with everything to get an “EV only mode”, just set it up so the computer sees it as a fully charged IMA battery all the time, and tries to reduce the charge by keeping the car constantly in “assist mode” until the gridded battery runs out of charge.

Assumption 4 – once the battery runs out of charge, the car would return to normal, 70mpg hybrid mode.

Assumption 5 – install it so it can be quickly unplugged and removed if the owner is considering a longer trip.

OK, now with all those assumptions in place, how much benefit would there be in fuel economy for the Insight?

First, the 80kg lead acid battery will be able to store about 2.25 kWh (80kg x 35Wh/kg x 80%) of electrical energy (16 cents worth) if we only allow 80% depth of discharge for good battery longevity.

Then, taking 70mpg as a starting point, we’re looking at a good 6-7 miles per kilowatt-hour if it were converted to a full EV. However, the way the IMA is set up, the battery can only assist the engine to the tune of its maximum IMA output of about 10kW (~14bhp).

Nevertheless, I reckon that under normal driving, the standard ICE output averages about only 7kW (estimate 6.5 miles per kWhr for average trip speed 55mph), so the additional battery should have no problem providing about half the power under normal (eg highway cruising) driving. Calling on 3.5kW from the battery, and the remainder from the ICE, the plug-in assist would be there for about the first 30 miles of any journey. Thereafter, it would revert to normal hybrid mode, but for the first 30 miles of the trip, the engine would barely have to work at all – if the electric motor is doing half the work, then the fuel consumption should be pretty much doubled – hence a potential 140mpg.

Of course there are loads of other issues to consider (most notably the concept of where and how the electricity is made, and how long the batteries will last), but I think it’s still an interesting way to achieve very high mileages. Use Lithium-ion instead of lead acid and you’d be looking at plug-in assist for the first 150 miles of every journey!

It would be cool if somebody made a prototype and showed Honda what could be done – plug-in assist might even appear as an option on future Insights! :)
 

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I think where they're getting a lot of the gain with adding batteries to the prius is with the electric only mode mod that you can add to the US models aftermarket. For freeway driving it won't do anything if in fact it does not reduce mileage because it would be charging off the engine longer to try and charge the larger pack.

The Insight is different, it uses much less electricity in the scheme of things. I really don't see much gain here unless you figured out a way to say electrify the rear wheels and add an electric only mode, then you'd have something. Plus I don't know if the Prius can tell it has extra batteries on board, but the Insight's SOC reading is based off of how much goes in and how much goes out so you would be confusing the IMA by adding extra batteries.
 

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Now that's just an electric car with gas assist! :lol:
 

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Rick said:
I think where they're getting a lot of the gain with adding batteries to the prius is with the electric only mode mod that you can add to the US models aftermarket. For freeway driving it won't do anything if in fact it does not reduce mileage because it would be charging off the engine longer to try and charge the larger pack.
Hi Rick. Well, this was the thing I was surprised about when reading about their Prius. Like you, I had been expecting that the major gains from such a conversion would be in city driving, when they can spend most of the time trundling around in EV mode without the engine running at all. But actually highway driving improves equally well, because the gas engine is barely working - the batteries are assisting all the time, so it's like driving with a strong wind behind everywhere you go - hence the 70+mpg on the highway.

Rick said:
The Insight is different, it uses much less electricity in the scheme of things. I really don't see much gain here unless you figured out a way to say electrify the rear wheels and add an electric only mode, then you'd have something. Plus I don't know if the Prius can tell it has extra batteries on board, but the Insight's SOC reading is based off of how much goes in and how much goes out so you would be confusing the IMA by adding extra batteries.
You're right, the Insight does use electricity much less in its current configuration, but that's only because the hybrid battery is small and programmed for shallow discharges and (comparatively) low power output. The Priusplus folk have hacked into the software (which wasn't really that easy it has to be said, but they appear to be in control now!) so that they can make the most of the new battery (ie solving the problem of the gas engine trying to recharge it). As you say, the Prius battery also determines its SOC by reading how much current goes in and out, so the Priusplussers have reprogrammed it to think it's always at something like 68% SOC - a sweetspot where they can always get full re-gen and full assist. (The OEM NiMH hybrid battery is completely bypassed when they run it in this mode.)

I think you're right that it wouldn't do much for the Insight without reprogramming, but now that it's been shown this can be done for another car, it may be possible for the Insight too. However, given that the electric-motor in the IMA is good for 10kW (14hp) - I reckon that with a big enough battery, the Insight could conceivably derive at least half its motive force from plug-in electricity.
 

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Interesting, keep us updated if you figure anything out. I will also say that when I attended the 03 NEDRA drag races in Las Vegas Rodrick Wilde of Wilde EVolutions (EV parts) said he had a customer who was looking in to putting an electric motor to the rear wheels of his Insight, but I don't know what ever happened with that.

Of course hacking in to the computer still remains the difficult part.
 
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