HV Mode: Does a Fuller Battery Improve MPG?

On a recent “road trip” (a little less than 200 miles each way), I saw considerably higher MPG numbers on the trip “to,” when I had GoM reading of ~16 miles, whereas it read only 3-4 miles on the way back.

So, I’m debating whether I’m seeing evidence that having a fuller battery might give you better MPG even when you’re in HV (eco) mode?

There certainly were other differences between the two trips, but they would, if anything, seem to favor the return trip, especially it being night, so conceptually less A/C power consumption; also less gasoline weight in the tank.

Although I don’t have any evidence to support this hypothesis, it’s *conceptually* possible that having more battery capacity to work with, even while operating in HV mode, could give it more leeway to use the MGs to supplement the ICE, and thus run the ICE at closer to optimum RPM, especially on hills.

Here are the approximate MPG numbers I saw:

On the way “to,” with fuller battery, in daytime:
77MPH = 54MPG
68MPH = 67MPG
66MPH = 69MPG
61MPH = 73MPG

On the way back, GoM reading 3-4 miles, at night:
62MPH = 65MPG
77MPH = 45MPG

Although driving in opposite directions on the respective roads, 65ish MPH readings were taken on the same, pretty-flat terrain. Similarly, the two 77MPH readings were taken on the same, somewhat-but-not-extremely hilly terrain.

Tire-inflation = ~40PSI all around on Bridgestone ECOPIA Plus EP422 tires.

The daytime (outdoor) temperature averaged somewhere around 8 Fahrenheit degrees higher than the nighttime temperatures. Air-conditioner usage was pretty comparable, and overall conservative in both cases.

Now, granted, my destination was at an altitude a few-hundred feet lower than where I live, so conceptually speaking, the trip to was “downhill,” and the return trip was “uphill,” but a few hundred feet spread over a couple hundred horizontal miles is pretty much flat (perhaps a 2000:1 gradient), so I really doubt that could have made much difference.

 

Perhaps a better way to put it would be, “operating in HV eco mode, if it has more battery capacity with which to assist the ICE, so that it can run the ICE at closer to optimal-efficiency RPMs, will it use that extra capacity — extra leeway — to do so?”

So, for example, upon climbing a hill, will it “dip deeper” into the greater battery capacity available to let the MGs do more work so that it doesn’t have to gun up the ICE as much?

FWIW, all in all, the trip-back numbers seem overall more believable than the trip-to numbers, from what I’ve seen anyway.

 

That reminds me of the Dilbert cartoon where the pointy-haired boss complained about his laptop being too heavy, and Wally suggested he delete some files...

 

I seem to recall that Honda worked with GM on the drive Clarity PHEV train, meaning that it’s somewhat similar with the Volt’s (mostly) series-hybrid architecture?

 

Ah, maybe that was it...

 

Yes that’s a very reasonable point, and it had occurred to me, but I didn’t mention winds because I don’t know. However, I didn’t notice any signs of particularly strong winds (trees bending or swaying, etc.), but I don’t know what the wind speeds or directions were.

However, two things along those lines are worth mentioning:

First, I’ve seen pretty much the same (lower) numbers returning when I’ve taken this trip before. What’s new this time is that I hadn’t really monitored MPG on the way “to” before. So, if wind is a factor, it’s been pretty much the same every time I’ve looked on the return trips, which is certainly not impossible although *maybe* unlikely.

Second, the trip consists of two legs: I-10 which runs pretty much east-west, and US-71 which runs more or less northwest-southeast. The 77MPH numbers I took on the US-71 leg between the towns of La Grange and Columbus, TX, and the 62MPH numbers I took on the I-10 leg, Northwest-Houston-Katy-Sealy.

 

Oh yeah, I just remembered another experiment I did on the trip back, and it, at least up to a point, contradicts the idea that a fuller battery improves HV mileage:

As I was driving back, it became apparent that I’d end up with too much gas in the tank. “Too much gas” in that I pretty much always drive on EV around town other than my comparatively-rare road trips, and I don’t want a large cold of gas in the tank to go stale over months of non-use.

Anyway, I therefore decided to go into Charge Mode for a while, trading off leftover gas for a shorter first charge after the road trip. I charged it up from pretty much zero on the Guesstimatron, to about 9 miles. I then switched back to ordinary HV Eco mode, and took another 77MPH reading. The 77MPH mileage reading was pretty much the same as when the GoM was reading all-but-zero — about 45MPG.

Just today my wife and I took ... not exactly a road trip, but something longer than would fit in he battery, and I was able to take a 76ish MPH reading. It ended up very close to 50MPG: pretty much in the middle between the two readings from my road trip Austin->Houston->Austin earlier. So, maybe it was just wind, or some other factor that evens out between the two trips.

So, what do you folks typically see at 75MPH in HV Eco?