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Lately I have noticed the Prius doing unexpected things with the battery. Keep in mind I don't think anything is wrong with it, I would just like to understand a little better what it's doing. The main thing I have noticed is that the computers seem to go out of their way to keep the battery at a relatively high state of charge. For example, sometimes I'll be cruising down the road with all but the top (green) bars showing on the pack and enjoying a very high instant fuel economy numbers. (50s, 60s, sometimes even 70s.) Other times I will be driving along under the exact same conditions but can't pull the instant fuel economy out of the 40s no matter how far I back off of the accelerator (until, of course, I go into glide or regen.) This is usually accompanied by charging and sometimes I will watch the SoC climb up to the next bar.

Here's why I ask about it: I know that there isn't much point in using gas to top-up the battery because my driving style is nowhere near perfect enough to avoid regen entirely. I also know that going from the fuel tank to the wheels via the battery is a lossy energy path. Yet my Prius seems to disagree at odd times. Is this just a byproduct of some other Prius drivetrain process? Could there be something funky about my driving style or environment that makes it decide to charge?

Just curious.

Without having a notion of battery *current*, it's harder to
envision what's going on. It's pretty well known that state of
charge always seeks 60% when convenient to do so, such as when
steady-state on the highway. When regenning or trying to stealth
along a little longer, obviously, that goal is set aside until
the demand ceases or a limit is hit. Probability of engine
startup increases as SoC goes down, and vice versa as it goes
above 60%, and more or less engine output is sent to the battery
depending. By being able to watch battery current you get a
good idea of which way you're trending the SoC immediately, and
more of a feel for what you'll have to make up for a few minutes
from now. It's very often hard to tell if you're pulling 5 or
50 amps while in stealth -- half a degree of rise or fall can
totally fool what your butt or eye thinks about the terrain.
.
_H*

Tim, let me attempt to translate what Hobbit just said. :-)

Not really. But here's my experience with the exact same thing.
The other morning, the temps were relatively up and the traffic was light. All the same, I just could not get Priapus to cooperate. Places where I normally stealth and glide, I was pulling a dismal 40s. It all worked out in the end and the commute wasn't a total waste and my tank average went up a few ticks, but it had me wondering what Priapus was thinking.

let me give you what I can give you judging from my prius miniscanner

The main thing I have noticed is that the computers seem to go out of their way to keep the battery at a relatively high state of charge.

When climbing down my first mountain, the first thing I thought is, "how long does it take to reach 80%? SOC?" well not too long....after not even 2 KM down the mountain my SOC was already maxed (started with 50%) and I was riding the friction brakes all the way down. Putting into B mode just wastes energy so I really had no choice when I put it back into D and had to accept 1 amp charge down from the large amounts I was getting before reaching 80%(like 10+20). By the time I reached down the mountain I'm sure my brakes were getting toasty and at the same time I managed to gain an extra %! bumping it up to 81%! Although once on stable ground no matter how hard I tried the battery was extremely assist happy... until I reached towards 60% again... things were back to normal....


For example, sometimes I'll be cruising down the road with all but the top (green) bars showing on the pack and enjoying a very high instant fuel economy numbers. (50s, 60s, sometimes even 70s.) Other times I will be driving along under the exact same conditions but can't pull the instant fuel economy out of the 40s no matter how far I back off of the accelerator (until, of course, I go into glide or regen.)

When SOC gets below 55% that's when the engine starts reving at least 500 extra rpms than usual and then the AMPS generated jumps from 1-5 to 10-20. Big difference... what you really notice is that the engine is screaming while at the same time... speed is barely increasing! Its almost like your throwing away free gas for no acceleration!!! Mr Bob Wilson (a prius classic fanatic) tells me the most efficent band for this state is usually 15-20 amps (2200 rpms?) and this will last until your SOC reaches a happy 58%. Once you do reach that SOC things will go back to normal and your engine won't rev as hard as if your battery was somewhat low.

Here's why I ask about it: I know that there isn't much point in using gas to top-up the battery because my driving style is nowhere near perfect enough to avoid regen entirely. I also know that going from the fuel tank to the wheels via the battery is a lossy energy path. Yet my Prius seems to disagree at odd times. Is this just a byproduct of some other Prius drivetrain process? Could there be something funky about my driving style or environment that makes it decide to charge?

you know what... there are times I try to hypermile this beast... and I get some crappy score... then you just drive like a normal citizen and you get some crazy score! So... now I just completely change my driving style... so what I try to do now is before I know my car is going to stop I decided (although I'm sure Wayne will disagree ;) to milk the best of EV to the best of my ability until I reach my destination. Since:

a) during a cold start, engine stays on no matter what until i hit 70degrees C (you HSD owners are lucky you get shutoff at 50!! grrr...)

b) since your engine is going to run, might as well charge the battery!

c) if your battery is pretty full on a cold start (like 60% above) I notice at a stop when the engine is still going and i'm sitting there.. i'm throwing away energy (-3 amps) rather than generating (+5 amps) if my battery was not as full (50-60%)

d) the worse part about my car (and I"m positive that the HSD fixed this problem) is that when my engine is nice and toasty and I let off the accelerator... my engine is still idling... WHY!?!?! the only time it will die is when i completely stop for 5 seconds or so then it will kill.... but then that means breaking my current momentum.... as opposed to if my battery was somewhat drained (below 55%) the engine reacts to pedal off/pedal on....


e) what a wacky car... sometimes I wish I can just reprogram how the car thinks... but I think its in its best interests if what's it doing is for the best then... I guess I have to work around its kinks...

other than that... its so good to see your score jump from 50 mpg to 60 mpg thanks to EV mode!!!! hahaha I still can't believe how wasteful this piece of crap ICE is compared to a battery and an electric motor... just unbelieveable waste.

Lately I have noticed the Prius doing unexpected things with the battery. Keep in mind I don't think anything is wrong with it, I would just like to understand a little better what it's doing. The main thing I have noticed is that the computers seem to go out of their way to keep the battery at a relatively high state of charge.

All very normal behavior. Also I have noticed that during cooler temps, the charge likes to stay 6-7 bars and in the warmer temps, it settles between 5 and 6 bars. Keeping the charge higher at lower temps seems to be a precautionary measure to ensure battery health.

Thanks for all of the replies! Phil, your experience with the miniscanner is really helpful and confirms Hobbit's assertion that it's tough to know what's going on without knowing what the current draw looks like. What I really ought to do is look for a simple CAN interface that will let me log data with my laptop. There are so many mysteries buried within this technical wonder of a car! I just installed my Coastal EV mode dongle, which adds yet another layer of mystery. (I gave my first impressions in a different thread.)