Attention Prius Owners, I believe I have discovered a previously unknown Mechanism of Low MPG.

Any updates on this issue? After taking a closer look it appears I too am stuck in the WEAK EV state. I also wonder if it has anything to do with 12V battery levels before / after a reset?

I would be curious if there is a trend where Prius has WEAK EV state and SKS functionality. Do you have the SKS key?

 

Hmmm. I have no data, but I suspect my Prius is in a High state, as it's getting 17Km/l (Worked out by distance travelled/liters refilled) which I believe is just under 40MPG (I do have the fan overridden to be on at all times via the Dr Prius app, as I'm in the tropics I generally don't allow the battery to go above 40C or 104F as the Prius throttles battery usage after that due to temp).

One thing I did do to this battery was manually cycle it in the car by doing the full charge trick of being in drive and holding the brake and accelerator. I used the Dr Prius app however to try and keep the charge rate at approx 20 amps by NOT putting the accelerator to the floor. Once charged, I switch to neutral and watch the app, ignoring any messages/warnings until the blade voltage drops to 7.00v (Need some voltage to restart the engine after all). I'd cycle like this 3 times, and then let it charge itself normally afterwards. When the engine finally stops, is when I would turn the Prius off. 3 cycles/day is to be kind to the battery as it does warm up (Don't want to cook it per se). Also did this in the morning and don't drive it afterwards to allow the battery to cooldown. Did this a few days to cycle the battery and apparently reclaim some capacity, but have noticed now that when we take off in the morning the Prius will run on electric for a short distance, and even drop the voltage as low as 6.3v (under 60a load or so) before deciding the battery was low and started the engine. I have seen it draw up to 80A in this state from the battery bank while taking off, almost like the EV button was being used.

I also have to do this when the 12V has been disconnected too, so maybe it's something with the battery ECU having to re-learn the battery's capacity.

I think I also may have found a way to get the Prius to "top level" charge it's battery pack, but still watching to see if this works. Seems to balance the cells a bit which is handy as there is no balancer in the ECU. It does seem to do it at 3-6 amps at high voltage. Will post more if I can get it doing this reliably without toasting my Prius.

 

anything new on this?

 

What kind of modifications have you done to this poor vehicle?

 

Some of those mods are pretty neat, I have been thinking of making an Arduino based blinker relay box, with two MOSFETs to PWM the LED units I got for fancy fade in out etc... Now, I'm thinking about blinking modes... >_>

I thought initially that a right hand drive prius like from here in AU might be easier for that job, then saw you use the glove boxes... maybe not.

 

From your OP I had the impression there were mods on the electrical side of the car and was thinking maybe they would have impacted the performance. Guess not. Thanks

 

Interesting theory. We just need two more things to support your hypothesis and that is what @Mendel Leisk provided in post #6: Number of miles driven and gallons of fuel at fill up.

Here are a few comments:

1. Are you feathering the accelerator to stay in EV mode? Because the hybrid battery is charged in a lossy process through gasoline, the most efficient driving is usually to speed up quickly and then coast. This would only affect city mpg and not hwy mpg. Forcing EV by speeding up slowly would decrease fuel economy.

2. 205 width tires do have an impact. Not 20% but some. My gen 2 is 185 OEM. Even if Toyota approves of the wider tires, do expect some change. If your stock size is 195, you're 2 sizes wider than the vast majority of gen 2's that get ~45 mpg regularly. That's a large difference between most Gen 2 Prius and yours.

3. The bars of SOC remaining is a guess as far as I'm concerned. Sometimes full bars gives me Max EV. Sometimes it gives me Weak EV. This is on both my Gen 2 and Gen 3, although Weak EV happens more often in the Gen 3. Anecdotally, Weak EV tends to happen if I charge up the battery through regenerative braking, i.e. battery bars fill up fast. If through normal engine recharge, then I get Max EV, i.e. battery bars fill up slow. My guess is the BMS reads SOC too high when charged quickly or the battery is too hot to give Max EV. Again, this would only affect city and not highway miles.

The Gen 3 is less likely to remain in EV mode down to 2 bars I think I read this was done on purpose to preserve the hybrid battery life in the Gen 3.

4. DW is driving in permanent Weak EV state. I got closer to 50 mpg and she got closer to 40. Different commute but straight to the hwy and back home. Driving style can pretty much account for the entire 10 mpg delta between some owners and others.

Please don't see this as disagreement. I'm just offering some healthy skepticism. Wish I could provide some data for your research. Good luck, OP.

 

The Gen 2 definitely maintains EV mode more readily. I can speed up more and stay in EV longer. It's like Max EV as you describe while the Gen 3 is a lot more sensitive and more often than not is in Weak EV. Hence, Gen 2 stays in EV mode. However, I can average 50 mpg in the Gen 3 but only 45 in the Gen 2. DW doesn't drive the Gen 2, only the Gen 3. I drive both. And between the two of us on the Gen 3, we differ by about 10 mpg. I don't know if she's going gangbusters 90 mph. She claims she drives at the speed of traffic.

I'm not skepticle of the two modes you describe. I don't have an opinion either way. They seem plausible and I've experienced both conditions. I'm skepticle on Weak EV mode as the source of low fuel economy.

 

Still, a few easy changes can make a big difference.

 

My 3rd generation is always in "weak" mode.

 

What I say three times is true.

 

Holy Shoot. My friend pointed me to this page after I complained about my acceleration never being the same after 2020. I hardly drive at all that year, and when I went back to driving at least once a week in 2021, the car just didn't feel right. It didn't seem to have the 'jump' or 'pop' that it did before. And I think this is it.

The most notable symptom, at least to me is that the care takes more petal to do the same, and it makes more ICE noise doing it.

My dad described it as seeming like the car was "missing a gear". Like it was stuck in 2nd when it should have been in 3rd, until it suddenly found 4th.

Unfortunately, I probably won't have an opportunity to highway commute it. But I did go one one cross-county highway drive from Chicago to North Dakota. Would that not have fixed it?

 

The only 2 things I noticed that directly improved mileage

1 Recalibration of anti-lock brakes. Details for this are in the forum to jumper OBD port. Also one other member accomplished improvement of the brake drag by hammering the brake and throttle at low speed. Both of these seem to work but the result does fall off after several weeks.
2 Replacement of a weak 12 volt battery. Results of this have held up for - 4 months so far….
Gen 2 / 220 K mi gets 45 mpg bought with 190Kmi
Gen 3 / 100 K mi gets 51 mpg
When I first got Gen 3 at 60Kmi it averaged 54mpg