To Drain the Traction Battery or To Not Drain the Traction Battery? That is the question.

I don't believe it will help long term MPG, due to the conversion losses as the car tries to refill up to 6 bars on the next trip. And the extra charge cycling and lower average SOC will likely take a small bite, or nibble, out of battery longevity.

 

Draining traction battery results in more fuel use and more wear and tear on the battery. Quite a no brainer here.

 

my commute allowed me to drive in ev for the last mile or so on the way home when i had my gen II. if i worked it, i could usually get my battery down to two bars pulling into the garage. and i assumed that since the ice had to start on the way out in the morning, it would recharge the battery for free as it warmed up. but i never saw any noticeable change in my mpg's and others warned me that it was not the best thing for the battery, so i stopped doing it.

 

This is a question for gravity. If you live on top of a decent size hill, you get a free battery fill-up on the way to work in the morning. In general it's a bad idea, as jacek says, but this would be a special case.

Richard

 

Generally the HV batteries want to be in the 60-80% charged range. Within that range, I don't think there is much reason to mark the stopped time as particularly important. In other words, drive it as if you weren't going to be stopping, as long as you are in the good range.

I live 1/2 mile down a road which is just off the middle of a steep hill. If I am coming home from the uphill side, braking down the hill mostly fills the batteries so I make sure to EV the rest, to bring the battery back to a more reasonable charge level.

 

Don't drain it and don't shut it off full. ICE warm up may need one or two bars in the battery icon (if idling during the warm-up). If you drive away immediately, you may even use up some battery since more electricity is used to protect interruption of the warm-up cycle.

If you want to prolong the HV battery, use it less. That means, drive it away during the warm up but take it easy. This will prevent unnecessary charging/discharging of the battery.

 

I never drain the battery at the end of a drive but what I have been doing lately is when I have to take the freeway I will use the battery heavily when I'm a mile or so from the on ramp, but I won't let it go below 50% SOC.
I will hold a glide using more battery or I will get up to a certain speed and try to hold it using battery only.
Then when I jump on the freeway the battery will charge back up to around 60%. It takes about 5-6 miles of freeway driving to build SOC from 50-60%.

Also sometimes I will favor the battery on my last trip at the end of the day coming home if it is a weekday and I have work the next morning.
The reason for that is I take the freeway to work and it is cold in the morning so the engine is going to run anyway to warm up on the way to the freeway (I live ~2 miles from the freeway).
So I favor the battery on the way home the night before for the last ~1/2 mile drive, but I only do this when I have work the next morning.
By doing this each day I think this helps at least a little for my overall MPG.
Not sure about how much of an effect on the HV battery long term but I tend to keep cars 3-5 years.

 

You sure to get a lot of opinions about EV driving. Funny thing is that the design has you driving on battery alone sometimes, not your choice. (I wonder if they do that to reduce your mpg As for draining the battery on the way home here is another opinion from How To Get Better Fuel Economy From Your Prius

Tip #1 - DO NOT DRIVE ON BATTERY ALONE!. I realize that this is again counter intuitive, but there is a good reason - 100% of the battery charge you are using by driving on battery alone is achieved by the use of gasoline, and there are always energy losses in every system. Even regeneration by braking requires gas to get to the speed you are slowing down from. I make one exception to this rule; the last 3 or 4 blocks into my home. By using the battery here I leave the battery in a discharged state, but it is a state to be recovered from the next morning. You see, the Prius computer will run the engine on start up to warm the emission system to its operating temperature.

 

I try and use up my battery before I get home. I figure that since it's cold here in the morning that the ICE has to run anyhow. So, when it's warming up the emissions system it's also recharging the battery. If I saved the battery for the morning I wouldn't be able to use it right away because of the warm up sequence anyhow.

 

That seems to make sense. Also I believe there is less self-discharge over night when the battery is at a lower level although that may not be very important.

 

If you drain the HV battery to anything below 47% when you get home, the next morning it will try very hard to charge it back up to normal levels above 50%. That 3% increase uses heaps of gas which would otherwise have been assisted by a normal battery charge level.

Also, during the stage 1 warm-up cycle , the battery only goes up at most 2% if it takes a long time for temps to reach 122F or 50C to end stage 1. You're better off covering some ground already while your foot is off the gas pedal (only if your driveway is long or there are no cars on the road who would get pissed off at a crawling prius ).

 

I don't agree. Your 100% statement is misleading and erroneous. Energy to the battery is derived directly from the engine, but it is also from recovered sources instead of just being lost..regen breaking, coasting etc. The trick is not to drive on battery and use more energy than was recovered but not to make the blanket statement/habit "Do not drive on battery alone."

 

I disagree here. I see little charging when ICE is warming up. Furthermore, if I drive when ICE is in a warm-up mode, the drive is mostly EV (to protect the ICE from cold engine wear I guess) depleting battery even more. Thus, it's good to start with a reasonable charge before driving.

 

That is my experience as well. Bob Wilson has also documented this.

At startup the car will drive mostly on EV power as long as you drive slow to low moderate. RPM stays right around 1,230-1,300 and the faster you go the higher your instant mpg. So instead of crawling around slowly until warmed up, try accelerating such that RPM stays below 1,300 but get up to a good cruising speed (speed limit). Watch your instant mpg at the higher speed vs. going slower during this same stage of warmup.

 

I don't agree with that statement either. Its not my statement, I gave the reference for the statement.

That's interesting, when the ICE is running in warm-up you can't go into EV at least the display won't show EV. The computer won't let you go below a "reasonable" charge in any case.

 

It won't let you go into full EV Mode but the car does run on mostly EV for the first minute or so while driving slow enough that you don't require too much ICE input. If you watch your RPM closely after pulling out of your driveway you will notice it likes to stay under 1,300RPM and MPG is relatively high for a cold start and acceleration. If you accelerate too hard during this tim the RPM will jump up and your iMPG tanks.

 

Yup, if you have SGII or Torque app and monitor HV battery draw, you'll see the amp goes up as you accelerate during the warm up. ICE will be "locked" at around 1,300 rpm with advance timing in negative value. That special timing decreases torque output from the ICE but it speeds up the warm up process hence lowering emission.

 

I don't see that part as misleading or erroneous. For those of us without plugins, all that recovered energy from regen braking or coasting previously came from gasoline.