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Headlight use and SOC

Discussion in 'Gen III 2010+ Prius Technical Discussion' started by snathanb, Apr 8, 2011.

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  1. snathanb

    snathanb New Member

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    Hi, I searched around a bit but didn't quite find an answer to my question.

    I've had my Gen III Prius IV for about 2 months. I've been getting great gas mileage on my 16 mile commute.. averaging 65MPG calculated over the first 2 tanks.

    My scheduled changed this week, so now my morning commute is in the pre-dawn hours. The last 4 miles of my commute, I barely need to use the ICE at all.. downhill.. low speed limits.

    I've noticed that in this stretch, that even though I'm really mostly gliding.. that SOC meter is dropping alarming fast.. like 4 bars in just a short period of time, when the ICE isn't being used at all.

    Since this is a residential neighborhood and it's very early.. I experimented this morning with turning the headlights off on that stretch and just using the running lights.. and sure enough.. the SOC plummet stopped.

    Is this normal? I would have thought the headlights would be run by the normal battery, not the traction battery.
  2. Flaninacupboard

    Flaninacupboard Senior Member

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    the HV battery keeps the 12V battery charged. The HV battery can easily power your headlights for a couple of hours before it is drained. More likely you are providing some power to the motor.
  3. BrettS

    BrettS Active Member

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    When a normal car is running, the headlights and other electronic accessories are really run off of the alternator rather than that 12V battery. The 12V battery is only really used to start the car and power the accessories when the car is off.

    It's similar in a Prius... the 12V battery is only used to start the hybrid system and power the accessories when the hybrid system is off. The HV battery powers everything (through the inverter) when the hybrid system is on and the motor generators provide power for the HV battery.

    However, that said, It surprises me that the headlights drain your HV battery so quickly. The headlights do draw a bit of power and will drain the battery faster than if they were off, but honestly I wouldn't expect it to make a noticable difference... at least not that noticable.

    Just to rule out other factors, I'd suggest driving until the HV battery is mostly charged, then pull over into a parking lot and stop, but leave the hybrid system on. Turn off the radio, the climate control, and other accessories, and leave the headlights on to see if the battery continues to drain quickly. If so, then you may have an issue. If not, then it may be the climate control that's draining the battery or you may be using the electric motor more than you think as that can definitely drain the HV battery pretty quickly.
  4. snathanb

    snathanb New Member

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    Thanks for the reply.. I don't really use the climate control at all.. it's a balmy 72 out here in the mornings. And, my driving behavior hasn't really changed. Same route, same style. All things being equal, the only thing that really changed is that I am now using the headlights, were before I was not.

    I'm gonna try your suggest about charging up the HV battery and turning the headlights on to see if it drops.
  5. mikewithaprius

    mikewithaprius New Member

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    Hey Nathan,

    Don't worry about headlight use. You'll be using about 1.2 amps to power those things. To give you an idea of how little that is, if the car is sitting in neutral you're at 1.0 amps. When you roll down your window it's 0.3 amps. A standard glide around 35 mph is around 5-6. Warp stealth is 11. Accelerating from a stop with electric or just cruising along at 35 mph in electric is probably going to get you over 20-30 amps output easily. When you take your foot off the gas going 35 mph you're probably getting 16 amps IN to the battery. So headlights are not much.

    If there is some problem with the battery, it's not the headlights.

    I have a feeling, though, that the drain over 4 miles, which is a considerable engine off distance, is probably normal. Something to try anyway - with the engine off, are you sure you have the bar completely toward the left side of the HSI for minimal battery use? Your engine could be off but if you're using even a little electric juice to keep going, that could easily drain it that much over 4 miles, which is quite the ways for a Prius. On my Gen II, if I'm gliding/stealthing for 2 miles, it's not rare for my bars to come down by 2 at night (so yes, slightly more draw with headlights, but really negligible), so I would imagine in a similar vein on a Gen III (there are 8 bars in both our displays, by the way), 4 bars down over four miles with some light EV use doesn't actually sound too farfetched. Sure, the car can do that, but it's not at all designed to be an EV.

    Great mileage by the way!
  6. spiderman

    spiderman wretched

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    To get that kind of mileage, you are most likely using the HV battery more than normal. It doesn't surprise me that the SOC shows lower. Remember the SOC meter is not linear at will drop faster at different levels. Plus it doesn't show the full range of the battery either.
  7. rpeek2

    rpeek2 Dry Ice Juggler

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  8. Teakwood

    Teakwood Member

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    Remember that the SOC relates to the HV battery, not the 12V battery. Since you are using the electric motor, the SOC is bound to go down.

    As someone else pointed out, use "B" or use your brake to regen the HV battery.
    Do not sweat the headlights - if you shut them off, you are asking for a ticket or, worse yet, an accident.

    Stop sweating the little things and learn to enjoy you vehicle.
  9. mikewithaprius

    mikewithaprius New Member

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    I forgot to mention something - I am not specifically telling you to do this, since it's probably illegal wherever you are, but for your knowledge, a glide in neutral with headlights on will use about 33% of the already very low amount of energy. The Prius will slow down more quickly in this case, though.
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