Please explain 12V system to me--how to keep battery charged while using accessories?

Discussion in 'Gen 4 Prius Technical Discussion' started by dpbsmith4, Sep 5, 2017.

  1. dpbsmith4

    dpbsmith4 Junior Member

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    I go camping with a CPAP machine with a rechargeable travel battery. It has an AC charger which I use at sites with electricity. At sites without electricity I have an adapter that charges the machine from a 12V auto socket. It takes about 3-4 hours. The stated capacity of the battery is 98 watt-hours. For comparison, the Prius 12V battery has a stated capacity of 45 ampere-hours x 12 volts = 540 watt-hours.

    With my older 2005 Prius, the (tiny, overpriced, gel-cell) battery was actually charged from the traction battery. Therefore, as long as the system was turned on, the 12V battery would stay charged. If I turned off air-conditioning, the car would sit there quietly with the traction battery state-of-charge slowly dropping; every twenty minutes or so, the traction battery would reach bottom, the ICE would start and run for a few minutes and charge the traction battery, and then stop. We've camped as long as five days at a non-electric site without driving the car, with no problems.

    This year, we took our 2016 Prius on a similar trip. One morning, when we'd used the Prius to charge the battery several days in a row... as well as lazing in the car with the AC on during hot parts of the day... the car briefly flashed "12V low" and would not start. AAA called and jumped the battery, and it started, and that was several weeks ago and no further problems.

    The 2016 owner's manual warns of the possibility of draining the 12V battery. I am beginning to suspect that in the 2016 Prius the 12V battery only charges when the ICE is actually running. If that's right, then turning off the AC to save fuel might result in the engine not running enough to keep the 12V battery charged when accessories are being used.

    Can someone tell me, on a 2016 Prius, how the 12V battery gets charged? And also, which systems in the car run off the the 12V battery (I assume computer, interior lights... but how about the headlights?) For better or worse, if I feel it is "necessary" to keep using the 12V battery to charge my CPAP battery for 3-4 hours with the car parked, what should I do to keep the 12V battery charged?
     
  2. goldfinger

    goldfinger Active Member

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  3. fotomoto

    fotomoto Senior Member

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    Highly unlikely since the OP stated they were running the a/c which would have been blowing hot air PDQ in accessory mode.
     
  4. Elektroingenieur

    Elektroingenieur Senior Member

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    Aside from an improved auxiliary battery, it’s supposed to be the same on the 2016 Prius.
    As Toyota’s New Car Features book (available by subscription to techinfo.toyota.com) explains, the DC-DC converter, part of the inverter with converter assembly, changes DC at the hybrid battery voltage (nominally, 201.6 V for Ni-MH and 207.2 V for Li-Ion) to DC at the auxiliary battery voltage (nominally, 12 V). The maximum output current of the DC-DC converter is 100 A, and its output voltage is regulated by the hybrid vehicle control ECU “in accordance with the charge state of the auxiliary battery.” The hybrid battery, in turn, is charged by the inverter with converter, which produces DC from the three-phase AC output of the motor-generators in the transaxle, turned by the gasoline engine.
    The 12-volt system supplies all of the electrical loads, except the air conditioning compressor and the two motor-generators in the transaxle.
    Keep the car in READY (not Accessory or Ignition ON), with the transaxle in P (not in N, which inhibits hybrid battery charging), the parking brake set, adequate ventilation for intake air and exhaust, and enough fuel in the tank. Obviously, this will work only if the total 12-volt electrical load, including any accessories, is modest enough that the DC-DC converter can keep the battery charged.
     
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  5. fuzzy1

    fuzzy1 Senior Member

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    His problem happened in the morning, while his writing suggests that the AC was used in the hot parts of previous days. So I'd still explore the likelihood of a user error in getting Accessory vs Ready mode on that particular charging run.

    This is where a voltmeter / ScanGauge-II / Other OBDII engine monitor will be quite useful. Seeing the actual 12V level should quickly provide a definitive answer about what is happening.
     
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  6. alanclarkeau

    alanclarkeau Senior Member

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    I'm not sure how long you'll be camping for, but I believe you can get a battery powered portable CPAP machine, and I think it runs for 3+/- days without needing charging.
     
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  7. fotomoto

    fotomoto Senior Member

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    Yeah, not exactly clear but the OP has been using this method for years and not a newb. If it is operator error, a cheap LiOn jump pack might be preferable to AAA.

    I also keep my SC II set to 12v readings along with traction battery SOC and temp. I've been using my car's 110v outlet to run cookware while helping with the Hurricane Harvey cleanup since there's still no working infrastructure. I use hybrid mode to get there and switch over to EV while on site.
     
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  8. Will N

    Will N Junior Member

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    I'd be willing to bet that within limits a Prius on Ready would be more efficient, and less annoying, than just about any generator. With Hurricane Harvey and Irma on the way, Priuses and other hybrids could be incredible resources for people who are displaced. I'll bet that sitting in a Prius is a more efficient option for four people than any building, trailer, generator options.
    My basis for this is experience car camping, several hours on Ready seems insignificant to gas mileage, and the software/sensors in the car. Caveat. I know electricity, but I'm no electrical engineer.
    The limitations of course would be how much power is needed and for how long. (A Honda generator just grinds away, a Honda hybrid is turning on and off as it needs with the battery as a buffer. End of a week, the hybrid hasn't powered any arc welders, but it's also consumed very little gas.)
    When someone builds a decent hybrid van, college dorms are going to be empty.
    Noisy roommate? Park somewhere else.
    I wish there was an electrical engineer who would just sort all this out. Toyota should do this as I think it would sell cars, be helpful in emergencies and of course car camping. I can see a cabin far off the grid. Pull up, park the Prius, plug in the cabin. All systems go. An App would let you know if you're running low on gas....
     
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  9. Mendel Leisk

    Mendel Leisk Resonant Resident

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    I've heard a general automotive statement, not Prius specific, that idling is not enough to maintain a 12 volt battery. This may go out the window with the Prius, no alternator and so on, not sure. But still I'd suspect incessant idling, coupled with accessory charging AND AC use, is pushing design limits.

    Personally the idea of using the most fuel efficient vehicle on the road as a personal environment bubble and power hub, during a "camping" trip: bugs me, lol.
     
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  10. ChapmanF

    ChapmanF Senior Member

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    It does go out the window with the Prius: the 12 volt battery is getting the same steady 13.8 to 14ish volts any time the car is READY, whether the engine is running or not, idling, revving, whatever. For that matter, when the engine does start up because the HV battery needs topping, it doesn't really idle. It runs at about idle RPM, but with the throttle farther open and delivering serious power into MG1 for charging up the battery. While it's not a high-RPM sound, you can hear the deeper note in the exhaust compared to actual idling. The best time to notice is when it finishes topping off the battery; the moment before the engine stops, you can hear it relax back to an actual idle as the system stops harvesting power from it, and then it spins down.

    I say, if you have something capable of being the most fuel efficient vehicle on the road and an environment bubble and a power hub, enjoy it while you can! :)

    With regard to the original post, if the 12 volt battery is not staying charged and the car is known to be in READY the whole time, it sounds to me as if something isn't working. I would maybe monitor the voltage, which should be a steady 13.8 to 14ish at all times in READY. If it ever drops below 13, that would indicate some issue, like the DC/DC converter shutting down for some reason.

    -Chap
     
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  11. goldfinger

    goldfinger Active Member

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    I don't want to highjack this into a generator thread, but the Honda inverter based generators are very efficient and quiet. They achieve efficiency by throttling up and down to match load. The inverter decouples engine RPM from output voltage and frequency. The EU1000i operates at 53 to 59 dBA, weighs 29 lbs, runs 7 hours on 0.6 gallons. It outputs a pure sine wave and won't fry sensitive electronics. It has a cap vent shut-off so it won't stink up the inside of your car with gasoline smell. Just saying.
     
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  12. fuzzy1

    fuzzy1 Senior Member

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    Somewhere back in the PriusChat archives should be a more detailed comparison. My memory of it is that the Prius did not shine all that well compared to a well chosen efficient generator, and was at a disadvantage outside a certain load range. The 400-ish watts of internal overhead is a handicap on the low load side.

    But the Prius isn't bad on this, it is fairly reasonable. Plus, it really shines on the noise and emission toxicity side. Few (if any) generators have automotive level emission controls, which is a reason why so many people get hurt by generator CO emissions during power outages. Modern passenger cars are far cleaner, to the point that they are no longer a dependable means of suicide.
     
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  13. ChapmanF

    ChapmanF Senior Member

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    For anybody who wants to pick up the math where this leaves off ... that Honda model is a one kilowatt output with a 0.6 gallon tank, with an advertised running time of 3.2 to 7.1 hours, depending on load. Taking the 3.2 hour figure to apply at the rated load of 1kW, that's an easy derivation of 3.2 kWh per 0.6 gallon, or, 5.3 kWh per gallon. I assume the 7.1 hour figure applies when you are drawing virtually nothing from it, and it runs itself dry in 7.1 hours just basically powering itself. That comes out sort of equivalent to a 450 watt base load, if I did that right. (Actually, I don't think I quite did, but you get the idea.)

    Googling for the noise of a Prius, I'm finding it much easier to get numbers for interior sound level. I assume the Honda generator's 59dBA is from a few paces away; so far I'm striking out on any comparable figure for a few paces away from a stationary Prius with the engine running to top off the battery.

    -Chap
     
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  14. FuelMiser

    FuelMiser Senior Member

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    I expected the same, so I took my 12V voltmeter (plugs into the 12V accessory outlet in the front console) in the car with me today and observed the following: first press of the power button without brake showed 12.2 volts (basic accessory mode). Second press without brake all electronics came on showed 11.7 volts. With brake pressed, goes into Ready mode showed 14.2 volts. So far, all this is expected. Now for the strange part. After about 10-15 minutes steady driving, it showed 12.5 volts and stayed at this level until I made my first stop (power off). I assume this drop occurs when the 12V achieves a full charge state. This behavior continued through the day of several stops, initial 14.2 volts after going into Ready, but then dropping to 12.5 volts after a few minutes of driving. I will attempt to try this in my 2014 Gen 3 and would appreciate anyone else trying this to confirm.
     
  15. ChapmanF

    ChapmanF Senior Member

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    I don't believe so. Sounds to me like the DC/DC converter going offline for some (not normal) reason.

    -Chap
     
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  16. FuelMiser

    FuelMiser Senior Member

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    So, today I tried the same experiment in my 2014 Gen 3. Same procedure, same route and the voltage at the 12V accessory outlet stayed 14.5 volts consistently in Ready mode and while driving. I will repeat for the 2016 Gen 4 in the next day or so.
     
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  17. alanclarkeau

    alanclarkeau Senior Member

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    Not sure. The Gen 3 and Gen 4 have different 12V batteries - maybe the "voltage regulator" is limiting the voltage once the Gen 4 12V is at a good charge level. It would be interesting to see a graph over time.

    There have been several iterations of them in cars over the years - generator with mechanical regulator ->alternator regulated by the car's computer - and each is different.

    +++++++++++++++++++++++++++++++

    Regarding camping - I'd be surprised if TOYOTA sees a CamperPrius conversion on a standard PRIUS as a priority - it would essentially be a very small market, probably for people camping solo or maybe 2-up. They would need a more appropriate vehicle.

    VW in the '50s with the Type 2 conversion, as far as I can recall, never made them themselves, they subcontracted the conversion (Westfalia?), and then many other conversion companies took it and ran with various conversions around the world (very popular here in Australia).
     
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  18. Elektroingenieur

    Elektroingenieur Senior Member

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    That’s possible, but it could just be the charging control system at work. In the New Car Features book, a diagram shows how the hybrid vehicle control ECU calculates the charging voltage to request from the DC-DC converter based on the electrical load and the state of the auxiliary battery. Toyota writes:
    • This system controls the voltage in order to maintain the capacity of the auxiliary battery optimal condition when auxiliary battery capacity is high, then reduces the power consumption of the vehicle by controlling generated voltage of DC-DC converter.
    • This control consists of battery sensor (battery state sensor assembly), DC-DC converter, switches and hybrid vehicle control ECU.
    • The hybrid vehicle control ECU stops charging control and the DC-DC converter switches to the constant voltage control mode under the following conditions:
    1. Low auxiliary battery capacity
    2. Low or high auxiliary battery temperature
    3. A cooling system for the inverter with converter assembly, the hybrid transaxle and the HV battery is in high duty, or high engine load
    4. Battery sensor (battery state sensor assembly) malfunction
    5. Communication failure (LIN communication between the hybrid vehicle control ECU and the battery sensor (battery state sensor assembly))
    6. Accumulated driving time becomes approximately about 20 hours.
    7. Voltage sensing failure
     
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  19. ChapmanF

    ChapmanF Senior Member

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    I am sure all of that is said in the New Car Features manual, but what it specifically means may need to be worked out by measuring somebody else's Gen 4 that is known to work. I don't believe it is normal for any Prius to, while READY, allow the 12 volt battery to discharge so deeply it won't start the car, and that's what the OP reported in post #1. Rather, the objective is to monitor the battery state and maintain it close to optimally, and that doesn't seem to fit this picture.

    -Chap
     
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  20. ChapmanF

    ChapmanF Senior Member

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    A question occurs to me: does the Gen 4 literature go into any detail on this "battery state sensor assembly", which sounds a bit more involved than the single temperature sensor in Gen 3, or the single remote voltage sensing of Gen 1 and Gen 2?

    In particular, does it say this sensor assembly includes a current sensor, or does coulomb counting?

    That would be a new technology since Gen 3 (for the aux battery anyway, though they have always used it on the traction batteries). And would be possible to confuse by adding an additional current draw, if it were added right at the battery posts, on the wrong side of the sensor.

    Again, just speculation, unless someone can confirm it in the manual.

    -Chap
     
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