Hybrid 12v (auxiliary) battery charge control

Discussion in 'Gen 4 Prius Main Forum' started by RGeB, May 2, 2021.

  1. RGeB

    RGeB New Member

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    Nobody at the rav4 forum seems to know, so I am hoping that longer experience with the Prius has led someone to wonder, perhaps even to answer:

    We know that the Toyota hybrid 12v (auxiliary) battery is charged from the DC-DC converter, and that charging is modulated (somehow) by the current sensed at the auxiliary battery negative terminal, maybe integrated with messages from other sensors.

    We can see that the voltage delivered from the DC-DC converter drops at times from 14.2v (bulk charging mode) to 13.6v (absorption / float mode).

    Can anyone provide detail on how this change is triggered, eg:
    Charging current (A)?
    Coulomb counting (Ah)?
    Does it count when the car is off?
    Does it correct for charging efficiency?
    Is it adaptive?
    Can it be programmed?
    If so, which parameters are programmable?

    It matters because the optimal charging profile or algorithm would vary with different battery chemistries, or potentially even with different battery sizes. I don't mind paying for TIS if the answer is there, but the material I have seen quoted from there is very general, which does not help to answer my specific questions.
     
  2. Elektroingenieur

    Elektroingenieur Senior Member

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    As far as I know, Toyota hasn’t published the details you’re seeking, beyond the general description I quoted in the earlier thread. Keep in mind that New Car Features and the other service publications are written for technicians, not as engineering design documents.

    You might try searching in the patent literature, but keep in mind that those disclosures may or may not resemble what’s actually implemented in vehicles. If you need the information commercially, I’d suggest checking with LTEC, which offers reports for sale on other aspects of the fourth-generation Prius design.
    I don’t recall having seen any field-programmable settings (“customize parameters,” in Toyota’s jargon) for the 12-volt charging system on any Toyota hybrid vehicle.
     
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  3. meeder

    meeder Active Member

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    My 2 cents is that it uses a standard charging cycle for lead acid batteries, so a CC-CV cycle. Constant current to 14,4V and then a constant voltage until the charging current is beneath a threshold which would turn off the charge cycle.
     
  4. FuelMiser

    FuelMiser Senior Member

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    Good reason to replace with the same spec battery.
     
  5. bisco

    bisco cookie crumbler

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    some are replacing with lithium, with good success. and at least one has its own bms
     
  6. kevinwhite

    kevinwhite Active Member

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    I don't know about the Prius Prime but attached is an example of how Chevy manages the 12v battery in the Spark EV. The Prius Prime is probably somewhat similar.

    They don't do Coulomb counting but the system voltage does depend upon many variables such as headlamp or windshield wiper status as well as temperature and even the odometer reading.

    Toyota does reveal a lot of technical details in their SAE submissions although I haven't seen any about 12v battery.

    kevin
     

    Attached Files:

  7. RGeB

    RGeB New Member

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    Thanks all. What a shame Toyota tells interested owners so much less than GM.
    Still, I guess that creates a market niche for players like LTEC. Thanks Elektroingenieur, I did not know anything about them.
    Meeder, I don't think it is that simple; see measurements below.
    FuelMiser, true, but no help. My rant is that an owner can set all the relevant parameters is a ‘smart’ battery charger costing $150. Yet the same owner is not permitted to know (let alone adjust) the same parameters in a hybrid car costing $40,000.
    Bisco, the LFP replacements are apparently being done 'blind'. Some appear to measure nothing, and do not even seem to think about charging current at low SOC. An LFP battery bms should be a defence of last resort if the protections in the charging system fail. It should never be the first line of defence. A high-voltage "sulfation" treatment (as shown by kevinwhite in the ChevySpark document) would be very bad news for an LFP battery.

    Here are a couple of my measurements (sorry not Prius but Toyota hybrid, so they are probably the same):

    Graphs.jpg

    I do not understand why the drop from 14.2v to 13.6v occurred at a charging current >2.6A with LA, but ~1A with LFP. Could it be Coulomb counting with an algorithm tailored to the inefficient charging of LA? Or some other complexity as mentioned in the ChevySpark document from kevinwhite?

    I have repeated the LFP measurements several times. As an aside: the charging current was dangerously high (>1C) at lower LFP battery SOC (I stopped charging from the $40,000 car and used the $150 wall charger that time)! My advice is never to let an LFP 'auxiliary battery' (placed against advice into a Toyota hybrid car) fall below 30% SOC. At least that remains so while we can not customize charging parameters in such cars.

    With a fully-charged battery charging from the car dropped from 14.3v @ 0.7A to 13.6v @ 0.3A within 3min after starting.

    But to understand if it might vary in a way harmful to an LFP battery, we need to know how the drop is triggered. Hence my original questions.
     
  8. alanclarkeau

    alanclarkeau Senior Member

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    Maybe give Professor John a query - in the comments section below this video - he's answered questions in the past ...

    (51) How to check a Hybrid/EV 12V Charging System - YouTube
     
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  9. RGeB

    RGeB New Member

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    Thanks alanclarkeau. I did find Prof Kelly's email at Weber uni, and I asked him before I started whether he had any knowledge or experience relevant to use of a LFP auxilliary battery in a Toyota hybrid car. I also asked Renogy and Power-Sonic and Toyota (Japan). They all gave me the same answer (no response).

    I understand of course that they are all busy, and for liability reasons they are unlikely to disagree with FuelMiser above, or the response received from Toyota USA: “Toyota does not recommend or assist with modifying our vehicles from the original factory specifications.” Fair enough, though I asked about my vehicle not theirs; and such advice ignores the millions of Toyotas out there with non-OEM modifications such as dual-battery systems, or even dual dashcams or battery chargers.

    Perhaps compatibility with LFP auxiliary batteries will feature in original factory specifications of Toyota hybrids in the future. In the meantime' any insight into how the charging voltage drop is triggered (or more broadly, detail of how the 12v charging circuit is controlled) will be greatly appreciated.
     
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