Just Installed BM-2 Battery Monitor and NOCO On Board Charger

Discussion in 'Gen 5 Prius Main Forum' started by VelvetFoot, Dec 9, 2025.

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  1. Gokhan

    Gokhan Senior Member

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    The BM2 battery monitor does not have a good way to estimate SOC, as it can’t measure and integrate the current, which gives the charge. The sensor in the car does that.

    Chances are that, since the car is always charging the battery when it’s cold, the BM2 battery monitor thinks the full charge is around 13.0 V, since the battery tends to be slightly overcharged because of constant charging.
     
    #41 Gokhan, Feb 2, 2026
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  2. VelvetFoot

    VelvetFoot Active Member

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    I think it was an anomaly of some sort.
    I was in and out of the car figuring out where to put the tolls transponder. :)
     
    #42 VelvetFoot, Feb 2, 2026
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  3. LRO

    LRO Junior Member

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    5th Gen EVAP Leak Check Conditions ... EVAP.jpeg
     
    #43 LRO, Feb 3, 2026
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  4. VelvetFoot

    VelvetFoot Active Member

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    Good info.
    I always seem to get a followup dip two hours after the first five hour interval.
    It is cold in the garage though, below 40°F, but they do say 'typical', so there's that. :)
    Could it be something else? Sure. :)
     
    #44 VelvetFoot, Feb 3, 2026
  5. Gokhan

    Gokhan Senior Member

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    I just verified that this is exactly what happens in my Gen 4 Prius Prime PHEV: 5 hours if the engine is cold and 7 hours if it is still warm.

    In addition, there is always a small dip at 50 minutes, which I don't know what it is.

    23:41 shutoff, 00:31 small dip (unknown), 16:41 big dip (EVAP). Note that this is an AGM battery, so the voltages when the car is off are about 0.25 V higher than with a flooded cell.

    You can also see the sub DC–DC converter in the AC charger in action in the first screenshot, between 18:00 and 20:00, keeping the battery at a float voltage of 12.92 V while the AC charger is actively charging the traction battery. The sub DC–DC converter turns off when the traction-battery charging ends. This is the only difference between Gen 4 and Gen 5—the latter does not have a sub DC–DC converter in the AC charger and instead activates the main DC–DC converter during active traction-battery charging.

    [​IMG]
    [​IMG]
    [​IMG]
     
    #45 Gokhan, Feb 3, 2026
    Last edited: Feb 3, 2026
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  6. VelvetFoot

    VelvetFoot Active Member

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    Cool
     
    #46 VelvetFoot, Feb 3, 2026
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  7. VelvetFoot

    VelvetFoot Active Member

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    Today I fooled around with the dashcams in the garage.
    Didn't drive anywhere.
    The engine started initially, ran for a while, and stopped.'
    I didn't have heat on.
    I had the car in Ready each time I worked on it.
    I think that was the only time it ran.
    I shut it down several times later, and the engine never came on.
    In the graph below, after the engine starts, not much charging going on.
    In the later times I was in the car, big charging, high voltage.
    It seems the 12v system was getting charged directly from the traction battery.
    I find this interesting.
    I wonder why not initially, after engine start, only later.
    I was under the impression only the PHEVs did this kind of thing.
    Pretty cool if predictable and repeatable.
    I'll have to try again, hopefully on another damn project, lol.

    upload_2026-2-21_19-49-34.png
     
    #47 VelvetFoot, Feb 21, 2026 at 7:50 PM
  8. Gokhan

    Gokhan Senior Member

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    Prius HEV/PHEV always charges the 12 V battery from the traction battery through the DC–DC converter, regardless of whether the engine is running or not, as there is no alternator. Running or not of the engine has no effect on the 12 V system whatsoever. There is also no difference between the HEV and PHEV 12 V systems other than what happens when the PHEV is plugged in and the traction battery is charging.

    The 12 V battery management system (BMS) started the charging (by canceling the voltage control on the DC–DC converter) later because the battery sensor detected that 12 V battery SOC had dropped too much.
     
    #48 Gokhan, Feb 21, 2026 at 11:32 PM
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