AGM battery for Gen 4/Gen 5 Prius/Prius Prime and observations on the 12-V charging system

Discussion in 'Gen 5 Prius Main Forum' started by Gokhan, Sep 20, 2024.

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  1. Gokhan

    Gokhan Senior Member

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    In my Gen 4, turning on and off of the A/C has no effect on the DC–DC-converter voltage.

    I did verify that the cigarette-lighter voltage tracks the battery voltage (but it is somewhat smaller, especially with load on the socket).
     
    #201 Gokhan, Dec 16, 2024
  2. Roy Peterson

    Roy Peterson Junior Member

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    Regarding the aux port voltage, it agrees with the BM voltage and what the scan tool reads. What I was intending to say was that I would put the car in ACC mode by pressing the start button with foot off brake. The voltage I see on the aux port voltmeter is 12.2 to 12.4 volts. This is with battery under a load in ACC mode. Comparing this to the BM voltage gives me a sense of the battery health. Along with actual reading. I monitor the trend. I’m not OCD - I did this type condition monitoring CM in my work career and traing and habits carry into your personal life thinking

    Drove the car yesterday but not today. Currently this evening, the battery voltage is 12.61 volts. Yesterday after driving with charge voltage 14.4 volts continuously, the BM voltage after car asleep for several hours was 12.80 volts. Most likely impacted by surface charge reading since BM voltage was 12.64 volts early this morning.

    So for a recent 26.6 minute trip with charge voltage 14.4 volts continuously, what I recorded was initial aux battery current started at 3.6 amps and over trip time dropped to 2.4 amps. Using 2.8 amps as an estimated average charge current, only @ 1.25 AH is put back into the battery (2.8 amps x .44 hours). This is the longest trip time typical for me. Often trip times less than 15 minutes. Thus even at 14.4 volts with my trip times, I doubt the battery would ever reach a true 100% SOC. Not a concern for me personally with this AGM battery.

    Now with the OEM battery, I would have a concern. Frankly, I don’t understand the Toyota statement of a full charge every 20 driving hours to prevent sulfation. How does that work if I drive a few short trips and not drive for a few days ? Maybe a clue as to why the OEM battery has reports of discharged state…..

    I may decide to do a full charge of the AGM battery once a month to improve battery life. It’s not a big effort plus I do that already with my truck AGM battery since my trips are same pattern as the Prius. 3 years from a battery is OK for me and I understand others feel different. I still have the OEM, it’s tests good and I keep it charged so I do have a backup of sort.

    Cheers
     
    #202 Roy Peterson, Dec 16, 2024
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  3. Gokhan

    Gokhan Senior Member

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    The lowest resting voltage I've seen is 12.61 V, and the aggressive charging kicked in the next day, raising it to 12.78 V. 12.61 V is about 77% SOC; so, the BMS seems to not allow below 75% SOC. Lately, the resting voltage has been around 12.61–12.65 V, which is about 77–80% SOC.

    One hour of continuous charging at 14.3 V should take the AGM battery from 75% to 100% SOC.

    In any case, from what is being reported, the Uplus AGM battery seems to be working much better than the OEM flooded-cell battery. Someone reported 11.9 V on his OEM Gen 5 battery here, which means it has already got damaged by sulfation.
     
    #203 Gokhan, Dec 17, 2024
  4. Roy Peterson

    Roy Peterson Junior Member

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    For the past several weeks, temperatures have been in the 30’s and low 40 range. During this period, the aux battery charging voltage has been a constant 14.4 volts. Morning battery voltage typically 12.6X volts per the BM.

    Chinook winds today with warmer temperatures. On 4th trip this morning, temperature at 58, the charging voltage dropped down to 12.8 volts. Temperature hit 62 this afternoon. All additional trips initial charge 14.4 volts but decreased to 12.8 volts within a few minutes returning to pattern I have observed prior to the recent drop in temperature. So the takeaway would be the outside air temperature has an effect on the charging voltage.

    Cheers
     
    #204 Roy Peterson, Dec 18, 2024 at 7:02 PM
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  5. Gokhan

    Gokhan Senior Member

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    I am not sure if this is because of the cold temperatures or because your voltage is dipping to 12.61 V. From what I see, when the AGM-battery voltage dips to 12.61 V, the BMS charges aggressively for half hour at 14.3 V.

    What I don’t understand is why you are still seeing 12.6 V the day after aggressive charging. In my case, it would be about 12.8 V or higher. Do you have an accessory that causes high parasitic drain?
     
    #205 Gokhan, Dec 19, 2024 at 4:08 AM
  6. Gokhan

    Gokhan Senior Member

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    Observations after weeks of testing:
    1. The BMS does not let the AGM-battery resting voltage to dip below 12.61 V (77% SOC). It does aggressive charging at 14.3 V for half hour when the resting voltage drops to 12.61 V. This typically brings the resting voltage to around 12.8 V.
    2. Unlike someone claimed here, the Prius BMS is not calibrated for a flooded-cell battery. It is clear (from observation 1 above) that it is agnostic to the battery type, as it can calculate the SOC using the history of the measured current and voltage values.
    3. The AGM battery works much better than the OEM flooded-cell battery in Gen 4/Gen 5 Prius. Virtually every owner is reporting quick capacity degradation and sulfation with the OEM flooded-cell battery. This is because of the miserly BMS—not parasitic drain. The AGM battery charges more quickly and efficiently, alleviating the low-SOC and subsequent degradation/sulfation issues.
    4. The only disadvantage of the AGM battery is the 5-lb extra weight. Other than that, the performance is far superior to a flooded-cell battery in the Gen 4/Gen 5 Prius.
     
    #206 Gokhan, Dec 19, 2024 at 4:19 AM
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  7. Roy Peterson

    Roy Peterson Junior Member

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    My car has no added accessories. I charged the traction battery yesterday evening and then unplugged the charge cable. This morning the aux battery voltage was 12.83 volts per the BM before first drive. Outside temperature 44F per car reading. 15 minute trip the charge voltage was 14.4 volts continuous. Return trip 13 minutes charge voltage 14.4 volts continuous.

    Cheers
     
    #207 Roy Peterson, Dec 19, 2024 at 4:00 PM
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  8. Roy Peterson

    Roy Peterson Junior Member

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    Battery voltage was 12.82 volts after drives this morning. Trip 3 this afternoon at 47F showed 14.4 volts for 15 of the 19 minute journey at which point it decreased to 12.8 volts. Return trip voltage went to 12.8 volts within a minute of starting car. Hour after arrival, current battery voltage 12.79 volts.

    Tomorrow morning temperatures forecast in the 30’s. It will be interesting to see if the charging system stays at 14.4 volts as was the case for the 2 week period where temperatures struggled to hit 40 F.

    Cheers
     
    #208 Roy Peterson, Dec 19, 2024 at 7:26 PM
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  9. Roy Peterson

    Roy Peterson Junior Member

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    Made 2 trips Friday about 10 minutes each way. Temperature 44F. Charging voltage constant 14.4 volts both trips.

    This morning voltage was 12.73 per BM prior to drive. Made 2 trips about 18 minutes each. Temperature 39F. Charging voltage 14.4 constant both trips.

    Cheers
     
    #209 Roy Peterson, Dec 21, 2024 at 7:21 PM
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