VGate Battery Assistant battery monitor

Discussion in 'Gen 5 Prius Main Forum' started by Paul Gregory, Jun 24, 2025 at 8:09 PM.

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  1. Paul Gregory

    Paul Gregory Senior Member

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    I wanted to keep an eye on my auxiliary 12V battery, so I installed this device:
    Vgate Battery Assistant Tester BLE4.0

    Not all that useful, but it does the job of showing the charge level. My only peeve is that some of the screen messages are in Chinese.
     
    #1 Paul Gregory, Jun 24, 2025 at 8:09 PM
  2. bisco

    bisco cookie crumbler

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    i've been thinking about getting one of these. nice to monitor the voltage without opening the hood.
     
    #2 bisco, Jun 24, 2025 at 8:37 PM
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  3. Paul Gregory

    Paul Gregory Senior Member

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    I can monitor my main 12V battery through a device plugged into the lighter socket. I needed a way to monitor the aux battery as well.
     
    #3 Paul Gregory, Jun 24, 2025 at 8:45 PM
  4. bisco

    bisco cookie crumbler

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    what is the difference between the main 12v battery and the aux battery? my prius only had one.
     
    #4 bisco, Jun 24, 2025 at 8:47 PM
  5. Paul Gregory

    Paul Gregory Senior Member

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    I added an auxiliary battery to my 12V system.
     
    #5 Paul Gregory, Jun 24, 2025 at 8:49 PM
  6. bisco

    bisco cookie crumbler

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    ah. i couldn't monitor the main 12 volt because the lighter socket was only live with the car on
     
    #6 bisco, Jun 24, 2025 at 8:53 PM
  7. Paul Gregory

    Paul Gregory Senior Member

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    I had one wired to the battery as an extra, fused and switched.
     
    #7 Paul Gregory, Jun 24, 2025 at 9:26 PM
  8. Paul Gregory

    Paul Gregory Senior Member

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    It's a bright LED readout that shows voltage and temperature, cost me about $10 on Aliexpress.
     
    #8 Paul Gregory, Jun 24, 2025 at 9:27 PM
  9. ChapmanF

    ChapmanF Senior Member

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    Just to keep things interesting, Toyota themselves use the term "auxiliary battery", by which they mean the 12-volt battery that comes in the car.
     
    #9 ChapmanF, Jun 24, 2025 at 10:23 PM
  10. bisco

    bisco cookie crumbler

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    so it would be an auxiliary auxiliary
     
    #10 bisco, Jun 25, 2025 at 1:29 PM
  11. Gokhan

    Gokhan Senior Member

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    This is what I have. It works well:

    BM2 battery monitor on Amazon
     
    #11 Gokhan, Jun 25, 2025 at 4:58 PM
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  12. Gokhan

    Gokhan Senior Member

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    I also noticed that the current drain by the device in the original post is 15 mA. That is unacceptably high. The BM2 battery monitor has only a 1-mA current drain, which is very good, and the BM2 app is written in English.

    BM2 battery monitor on Amazon
     
    #12 Gokhan, Jun 28, 2025 at 6:17 PM
  13. Paul Gregory

    Paul Gregory Senior Member

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    It's coming in a little handy so far. It shows changes in the voltage level to two decimal places.
    Today I took a road trip of a few hours, and I saw the voltage creeping down slowly, after an initial slow climb. I still don't know why it went dead the other day, but based on the slow decline in voltage today, it's a step in the right direction for understanding what happened. I am running a 12V cooler, rated at 5 amps, but the car's 12V system should be able to handle that.

    The main purpose for the auxiliary battery is to keep the drinks cooler going while I'm parked.

    I checked the connections and they seem OK. But for some reason, the voltage will creep up slowly for about an hour, and then slowly decrease until the battery is dead. I suspected the isolator may be bad, but it does allow the battery to charge for the first hour or so.

    I do have a workaround; I can plug my 5 amp battery charger into the inverter socket if I have to, to charge it as I drive, but I'm still unraveling this mystery.
     
    #13 Paul Gregory, Jun 28, 2025 at 8:07 PM
  14. Paul Gregory

    Paul Gregory Senior Member

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    I may have something...
    I was looking at isolators on Amazon, and some of them say, "not suitable for lithiun batteries, which is my auxiliary battery. The isolator I currently have makes no mention of suitability for lithium so I ordered a different one that does say it's compatible. It may have something to do with incorrect triggering voltages.

    This one is adjustable, so that may be the answer.
     
    #14 Paul Gregory, Jun 28, 2025 at 8:15 PM
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