2024 Prius Prime XSE 12v battery monitoring and charging

  1. I suspected that my 2024 PP XSE 12v battery was not getting charged to 100% State of Charge (SOC) and was remaining at a low voltage most of the time it was parked (not in READY mode).
    I found the previously purchased and uses Battery Monitor BM1 in the garage. It has been used on a Jeep battery that spends some time parked in the driveway waiting for the next adventure.
    So I checked the Prius Battery voltage with a mlti meter after sitting for one night in the drive way. It was close to 12 v. Not good. I installed the Battery Monitor BM-1 direcly on the battery posts.
    I re-researched the CTEK Multi-US-7002 charger that I use on my Ford F250 5.0 L diesel pickup to keep the batteries in it ahead of the parasitice discharge found in that vehicle that sits for weeks without being started. It is a 120v ac input 12v battery charger that can be set to STANDBY, NORMAL, SNOWFLAKE, SUPPLY, RECOND. NORMAL allows 14.4 v Bulk and Absorb voltage. These are the setting for Flood Lead Acid battery charging. The charging current is rated at 7A.
    The Amprobe I used to monitor the charging current was a cheap one, might not be the most accurate.
    The BM-1 seems to indicate about 0.1 volt low, compared to the multimeter digital meter I used in the 14+ v range. Good eough for me to see if the charger was charging the battery.
    I chose to charge the ‘low charge state prius battery’ when it was at rest (vehicle turned off, not in the ready mode).
    It accepted about 8A continuous charge current from its low charge state in Bulk charging untill the battery reached the Absorb mode (about 14.4 v). Then as time passed, the absorb current reduced to nearly 0 amps at 14.4 v.
    The Float charge voltage took over at 13.3 v.
    The charger is a 8-stage charger, desulfation, soft start, bulk, absorb, analysis, float, pulse. By selecting mode I eliminated a few fo the stages.
    It took hours to charge the 12v battery.
    Next morning the battery was at about 12.8 v.
    I set the vehicle to the READY mode and the battery instantly went to 14.4 v. Stayed there for several minutes, charging from the traction battery. The ICE started and ran for a minute or two during the charge stage. Then it stopped running. The voltage at the battery went to about 13.6v and stayed at that level as i drove to the restaurant for breakfast. Upon getting back in the vehicle and putting it READY mode, the battery again went to 14.4v charge voltage, no ICE start, then in a short time went back to float, 13.6v.
    Got home, parked in the driveway, let it sit an hour or two to cool down. Applied the 120v ac charging cable and started a home recharge of the traction battery. The 12v battery went immediatly to 14.4v, indicating that the 12v battery was being charged up.
    Much of what I observed above is different than reported or speculated about 12v battery chaging by others in this forum.
    Any comments ?


  2. RandyPete

    Note the clear battery state of charge eye at the end of the blue arrow. It’s one of the battery fill point hole threaded plugs.
    I remember the old AC/DELCO 12 volt lead acid flood batteries with the delco state of charge eye, if it turns yellow/red the battery needs charging.


  3. RandyPete

    I do now see that there have been many detailed excellent posts about the BM-1, Toyota 12v batteries, battery charging in the past. Unfortunately I was only searching the Gen5 PP forum. This is an old problem/subject that has been fully vetted in the past. Only difference might be the software that controls battery charging in the READY mode or during 120v AC home charging of the traction battery for a GEN5 prius prime as compared to a pre-GEN5 Prius prime.


  4. vvillovv

    Mar 19, 2013
    2017 Prius Prime
    Prime Plus

    I noticed you didn’t provide a floating voltage of the battery before you hooked it up to the charger, after letting the battery cool for an hour or two.

    One thing you’ll probably get used to after enough reading here at priuschat, is that agreement on many causes of issues, instrument / sensor readings and behaviors exhibited are rare.


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