12V Memory Saver

Discussion in 'Gen 4 Prius Technical Discussion' started by HighSpeedTofuDelivery, Apr 9, 2021.

  1. HighSpeedTofuDelivery

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    Hello everyone,

    I have a 2017 Prius (gen 4) that is starting to show some signs the 12V battery will need to be replaced soon. I am monitoring the charge via DC voltage with a multimeter in order to help prevent unwanted surprises. I am considering doing the replacement myself but in the interest of not damaging the computers, I am researching OBD2 memory savers that I can plug in to allow me to remove the 12V without risk. Does anyone have direct experience with using these memory savers, which one do you use (that I can still buy), and do you have any tips?

    I am mainly concerned with this tool as other threads cover mostly everything else in great detail but this important tool is hard to find details on. I saw a Scotty Kilmer episode with a simple OBD2 plug and positive and negative jumper leads that is obviously appealing for the price. But again, not going to plug anything in that I am not 100% sure about. Thanks in advance.
     
  2. bisco

    bisco cookie crumbler

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    you can't damage the computers by changing the 12v
     
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  3. HighSpeedTofuDelivery

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    Have you used an OBD2 memory saver/replaced your accessory battery? I need to know more specifics like what actually happens when you disconnect the 12V in a Prius to weigh the risk. I've done this job on older cars but they weren't exactly filled with sensitive electronics.

    Does anyone have access to the Toyota Repair Manual on battery changes/does it say anything specific regarding disconnection of the terminals? Sources would be helpful.
     
  4. fuzzy1

    fuzzy1 Senior Member

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    I'm not aware of any damage from simply disconnecting the 12V battery, only loss of non-essential computer memory values.

    As a "memory saver" procedure, some of us just replaced the battery while the car was running, in Ready mode. Once in Ready, the car (at least numerous Prius models, I don't know about every single one of them) will continue operating without a 12V battery in place. I also turned on the headlights to ensure the HV --> 12V inverter would have a minimum load to keep in regulation, but don't know this to be necessary at all. There are many other loads on the 12V bus that may well cover this.

    Some less adventuresome owners, wanting a more cautious approach, have instead left the car turned off and simply attached a battery jump-start box somewhere to the 12V bus -- at the underhood jump connections of Gen2-3 models, which your Gen4 probably lacks. This will provide enough power to maintain computer memories.

    I would not hook up an old-fashioned battery charger as a memory saver. Without the battery present, some will drive a too-high voltage. Newer better designs might be better behaved, but I don't know which ones they are, only that my old dumb charger contains no protection.
     
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  5. rjparker

    rjparker Senior Member

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    I have paralleled a second 12v battery at the front jump points prior to replacement of a Prius aux battery with no loss of current data. However, I usually just disconnect without concern. I only noticed the radio restarts after a power cycle. All the presets remained as did all of the easily observable settings throughout the vehicle.

    Modern ecus save most data in non-volatile memory. There are many procedures in the repair manual requiring negative cable disconnects prior to removing some piece of electronics without any corresponding reference to using a memory saver. This is to prevent accidental shorting of exposed pins. The most reassuring aspect of the Prius electrical design are the many cases of reversed battery or jumper cable polarities that are resolved by replacement of fuses and fusible links.
     
    #5 rjparker, Apr 9, 2021
    Last edited: Apr 10, 2021
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  6. HighSpeedTofuDelivery

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    I have a Dewalt lead acid booster pack which I could hook up to the jump points under the hood. So long as I can maintain the positive connection while I remove and replace the battery, would you say that this should do the job and prevent any memory loss? I can't see why it wouldn't. That connection just doesn't seem very secure but I could make it work.

    While the idea of powering the car with the traction battery in ready mode while I change the 12V is an attractive option, I prefer not to risk getting shocked to death. Lol
     
  7. fuzzy1

    fuzzy1 Senior Member

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    12V doesn't normally shock people to death, or even give a noticeable sting. Though a long-ago coworker got a very nasty burn when his wedding ring was briefly stuck between terminals of a starter.

    Gen3 and earlier Prii were better setup up for connecting a jump pack at one point while changing the battery at another. But I had to be sure to not allow the positive cable terminal to hit any ground point, so I shoved it inside a work glove.

    Lacking a separate jump terminal, Gen4 will be a bit more tricky to do this without inadvertently losing the connection.
     
  8. HighSpeedTofuDelivery

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    Yes losing that connection is my concern, which I why I wanted to look into the OBD style 12V supply.

    As for shocks, I was referring to the traction battery being active from ready mode and supplying 200V close to where I'd be working. 12V is just enough to tickle. Lol
     
  9. rjparker

    rjparker Senior Member

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    Be aware that a small OBD2 memory saver may not have enough current output capability to operate cabin lights or other accessories that may be on while the battery is being swapped.
     
  10. Elektroingenieur

    Elektroingenieur Senior Member

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    There are only a few items that have to be re-initialized when the auxiliary (12-volt) battery is disconnected and reconnected, and in my opinion, connecting an extraneous device is more likely to cause a problem than prevent one.

    Just do the battery removal and installation procedures as written in the Repair Manual (more info).
     
  11. HighSpeedTofuDelivery

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    Thank you for clarifying.
     
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  12. fuzzy1

    fuzzy1 Senior Member

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    The 200V should be extremely well contained and protected from you, and from ordinary mechanics, and from first responders. And it should have GFCI protection too.

    And as others have pointed out, there really isn't that much memory lost if you forego all this and just skip all this 12V-support effort. Nothing essential.
     
  13. HighSpeedTofuDelivery

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    These are good points. Honestly glad to find out I don't need that stupid device to get an otherwise simple job done!
     
  14. bisco

    bisco cookie crumbler

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    in 17 years on priuschat, i don't recall anyone damaging an ecu by changing the 12v. some have used memory savers, but i would say most have not.
    some lose their window preset, others radio presets.
    i did mine 2 years ago and lost my trip meter
     
  15. Mendel Leisk

    Mendel Leisk Sand Pounder

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    Every time I do brake work I purposefully disconnect the 12 volt. Subsequent start ups have a few minor hick ups. Typically widow auto-up is ok, but radio presets and trip meters are gone. First start up, the start button is a little quirky, and I’ve noticed the engine does some sort of rev up (recalibrating?), for 2~3 start ups.

    If you’re mothballing a car, it’s a common strategy to disconnect the 12 volt, saves it from being run down by the cars constant background demands.

    When I do use a memory saver, it’s a Clore JNC660 jump pack with a Solar OBDII Memory Saver Cable.

    The notion that the car’s electronics will implode with battery disconnect? Reminds me of this movie:

    Happy as Lazzaro - Wikipedia
     
    #15 Mendel Leisk, Apr 10, 2021
    Last edited: Apr 10, 2021
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  16. sam spade 2

    sam spade 2 Senior Member

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    Yes, indeed it should......unless you make a mistake.
     
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  17. Fester

    Fester Active Member

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    You shouldn't have any problem with a simple "disconnect and replace" on the 12v aux. battery. I've had no problems (yet) with my 12v on my late 2017 Prius 2, although I monitor it with a Scangage. My earlier 2008 Gen 2, would on a handful of times exhibit some strange behavior like abnormal running of the brake system boost pump. Disconnecting the 12v, waiting a few minutes then reconnecting fixed it. No such problems with the gen 4.
     
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  18. HighSpeedTofuDelivery

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    Question: does the odometer and overall fuel economy score get wiped? Or just the Trip A/B, etc?
     
  19. rjparker

    rjparker Senior Member

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    Trip A and B only
     
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  20. krmcg

    krmcg Lowered Blizzard Pearl Beauty

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    YMMV. I have never lost anything but the time on my clock on my 2016. Replaced the battery, had multiple upgrades that have had the battery disconnected for hours at a time. My "B" tripmeter is my lifetime indicator and has never been reset.
     
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