ONE GOOD WAY TO KEEP 12V BATTERY CHARGED UP

Discussion in 'Prime Care, Maintenance and Troubleshooting' started by srivenkat, Jan 18, 2021.

  1. srivenkat

    srivenkat Active Member

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    [this method was already presented by others in the other threads, but perhaps placing it in its own thread will bring it better visibility]

    ONE WAY TO KEEP THE 12V BATTERY CHARGED UP (i.e., keep it from getting discharged too low), if the Prime isn't being driven much, is by switching on the Prime and leaving it on (READY mode) with sufficient charge in the Hybrid/EV battery to last the charge session. But the Prime automatically switches off, if the car isn't driven within some time of switching on the car. But as noted by others (augmented with my experience in the below post), the auto-switch-off can be gotten around by locking the doors using the metal key in the keyfob (can't lock the doors with the keyfob buttons while the car is in READY mode, hence the metal key needs to be taken out of the keyfob and inserted into the door handle and turned counter-clockwise to lock the doors) and then the car continues to remain in READY mode without automatically shutting off.

    The traction battery is ideally charged up enough to NOT bring the engine on during the above 12V charge session. Also, a most important commonsense SAFETY PRECAUTION would be to leave the garage doors open in case the hybrid battery charge gets depleted before the car is manually turned off at the end of the above charge session and the engine comes on to charge the HV battery. Also, I would switch off all the accessories, including seat heaters and the display also (on the XLE go to Home->settings->Display off) during the charge session so the EV battery charge lasts long enough to charge the 12V battery.

    So, NO NEED TO UNNECESSARILY DRIVE THE PRIME BURNING GAS IN HV MODE JUST TO KEEP THE 12V BATTERY CHARGED UP!

    EDIT: noted that the above procedure was already presented by others in other threads, i.e., it's not a new way. For example, the below by @Salamander_King :
    #293
    and
    #298
     
    #1 srivenkat, Jan 18, 2021
    Last edited: Jan 18, 2021
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  2. srivenkat

    srivenkat Active Member

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  3. john1701a

    john1701a Prius Guru

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    What supports the assumption that keeping the 12-volt battery "charged up" is necessary?

    Why is not being at 100% capacity considered a requirement? It's not used for starting the gas-engine. It only powers low-draw components. Just driving in HV from time to time.... which is necessary anyway... will ensure a low state-of-charge doesn't happen.
     
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  4. srivenkat

    srivenkat Active Member

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    I have edited the post to clarify that by "charged up" I meant "keep it from getting discharged too low". Thanks.

    I think the consensus is that Prime is placing parasitic loads on the 12V even when off, and repeated short trips wouldn't give the system enough time to replenish the 12V charge, while depleting it in small amounts each time the car is switched on.
     
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  5. Salamander_King

    Salamander_King Senior Member

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    Although this is not specific to Prius, the problem is that you often don't know how deeply the 12v battery is discharged until you find one morning that the 12v battery is dead. In my experience, if the car is being driven at least once a week for ~1 hour, the 12v battery is maintained above 12.0v but not much higher than 12.4v. If the duration of inactivity is longer than a week or you drain the 12v battery by operator error such as leaving a door open, keeping the car in ACC mode too long, and if you don't own a smart charger to maintain the 12v battery charged, this method does keep the 12v battery charged without driving the car. Also, my understanding is that keeping the flooded lead-acid 12v battery in a low state of charge for a prolonged time will shorten the life span, so to prolong the service life of the 12v battery, it is recommended to charge-up the battery when it is drained to the low level of SoC.
     
    #5 Salamander_King, Jan 18, 2021
    Last edited: Jan 18, 2021
  6. Mendel Leisk

    Mendel Leisk Senior Member

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    You have a garage? Get a charger?
     
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  7. john1701a

    john1701a Prius Guru

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    The recommendation to run the engine every few weeks has proven effective to address a variety of exposures. KISS

    Going out of your way to address an issue that impacts so few is often counter-productive. Look at how often "90/10" applies to a variety of topics. In this case, the situation has an even smaller reach.
     
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  8. srivenkat

    srivenkat Active Member

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    In my mind, the charger is the next step up from READY mode charging. I think learning to keep READY mode on for an extended period of time is perhaps also useful, if one wants to keep AC/heater running while parked (haven't tested AC this way but looks like can be kept on this way).
     
  9. Salamander_King

    Salamander_King Senior Member

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    I am not recommending anyone to follow the procedure. It is just an observation I made while experimenting with the 12v battery charging system in PP during COVID-19 inactivity. It is one way to charge the 12v battery without using an external charger or driving the car. I kept my PP's 12v battery charged up with a charger when notified by a Bluetooth battery monitor while it was mostly being parked on my driveway. But in retrospect, I could have gone without charging the 12v battery for a longer period. I no longer own a PP, so no more worry about a dead 12v battery in PP.

    BTW, what KISS means at the end of your comment?
     
  10. ChapmanF

    ChapmanF Senior Member

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    In context, I would guess at "Keep It Simple, Salamander!".
     
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  11. jerrymildred

    jerrymildred Senior Member

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    I don't get the obsession with keeping the 12V at 100% charge. Some people worry way too much. Mine is almost never at 100% when I've checked it. Which I do typically every three months or so. That's about how often I think to look at the voltmeter/USB charger plugged into the 12V outlet hidden under the center armrest overhang. It shows the voltage sag during startup inrush, then a brief look at post start voltage before the DC/DC converter engages. Then later, if I think to look at it, I'll see that the car is satisfied with the charge when the volts drop down from 14.1 or whatever to something closer to a maintenance voltage. Two or three times a year, I'll check it with a multimeter, especially now that it's about four years old.

    But, honestly, I rarely think about it. My driving situation is way different from the people who are having to survive under house arrest since I drive to the office, do shopping, and all the other stuff like we've always done. I average about 500 miles a month these days. It's less than the 800-1,000 I used to do but that's because of a change in our church's location and a reduction of how many meetings I need to go to. If you're only going for a 2-mile drive to the grocery store once a week, then yes, you need a charger.
     
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  12. Salamander_King

    Salamander_King Senior Member

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    I agree with you under my past operational mode pre-COVID. I did not even once check the resting voltage of the 12v battery in my 2017 PP for 2.5 years. Neither did on 2015 Gen3 for 2.5 years. Never had a problem with the 12v battery in those cars. In fact, I never did check the resting voltage or did any maintenance on the 12v battery on my 2008 HCH for ten years, until one cold winter morning the car would not start. That was the OEM battery which came with the car and it probably died of aging but could have lasted a little longer if the car did not have the failing traction battery. On HCH, a hybrid battery cranks the engine normally just like Prius, but when the hybrid battery does not have enough juice to perform this task, the 12v auxiliary battery takes over the task just like a regular ICE car. I had a failing hybrid IMA battery in the car that needed to be replaced.

    If a car is driven daily, I don't think there is much to worry about the 12v battery if it is less than 4 years old. But COVID-19 changed the situation. If the car sits longer than a week and only driven very short time, it is wise to keep eye on the health of the 12v battery more frequently IMHO.
     
    #12 Salamander_King, Jan 18, 2021
    Last edited: Jan 18, 2021
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  13. Mendel Leisk

    Mendel Leisk Senior Member

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    Often driving just once a week, I just keep mine on “life support” (aka smart charge) all the time.
     
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  14. Salamander_King

    Salamander_King Senior Member

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    You also plug-in the engine block heater year around too, don't you?
     
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  15. Mendel Leisk

    Mendel Leisk Senior Member

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    Most every cold-start, preferably for 2 hours min.
     
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  16. jerrymildred

    jerrymildred Senior Member

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    Pretty much what I said at the end. ;)
    My main point is the obsession with 100% charge and that's not just since COVID. So I guess this is a bit of a tangent, but still related. Of course, if you don't drive it, it needs a charger, as I said. But there are countless posts saying 12.4 V (for example) is going to cause trouble. No, it won't. The car will start fine. Now, if you drive it about two miles and park it for another week, the battery won't be at 12.4 volts next time. It'll be lower. If you keep doing that, it'll keep getting lower. That should be a given. But all the blanket statements over the years saying that it's important to have a fully charged 12V are causing a lot of anguish.

    So again, if you're under house arrest, lost your job, or for some other reason don't or can't drive regularly, keep it on a charger. Putting it in READY mode will also charge the 12V but that's kind of like using a window screen to swat a fly. The charger takes way less power and you won't be forced to remember to turn the car off and on again or to figure out when you need to do so. It'll cost less in the long run and just might save your bacon someday.
     
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  17. Heimhenge

    Heimhenge Junior Member

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    Here's what the tech at our local Prius dealer told me regarding keeping the battery charged if you don't drive it enough. This is a continuation from another thread: 12 volt battery charging regimen | PriusChat

    You can follow that link for more details, but bottom line: The car gets driven maybe 1-2 times a week, and for such a short distance that it never gets outa electric mode.

    Tech told me the best way to keep that 12V battery topped of is to "go out every other day and run the gas engine for 5 minutes." It's under a carport, so no fume worries. Sounds like an easy enough solution to ensure my new battery lasts more than 3 years. Should be able to do that via the app, but haven't DLed it yet. Has anyone else done this with the app?
     
  18. jerrymildred

    jerrymildred Senior Member

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    The tech either doesn't know you have a Prius or he doesn't know anything about a Prius. The engine doesn't charge the 12V battery. It charges the traction battery. The traction battery charges the 12V whether the engine runs or not. I drive my Prime almost every day. I only run the engine about once a week. Sometimes not even that much.
     
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  19. Salamander_King

    Salamander_King Senior Member

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    Obviously, the tech has no idea how the 12v battery in Prius is charged. There is no need to have the engine running to charge the 12v battery as pointed out by @jerrymildred.
     
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  20. Mendel Leisk

    Mendel Leisk Senior Member

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    Running the car every other day for 5 minutes? No thanks. I’ll just leave my CTEK 4.3 hooked up, around the clock. It’s safe to do and convenient. It’s an automatic charger, so after the main charging session it falls back to low amperage maintainer.
     
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