I had to jump my 12v battery today - How do I maintain a healthy battery?

Jonathan NCSP · Jul 2, 2024 at 5:54 PM

Fenichel said: ↑

For what it's worth, here is my summary of the 12V-battery situation, based on my experience and reading multiple threads here.

The 12V battery is charged from the traction battery. That happens when you have turned the car on (READY mode), or (less commonly) when, during charging of the traction battery, the battery heater needs to be turned on. You might think that during charging of the traction battery, the 12V battery would always be topped up, but it isn't.

A large number of 5G Prius Primes were delivered with defective 12V batteries, replaceable under warranty. I had one of these.

Accessory Mode is available in ICE cars so the user can run the lights & audio without burning gas. There is no reason ever to use accessory mode in a Prius Prime. Its presence is a design error.

Just sitting in your driveway with everything nominally off, the 12V battery is slowly draining, with a half life of ~2000 hours. That will get it from a full charge down to ~12V in about a week. The drain is faster (but still pretty slow) during charging of the traction battery (except, as noted above, when the car decides to run the battery heater).

For a few dollars, you can get a little voltmeter that, wired across the battery, can be arranged so that it is readable without opening the doors or otherwise waking the car. The drain from the voltmeter itself is negligible.

If the car is on (READY) and you open the door to leave, it will ask you to approve its (a) staying on for another hour, and then (b) turning itself off. This obscure feature turns out to be useful.

When my Prius has been idle for a few days and the VM shows less than 12V, I get in, turn the car on, approve the automatic shutoff, and then get out and lock up. When I next come by, the 12V battery is putting out 12.4V or so.

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I found this post and other related posts to be extremely helpful. Thanks, all. I just got back from my dealer for 12 Mo. service and had them check the electrical system because my 2023 Prius Prime shut itself off after listening to the radio for 5 minutes while waiting for my wife's train. I was told to go to the dealer for this check when I called Toyota Customer Care. Based on the thread here, I plan to tell TCC to change the name of this mode when they call me tomorrow to follow-up on my concern. The dealership says ACC mode is only there for their mechanics to run tests under that setting. So why does Toyota use a term that most (older?) owners would associate with using power without the engine on? And I could find nothing in the Owners Manual warning about draining the 12v. The only thing I did find said, "Do not use the power outlet longer than necessary when the hybrid system is off." which conflicts with the advice I got from the dealer NEVER to use ACC. The previously posted advice to use READY mode instead seems to be spot-on.

Fenichel · Jul 3, 2024 at 1:41 PM

Jonathan NCSP said: ↑

I found this post and other related posts to be extremely helpful. Thanks, all. I just got back from my dealer for 12 Mo. service and had them check the electrical system because my 2023 Prius Prime shut itself off after listening to the radio for 5 minutes while waiting for my wife's train. I was told to go to the dealer for this check when I called Toyota Customer Care. Based on the thread here, I plan to tell TCC to change the name of this mode when they call me tomorrow to follow-up on my concern. The dealership says ACC mode is only there for their mechanics to run tests under that setting. So why does Toyota use a term that most (older?) owners would associate with using power without the engine on? And I could find nothing in the Owners Manual warning about draining the 12v. The only thing I did find said, "Do not use the power outlet longer than necessary when the hybrid system is off." which conflicts with the advice I got from the dealer NEVER to use ACC. The previously posted advice to use READY mode instead seems to be spot-on.
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I'm glad you found my post helpful.

I should add to what I said then to say that I'm no longer using the parked-but-READY trick to top up the 12V battery. Instead, I bought a Noco Genius 2D battery charger, permanently mounted it in the trunk, and permanently wired it (just 2 M6 nuts) to the 12V battery. When I see that that battery is low, I open the trunk, run an extension cord into it, and plug in the charger. I can even close the trunk over the extension cord without apparent injury to the car or cord. I can leave the charger plugged in indefinitely, and not have to think about it.

Xterra72 · Jul 3, 2024 at 3:02 PM

Mendel Leisk said: ↑

Are you garage parked, secure, with 120 volt AC outlets? If so: get a decent smart charger, maybe wire a quick-connect harness, connect and leave connected, any time the car’s going to be idle more than a day.

2023 Owner's Manual doesn't seem to have a guidance for max amperage charger spec, though I might have missed it. Gen 4 says 5 amp max IIRC, FWIW. And you definitely want a "smart" charger, one that runs through a charging regiment, and can be left on indefinitely, won't overcharge.
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On my Toyota Prius the oem 12 volt battery has the sticker warning ⚠️ stating that the battery can be on a battery charger at no more than 4.2 amps and fully charged in 10 hours.

So my question is, if I have a smart charger, is it ok to keep the 4 amp battery charger plugged in and connected to the 12 volt battery for more than the 10 hours?

I had to jump my 12v battery today - How do I maintain a healthy battery?

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I had to jump my 12v battery today - How do I maintain a healthy battery?

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