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

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.

 

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.

 

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?

 

• "Dashcam plugged into the cigarette lighter is draining the 12v battery (shouldn't be right? Accessories are turned off when vehicle is off)"

Yeah that outlet gets NO power once the car is off. It can't drain. The only thing that MAY cause a drain is if there is some power source in the dashcam feeding back into the 12v and keeping the car awake.

• Second smart key might have been pinging the car, I keep it on the second floor with the car parked directly below

Get a small faraday box to store your FOBs in. This prevents pinging AND as a bonus no one can remotely jack your car using a large repeater antenna. I have one and it's just nice knowing it's that much harder to steal the car.

Another suggestion, get a cig lighter USB thing that has a voltage readout. I use B01M9IKYVH (amazon) and have had it in this car and my last car. This tells me what the 12v system is running at. When the HV is charging the 12v I get between 13-14v. I assume 13v is when it's tapering off. When it's not charging but the car is running it's around 12.7v. I assume it does this occasionally to load test the 12v battery and see if there's significant voltage drop. In my old car, this adapter specifically was about 0.2v off from a multimeter at the battery terminals. So the read out was good. Haven't checked it vs terminals in the Prius though.

 

A smart charger should drop to maintenance charge once the battery is full.