12v battery voltage?

With the COVID-19 pandemic, I am driving *way* less frequently... My 2017 Prius Prime Plugin normally gets driven 4-5 times a week, for 10-30 miles... But now it sits for days on end. I do keep it plugged in, but of course this by itself does not keep the 12v charged, apparently.

I do *not* want to buy a 12v trickle-charger just for what I hope to be no more than another few months of life disruption... But I also don't want to have to take a 10 mile drive each week just to keep the 12v charged.

Any ideas? My current (in progress) test is to use the Key FOB to turn on A/C conditioning once a week (leaving it plugged in, and never bothering to drive anywhere). Does the A/C do anything that will keep the 12v Charged from the Hybrid Array? Or worse, am I just draining the 12v even faster?

--Backstory: I left it undriven for 69 days, and had to jump start it last week (mid April 2020), so I could drive about 30 miles. It was *totally* dead... Had to use the physical key, pop the hood, and use my portable lithium battery jump kit...

 

Welcome, Brian. Every car owner should have a 12V battery charger because you will almost for sure need it someday and it'll probably be really inconvenient to have to buy it in a hurry. In your situation, a battery tender is absolutely the best option.

That said, there's no need to drive the car to charge the 12V. Just put the car in READY mode once a week for maybe an hour. It doesn't need to be moving for the DC/DC converter to do its job. But it's much easier to just have a battery tender connected so you don't have to fuss with it. They are way cheaper than replacing a 12V that died an early death because it went too flat too many times.

Also, it's not a good for the traction battery to be all charged up with no place to go. The owner's manual says to charge it before you leave, preferably with the scheduler. The battery should maintain good capacity much longer that way.

Aside from the battery concerns, I'd still not want my car sitting that long without being driven. That's hard on a car.

 

Spring for the charger.

 

This.
It is useful in various circumstances......including sharing with your neighbors and friends.

 

disconnect the 12v neg

 

To jump in with my own questions about 12v battery drainage due to lack-of-use lately...

I just bought a Noco genius battery charger/maintainer to recharge my 2017 Prius prime's 12v battery, since I also was not thinking and left it too long without driving due to working from home all the time these days. Car doesn't turn on at all right now. Before I go and hook this charger up, I had some questions regarding the instructions in Toyota's manual (sorry I can't link directly to the manual, I don't have privileges to post links yet!) that hopefully you fine folks can help me with:

• The instructions on p. 587 for recharging the battery say that if you're recharging it while still connected to the vehicle to first disconnect the ground cable. ...Which cable is the ground cable? It's not labeled on the diagram and I didn't see something obviously labeled ground when I was looking under the hood. I can't find any videos online that point it out either. I've seen some other posts on this forum mention a quick-disconnect for the positive terminal. Is it that?
  
  
• My car is parked nose-first in my garage, and the manual says not to charge the car in a garage or closed space due to the risk of hydrogen gas being emitted from the battery while recharging. Is it fine if I just leave the garage door open while charging, or do I have to find some way to move the car out into the driveway before I charge it? The doors are unlocked so I can get into the car, but I'm not sure if I can change the transmission to neutral and try to push the car out manually if there's no power?
  
  
• Once the battery is recharged, is there still a risk of releasing hydrogen? i.e., if I'm not out driving frequently and I want to use the Noco maintainer to maintain the charge of the battery, would it be safe to do so in the closed garage?

Thanks for any advice anyone can offer!

 

The negative terminal on the battery is always the smaller, if you follow that cable you will see it always connects to chassis ground. Polaity in DC is very important. Buy a inexpensive multimeter, I’m sure there are numerous on line sites for using a multimeter. As hydrogen, scientist, inventors have been searching for over 150 years for cheap ways to obtain it, nothing yet. The amount of hydrogen obtained from charging is infinitetessiimal and of no danger. I remember my first science class many years ago the teacher collected
Hydrogen and oxygen thru electrolysis and was able to show the characteristic of both. Oxygen burns and
HYDROGEN EXPLODES, however in a small test tube, the lit match just produced a little pop! Anyway alway be aware of which battery lead you are handling. The Neg is safer. Should you inadvertently touch ground
(The chassis) with 12V + say with a wretch on the terminal, the tool will get quite hot and burn your hand possibly, so be careful. Have Fun Drive your Prius at least 1-2 X a week, you have a home charger in the car, it’s flawless.
Or just keep it plugged in to you 120V AC outlet. Always put your State location as it could be
Be relevant to your question. Welcome to Prius Chat.

 

Welcome, @braden. Let me try to shed some light.

@Andyprius1 mostly answered your questions, but a couple things need clarification.

First, lawyers wrote the part about disconnecting the ground (aka) negative battery cable before charging. There is no need for that. However, when removing or replacing a battery, you should remove the negative cable first. That's the one that connects the battery to the car body (or "ground"). It's always black. The positive is always red. The negative battery terminal should have a "-" symbol and the positive has a "+" symbol. This way, if you accidentally touch the car's body with the wrench while it's also touching the positive terminal, you won't burn your hand or poop your pants.

But, when charging, I don't disconnect it and I don't know anyone who does that and there's no valid reason to do so. As Andy said, the amount of hydrogen that might get released, if any, is minute. But here's one clarification. Oxygen does not burn. It's a catalyst that makes burning possible. Hydrogen burns in the presence of oxygen. See the old news reel films of the Hindenburg disaster for a spectacular example. If the hydrogen is in a confined space (or greatly restricted, like a test tube), it will explode. (Gun powder behaves exactly the same way.) For it to burn or explode, the mixture of flammable gas to oxygen has to be correct, just like in the engine of a car. It's also why trucks carrying liquid oxygen have green warning signs without the word "flammable" and trucks carrying gasoline or liquified natural gas have red warning signs with the word "flammable."

You are correct that you won't be able to put the car in neutral to move it when the 12V battery is dead or disconnected. The pawl that engages the teeth that lock the transaxle is powered by a 12V motor.

If the charger is a smart one, you should be able to just leave it connected and plugged in whenever the car is in the garage as Andy said. Also, his advice to give a location more specific than just somewhere within a 3,796,742 square mile area can be very useful in many situations.

And I agree that you should drive and enjoy your amazing car. I've never had a car as fun to drive as my Prime.

 

NEITHER of those things are really important.

Your "smart" charger should be just fine connecting to the battery just like it sits......with everything still connected.....as long as you don't try to start the car.

AND unless you are using an industrial capacity charger, the amount of H released will be insignificant.
If it is an AGM battery in the trunk or under the back seat, the amount release should be ZERO.
But yes, crack the door open if it will make you feel any better. Hope no "critters" get in.

It is not really necessary to leave a tender type charger connected ALL the time that the vehicle is not used.
Overnight once a week or so is plenty......IF your battery is still healthy.
Having run it down once already might mean that it is NOT healthy anymore.
This is more likely the older that it is. More than 5 years, probably should be looking for a new one.

 

Great answers, Jerry and Sam

 

Great, thanks for the advice & input! I figured the instructions were probably erring heavily on the side of avoiding liability, but I wanted to be sure before I just started hooking things up. The car is only ~2.5 years old so I figure the battery drain is just due to not having driven the car for more than a month or two, but once it's charged up I'll get it looked at to be safe.