Dead 12v Battery?

You can buy a trickle charger for the 12v battery, but needing jump started after one week is unusual, I've had my car for 3 years and have let it sit for a week (or slightly more) several times in that span and never had to jump start it. I'd try and get it replaced. (warranty replaced)

 

This too happened to me several months after taking delivery of my 2018 Prime Advanced.
Recharging the battery took care of the problem.
As to why it happened I can not rule out leaving it in Accessory mode with the headlamps on.
Things I have done and do differently now:
As the car is in the attached garage I no longer lock the car so as to not leave the Smart Key
system active. I purchased a battery "maintainer" which I leave running overnight once every
6 months or so. I don't think this is needed because I also picked up a $10 voltage monitor that
plugs into the accessory (cigar lighter) port and the voltage has been in the normal range despite
sometimes leaving the car idle for days.
I also purchased a modestly priced NiCd Jump Starter.
I haven't used that on the Prime but shortly after getting it one of our coffee clutch friends had a dead
battery at Panera and was going to call for a boost. I asked if I could try my new "gizmo" to see if
it could start his car. He agreed and it worked amazingly well. I now keep that in the trunk I fear
what might happen without it if I get a repeat and have no way to start the car while on a trip etc.
I hope I never need it myself.
J

 

Any modern car with all the stuff that's running when it's "off" like a Prius, if only driven once a week, will run down the 12 V battery unless it's driven long enough to completely recharge it, which means you'd need to keep the car in "READY" mode for probably about an hour.

Or do what @Bob Comer and other experienced drivers do and get a trickle charge so you don't have to be concerned about it.

It's theoretically possible that they could have designed the car so that plugging in the EVSE would enable the 12V charging portion of the Prime's charging system to trickle charge the AUX battery. But that's kind of an expensive to design and implement feature compared to a $50 Battery Tender.

 

Ditto to all the above except the friend's car was at their home And I just got my monitor about two months ago.

Edit to add that since the 4th gen charging system is a smart one, with that monitor you can see when the system determines the battery is full because the voltage indicated will drop to it's maintenance level.

 

For my 2017 PRIME, I never had any 12v battery problems. But I have read many threads here with dead 12v battery issues. So, I did buy a Li-ion jump starter just in case, though I never had to use it.

Now, I have a 2020 PRIME with less than year old 12v battery in it. But with the lock-down order, I am not driving my car as much. Although I know there is going to be some parasitic draw on PRIME while parked, I have not had any 12v battery issue with my new 2020 PRIME with once a week ~30miles drive schedule now into 4th week. Still, I am bit more worried now. Especially after reading this thread yesterday, 12v Battery Is Discharging, Re-Charge Now ! | PriusChat. As a result, I decided to install a BlueTooth enabled Battery Monitor.

I am currently checking the battery charge level in my car over the course of week. It will be interesting how low it will get. One good thing about Battery Monitor is that even though it must use a slight amount of battery charge for transmitting the BlueTooth signal, it will notify when the battery hits a certain level which can be set at any level. Mine is currently set at 70% and 20%.

 

Correctamundo.

 

Using my Accessory/Cigar Lighter Port monitor and Digital VOM at the 12 battery posts I regularly see the battery in the upper 11 volt range after putting the car in IG ON or ACCESSORY mode after sitting for a while (hours and days). Sounds low but hasn't caused issues knock on wood. As soon as I go into READY it increases to 14.5 volts and will stay there until it finally drops to about 12.6 volts after an hour+ of driving.

 

That sounds normal behavior. Since you have to enter the car and turn the IG ON to see the meter. I saw a drop in voltage from 12.6v to 12.1v immediately after opening the door. The good thing about BlueTooth enabled battery monitor is that I can check the battery voyage 24/7 without even toughing the car. The hood can remain closed, the door does not need to be opened. In fact, my car is parked close enough to the house, I don't even have to go outside to check the 12v battery SOC. And it keeps a histrical log of the SOC change measured every 2 mini for 30 days.

 

There are other threads that discuss these items. The L2 charger does not keep the 12V battery charged up. You need to get a separate 12V maintainer for that purpose. You don't want to let it go flat and jump start it repeatedly. As for keeping the traction battery fully charged/plugged in all the time, I do not believe that is good practice either. Generally you want to keep battery packs at 80% or below when sitting. Only fully charge when you need it for driving and bring it up to full as close to the trip as possible. That's why you have scheduled charging available.

 

I don't use one, haven't needed it. My commute is 45-50 minutes so it may be getting charged enough that way, even if it sits a week or more.

 

We have experienced that if the rear hatch is not fully latched, the car thinks it is still open and it keeps the interior lights on. If left overnight like this it will run the 12vdc battery dead.

 

I have one, but I think I've only used it once in the last few decades. I am keeping a closer eye on things now with the reduced driving opportunities.

 

Even before COVID our 2010 would sit idle 3, 4, 5 days at a stretch. And then it might just a short run for groceries. with COVID it's often a week or more between start ups.

If that your scenario, hook it up to a smart charger 3~5 amp range, let it run through a charging cycle, and leave it hooked up. You'll have no problems.

If the only way you can start the car is with a jump start, you're kidding yourself that you're battery's ok. Get it tested with an electronic load tester, see what's going on with it.

 

When off, hybrids and plug ins disconnect the traction battery from the rest of the systems. Only a few keep the 12 volt charged. I only know of Tesla doing this, and that is periodic check and charge as needed, as opposed to a constant trickle charge to the 12 volt.

Instead of getting a battery charger, you could turn on the car ocassionally, and it will charge the 12 volt from the traction pack. As mentioned, leave it on for an hour to make sure it gets fully charged, and also turn off all the accessories.

Came across this in the 2018 manual under preventing the 12 volt from discharging, "Turn off the headlights and the audio system while the hybrid system is off. Turn off any unnecessary electrical components when the vehicle is running at a low speed for an extended period, such as in heavy traffic." Regarding the first, just because the the car is off, doesn't mean things like the lights and radio are fully off if left on. Usually not a problem for the auto light sensor and such to draw power, but over extended periods it could be. The second line is telling me that the 12 volt may not be fully charged even after a drive length that should charge it fully.

Toyota has had some 12 volts discharge on past hybrids. I recall a Auris owner posting about dead batteries here. The problem seemed to only be with the high trim model. The batteries may have been undersized for the load, but defective ones can't be ruled out.

 

...................
New 2020 Prius Prime XLE and my battery went low while installing a 3rd brake light fasher. It took all day with the doors open and the brakes being continually tested for the right number of flashes. By the time I got it done and wanted to take in for a spin, I got a low battery and unable to get the READY. As JerryMildred mentioned, fully charged traction battery has bout 600V and I can't start the car because of the 12V battery is low. Yes they should have the traction battery charge the 12v at a set level if the traction battery is above a certain level or maybe a direct bypass just for starting or no 12v battery at all. This seems dumb to me.
So Jerry said, the ready must be on probably about a hour. Are you saying the 12V battery is charged when in ready mode or in the process of running. What charges the 12 V battery as someone said their is no alternator.

 

Since March 10, I've driven about 850 miles. The engine ran for about 40 of those miles. My 12V is just fine, so the engine is obviously not what charges it.

The DC/DC converter which is part of the inverter charges the 12V battery whenever the car is in READY mode. This applies to all hybrids -- Prius Prime, regular Prius, Ford, Chevy, you-name-it. Otherwise we'd be having to charge the 12V almost every day with an external charger.

This video shows the details.

Having the traction battery maintain the 12V when the car is off would be an expensive proposition involving extra wiring, getting power from the traction battery to the inverter and the extreme danger presented by having a parked car with 600V live wires in it if the garage caught fire, etc, etc. And then, if you left it too long, the job of maintaining the 12V would eventually use up the traction battery charge possibly making the car undrivable without some expensive repairs. At $50 battery tender is a fraction of the cost and far safer.

 

I'm with that 100%, ours is pretty much always hooked up to smart charger.

Tough for people with no AC outlet though. Not sure what works there. Is a solar panel charger effective/practical? Definitely not in underground parking, lol.

Or maybe install a disconnect switch on the negative cable? Or just disconnect it, quick solution. It's inconvenient, toasts a lot of memories, but what can you do.

 

What is the side effect of doing this? Things that come to my mind are radio presets and time of day clock get lost. What else?

 

I've done this a few times myself recently, so it's fresh in my mind:

1. Window(s) auto-up might need to be re-taught. There is a procedure. I haven't had to do it though.
2. Trip meters are gone. Maybe other records, though trip meters are all I pay any attention to, not much at that.
3. The next few start ups will be a bit weird; a few seconds in, the engiine revs up dramatically for maybe 5 seconds, like it's recalibrating something.
4. The first start up post disconnection, something weird happens when you hit the start button. I forget what exactly, but shutting down and trying again it's fine.

Yeah disconnection of the 12 volt is a pain. With generations with battery under the hatch floor it's even more complicated, requiring a crawl into the hatch area (to reconnect), or you can avoid that I think if you hook up a jump pack to the jump start point under the hood, that'll give you enough power to pop the hatch.

This'd be good for someone who's really behind the eight-ball: no AC outlet, and the car likely to be out of commission for many weeks. A disconnected battery, especially if it's well charged at the outset, is relatively stable.

You can just pull the battery right out of the car for the downtime. That might be necessary to do the charging as well.