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

it's probably the 10-20 km/week. keep it on a smart charger, about 30 bucks like this:

ref=asc_df_B07W46BX31

that way, it's always topped up and you don't have to worry

 

You can get radio shielded bags or boxes to cut off communication between the car and the key fob, that might help.

The thing that would help the most is more utilization, imo. These cars are built to be driven.

 

There are about 3-5 threads about similar issues that are currently active on this forum. I'd suggest reading them to see what's going on and get ideas. This is happening much more frequently to Prime owners(you may be the first HEV owner I've seen on the forum having this problem).

Current theories are a batch of bad 12V batteries, a software/firmware bug that sometimes keeps the car from going into deep sleep, user error in regards to PHEV charging(doesn't apply to you), user error of some other type, swamp gas from a weather balloon trapped in a thermal pocket and reflecting the light from Venus. Honestly, your guess is as good as mine at this point.

If it happens a couple more times, you need to go in and force your dealership to do a full test on the battery as it will likely be damaged at that point. If they are hesitant, get the Toyota Brand Engagement Center involved and get a case started. It will likely force your dealer to replace the battery, but more importantly, it will be another data point for Toyota global in regards to troubleshooting the problem. This will never get fixed if owners don't force dealers to contact Toyota and let them know what's going on.



FYI, 90% of my trips are 2km or less and I've never had a problem. (10 months of ownership)

 

I’ve actually wondered “is there something wrong with my garage or electrical system?” A lot of the infrastructure in my older home is fairly well-maintained and up-to-date, and the garage wiring is all-new professionally updated only a couple of months ago.

“Anomalies in my own home” are hard to test, and even harder to control for. I’m not turning my home into a money pit for an electrical investigation, and I’m probably not moving or relocating my car to test out issues with charging. Still, I wonder if I have some unusual stuff with EMF, wiring, or materials that cause the car to not sleep properly. Unlikely, but I’ll have to consider it.

 

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.

 

I posted some tips and shared experience in the “who hasn’t had an issue” thread: Sunday at 7:19 PM

Pasted below as well:

Spoiler: things to do/not do (speculative and Prime-specific, but should apply to any gen 5 Prius for the most part)

Things that I don’t do:

• no long charging sessions
• not driving exclusively or excessively short drives, and do drive longer distances at least weekly (735 miles at first failure, now up to 1300+ miles after a couple of months of ownership)
• no accessories plugged in constantly (a couple of USB plugs are plugged into nothing at all except while driving)
  
• no remote starting – only done once to diagnose the 12v issue
• no dash cam
• no dealer accessories

Things that I have probably ruled out (only for me – anyone else will have to rule these out individually):

• key fob is not kept nearby the car
• key fob is usually turned off to prevent interacting with the car
• dealership says no constant parasitic drain – although what about large spikes of activity when garaged?

Things that it still could be:

• the Toyota app, particularly since my issue started up immediately after the v2.5 app update, and my bluetooth “remote key” kept removing itself
• slow/inadequate 12v charging while driving – let’s say that hypothetically it normally charges up, but on some drives it was under-charging for some hard-to-diagnose reason
• cold weather – not as a sole cause, but as a contributing factor
• bad 12v battery (I guess – I don’t find this convincing)

Things that I am now doing to mitigate any possible issues:

• monitoring any failures or abnormal behavior (smooth sailing for 500+ miles since a new battery)
• documenting my drives so I know how long/short drives are so I can rule that out as a cause
• got a battery maintainer/charger to prevent having to jumpstart my car in case of failure
• got rid of the bluetooth key – it was de-registering in the Toyota app almost daily so I just stopped configuring it

 

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.

 

There's a TSB out, because Toyota received a bad batch of batteries form their vendor. Take your car back to the dealership and have them run "documented tests" on the battery and electrical system. You need to focus on the dynamic load test results. Don't take their word for this - depending on the honesty of your dealership. If you clean out your trunk and allow access to the battery door, any auto parts store, down here in the lower 48, will run a free battery test for you. Their readings should be within 10% of the factory battery specifications.

Good Luck and hope this helps....

PS, on post #3 - Leadfoot is talking about a Faraday bag. You can find them on Amazon or Ebay for less than $5 or just turn it off

 

Is the TSB on the batteries available online?

 

More and more cars (not just hybrids) are including electronics that need power even when "everything" is off.

The Prius has a smaller 12v battery than most other cars, but is tasked with supporting more or less the same whizbang feature set, and it is generally using the same technology as those non-hybrid cars.

So you'd see something similar with just about any late-model car, but it's a bit more pronounced with a Prius.

I like the suggestions for a maintenance charger and voltmeter. I believe there are many ways to skin that cat.

 

I'm going to be a bit contrary and say I don't like the idea of a battery charger or tender to "fix" this problem.

Hear me out.

I'm worried that using a charger or tender to baby the battery is a case of treating the symptom and not addressing the underlying problem.

If it's a defective battery, then you might end up babying it long enough that you end up out of warranty and have to replace the battery on your own dime. Better to let it fail completely and get it replaced under warranty.

If it's a software/firmware glitch, using a charger/tender deprives Toyota of good info that could be used to track down the problem. Every case of a battery dying needs to get reported to Toyota North America and Toyota Global so they understand the severity of the issue and how widespread it is. Every owner that solves or minimizes the problem on their own is one less datapoint. This includes affected owners making sure their dealership is reporting the problem up the food chain in a timely manner.

Anyway, those are just my thoughts on the issue.

 

5th gen owners cobbling spare tire solutions has similar impact?

 

I predicted this reply. lol. I thought about preempting it in my post, but decided to let it go.

Yes, they are different. Toyota not including a spare is a deliberate choice by them. They ran their numbers and had their focus groups and consciously decided that not including a spare was the better move. We can disagree with that decision, but I don't think we're going to alter it. There just aren't enough who find the spare important enough on which to base the decision to buy or not.

The battery situation is different. Obviously Toyota didn't decide that having 12V batteries fail left and right would be a good idea. Something has failed somewhere along the line and Toyota needs to know about it to fix it. Owners finding ways to fix it on their own either covers up the problem completely, or at least makes it seem to Toyota that it isn't as prevalent as it really is.

 

So far so good.

I agree. Reporting major issues to Toyota via dealerships is key. I spent a lot of time going after the problem and I recommend anyone else do the same. To their credit, my dealership did replace my failing 12v battery, so hopefully that is passing the message on to Toyota.

I did get a battery maintainer since I will need one in the longer run, but I wouldn’t suggest that anyone approaches this issue with a maintainer as a patch. My rationale for getting a maintainer is so that I am set up for not driving the car for a week or two if I ever need to, and so that I can avoid unnecessary jumpstarting in the future.

 

I see your point, and I agree that it is important to let failure data get back to Toyota via established channels.

That said, going from the first post I came away with the understanding that this car has been driven 400km (248 miles) since it was purchased new in December 2023.

I would regard that as exceptionally low utilization. I'd expect a fair number of late-model cars to have issues with healthy 12v batteries when used so little.

 

Phhht, more like “they figured they could get away with it” lol. Anyway, sorry for butting in, off-topic.

 

i agree on making sure the battery is good and nothing wrong with the car. if those pass, it's likely the small amount of usage and a maintainer becomes necessary, short of disconnecting the battery.

 

See https://priuschat.com/threads/third-day-with-new-car-dead-12v-battery-second-day-in-a-row.242574. I posted on page 7. The best thing you can do to charge the battery is put the car into READY but not drive it for 30 min. The worst thing you can do is actually drive the car... eventually it will charge it but only when it hits 60% and even then will only charge it to 85%.