Prius Prime 12v dead, what to do after?

First things first, let's see if you can get it back up to ~13 volts.

Hopefully you can...

Question: How many days was it parked for when you discovered it was dead ?



Rob43

 

What do you mean by “trickle charge”? How do you do that in a Prius?


iPhone ?

 

You attach a trickle charger to the 12volt battery under the hood.

 

Doesn’t the EVSE charge the 12V as well when plugged in?

I drive about once a week since the lockdown for essential errands like groceries and pharmacy needs, and I haven’t had problems with my 12V. I try to keep my EV pack charged.


iPhone ?

 

This happened to me. I remember I lost my radio presets. I did not loose the clock or charging schedule. I don’t think I let the 12volt go as low as you did. There was enough juice left to display the low battery warning on the MID etc. but not enough to start the car.

The consensus here is it maintains the 12volt’s level of charge but not much more and only does that while the traction battery is actually being charged. After the traction battery charge completes the 12volt is on its own again.

 

That is too long. As long as you can bring the 12v battery back to life, it should be just re-initializing the car. I haven't done it myself, but similar to what you had to do the first time you bought the car. Following is the re-initialization steps needed after 12v battery reconnection. It is from the 2020 manual, so the page referring may be different for your model's manual.

If your 12v battery is healthy, once a week drive for ~1hour is enough to keep the 12v battery charged. As said above, the plug-in does maintain 12v while actively charging the traction battery, but will not bring the dead battery back to life.

 

5V??? For a 12V battery???

That battery might never recover. If you can get it to take a charge, have it load tested before next winter. Depending on where you live, it might let you down some cold winter night.

Ed

 

That 14 volts is most certainly read while the car was in ready mode, maybe the dash display? You're not seeing the true voltage of the battery under those conditions; not even a brand-new fully charges battery would read 14 volts.

Let it sit overnight, with the hood popped if practical (you have secure parking). If not, just pop it in the morning, but do go in through the passenger door, wake the car up as little as possible. Measure the voltage with a digital multi meter, see what you get.

 

What’s interesting about that is that you can’t charge the traction battery if the 12V is dead because the charging system itself can’t start. I find that laughable. Why can’t it start the computers from the AC charger? What this means is that you have to jumpstart the Prime if you want to plug-in to charge the traction battery. LOL!


iPhone ?

 

Yeah, but even if it can charge the traction battery with a dead 12v, what good would it do if you can't start the car without jump starting anyway.

I have had numerous dead battery incidents with all of my previous cars, but never had it happen on my PRIME yet. Of course, most of the dead battery incidents happened in middle of winter on a car with an old battery, but still, I've had better luck with PRIME to date. That being said, reading on many cases of dead 12v battery in PRIME, I have purchased Li battery jump starter just in case, and now with COVID-19 lock down, I installed BT Battery Monitor on my PRIME, so I know exactly what SOC my 12v battery is at 24/7. I am prepared.

 

It's been stated many times in this forum that while the traction battery is charging from the AC, the 12V gets some recovery juice too until the traction battery finishes charging. Is that assessment incorrect? If you have a borderline dead battery, which is the most common case, completing a traction battery charge should be sufficient to give the 12V some juice to start the vehicle after you finish charging the traction battery.

 

Since I have not had my 12v battery depleted low enough to benefit from L1 EVSE when I plug in my car, I don't have good data. My car is almost always driven right before or at most within 6 hours prior to charge start time, the 12v battery is already replenished from the onboard charging system just from driving. From my 12v SOC observation after having PRIME sit for a week, the onboard charge system works much better when the car is driven (or just in READY mode) than when the car is off but plugged in. I am not sure how much of juice it would provide to a dead (or near-dead) battery just from plugging in.

Humm, that's an interesting question. My car is now parked and sitting at 40% SOC traction battery for the past 3 days. The 12v has gone down to 12.4v 65%. If I just plug-in for a few hours before driving next time, would it replenish 12v SOC up to 100%? I will try that and report here.

 

I had almost the exact scenario a couple of weeks ago and I was very disappointed in the results as the 12 volt didn’t come up to 100%. Someone on here (can’t remember now) told me the EVSE basically only maintains the charge in the 12 volt. But good luck anyway!

 

Yeah, that is my understanding. EVSE charge only maintains 12v SOC level but does not replenish it. So, if it is near dead, it will be near dead at the end of charge. Or as mentioned by @mveras1972, if the 12v battery is dead, even if EVSE can be used to charge the traction battery, it would not help bringing the 12v back to operational SOC.