2021 Prius Prime 12V Battery Low

A one time occurrence won't harm the 12v too bad as long as you charged it back to full soon after but after 5 more times your 12v is likely toast. You can stop in at many parts stores (napa, autozone, advanced auto etc) and they'll do a load test for free to tell you for sure. If you're still under 3 years 36k miles they'll probably replace the 12v for free at the dealership. You can probably ask them to double check there isn't some zombie drain going on.

If you only got a jump start and only took short trips after, you may never have gotten the battery fully charged again and that's why it died so early. It can handle being discharged like that 1 time but if it remained low afterwards without getting back to charged enough that's probably what did it in.

It wouldn't be a bad idea to have a plug in battery charger (not just a jump pack, something like a battery tender) handy in case it happens again that way you'll get the battery back to a full charge. You likely don't drive enough to get it back to full on it's own.

 

The original battery is covered under warranty and should be replaced by the dealership without cost to you if it fails in the first 24 months.

 

How short?

 

The 12v battery may have not been fully charged from the first day it was driven off the dealer's lot. There are some people on this forum who believes PP's charging system is superior to any aftermarket battery charger/tender that can fully charge the 12v battery, but I believe the PP and other modern cars have very high current draws when the car is not being driven thus on-board charging system is not adequate for replenishing the 12v battery charge. On my PP, a fully charged battery loses 0.1v/day and usually drains down to 12.4v within a week without substantial driving time (at least a 1-hour duration). I believe, your daily short EV driving is not long enough to fully charge the battery.

Yep, get it replaced. And if your driving habit does not change, I would think it is a good idea to use an external battery maintainer on it. Read more in the linked thread below if you care to learn the detail.
12v Battery Is Discharging, Re-Charge Now !

 

JUST A QUESTION. If start off driving using your main battery only and then at 15 miles a hour your sensor computer tells the engine too start.if your 12 volt battery is dead your stuck still driving at 15 miles a hour ? My prius is a generation 3 2015. What happens with a dead 12 volt battery ??

 

see my answer in your other thread

 

The 12v battery is needed to boot up the computer on board dnd start the car, but it is never used to start the engine. The traction battery does that part. If the 12v battery is completely drained, then the car will not go to READY mode... or even before that, you can't open the door without using the physical key if locked.

 

If the traction battery is used to start the engine, why does it need a 320 CCA 12 volt battery? A small motorcycle battery will run the electronics.

 

Do you unplug the car, once the traction battery is fully charged? Is your short trips less than 15 minutes?
You'll draw down your aux. battery if the car remains plugged in after a full recharge. When your car is in "Ready mode" the aux battery receives a charge; but on short hops (<15 minutes); your not charging the battery long enough to overcome the initial start-up draw on the battery. So your battery is always at a deficit. You'll either need to recharge the aux. battery every month or other month to make up for the deficit or install a "smart" maintenance RV battery charger.

Hope this helps...

PS The cold weather isn't doing you any favors either. .

 

It's not just 'electronics'—pretty much everything in the car that isn't the traction motors or the A/C compressor still runs on the 12 volt system, and sometimes people leave things like the wipers and heated seats and stuff turned on, so those all come online when the button is pushed, and the brake pump is likely to run, and a bunch of the electronics may have capacitors making a bit of inrush. Somebody datalogged and graphed the 12 volt current during the car's startup (an earlier generation) and it was around 20 amps mostly for a couple of seconds but with brief spikes as high as 40, 50, even 100.

 

The reliability and longevity of the 12v would be even worse if they reduced the capacity. I wish they had a better way of handling this, but it seems like using a 12v system for accessories keeps costs down (sharing parts with other similar Toyotas). Something needs to get the car to “READY” even if the system has fairly modest needs compared to a conventional ICE car.

There is something absurd about a car that has a massive high voltage battery with plenty of charge that can’t keep the 12v alive except when driving or charging. I wish they had a system where the traction battery could intermittently maintain the 12v on its own for up to a month – future EVs would benefit from this as well.

 

It makes more sense from a safety perspective, for both mechanics working on the vehicle, and for first responders disassembling the vehicle to extricate trapped patients. At the time this car series was introduced, they already understood how to be safe around the low-hazard 12V systems, but many were unfamiliar with and afraid of the high voltage systems.

This system was designed so that disabling the 12V supply, which they already knew how to do, automatically disabled the high voltage system and contained the remaining hazard inside the battery case. This approach was an essential part of getting many in the first responder industry to even touch a hybrid car, for fear of electrocution.