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2010 Toyota Prius - Battery issues

We mostly use our 2010 Prius for short trips, and during very cold and/or snowy weather, it may sit in the garage for up to a week or two at a time. During last winter’s polar vortex, the car battery (not the hybrid battery) died 3 separate times. I started the car with a jump from our other vehicle and took it to the Toyota dealership. They couldn’t find anything wrong with the battery or anything else. I got the battery fully charged, and it ran normally throughout the spring, summer, and fall. Now with winter here again (and a two week vacation), the battery has died twice more. Is this preventable or something I now have to deal with each winter?

I suspect your starting battery is weak, especially if the original. The dealer may not have done a complete test of the battery. That it died three times shortens the life of the battery.
You could have a parasitic drain causing the battery to drain.
A good battery without a parasitic drain should be fine for two weeks.

The 12v battery is not the starting battery.
How old is the 12v battery?

If the battery passes a load (or conductance) test. the problem must be that some circuit is discharging the battery as the car sits unused. Make sure no lights are left on when the car is parked and the key removed. Check the glove compartment, trunk, engine compartment etc. A faulty door switch can cause this on newer cars like yours. The computer thinks somebody is getting inside to start the engine and it wakes up all the car’s computers to get ready to go, when if fact its just a faulty door-open switch.

If I had that problem and couldn’t ID a reason for an unusual battery drain and the battery & alternator checked out ok I’d just charge the 12v battery one night every week using a battery charger. It may be you just aren’t drivng the car enough to ever get the battery fully charged. The problems from that turn up in the winter.

Just hook up a battery tender tp it in the winter. they are not terribly expensive and it will keep your car ready to go without overcharging it.

+1 to @oldtimer_11 advise on connecting battery float charger

pay attention that Prius’ low-voltage battery (not “starting” one, it starts off high-voltage one!) is a very fragile animal, it is not supposed to be charged with currents above 4 Amps or it may be damaged

you will have to buy charger with the max rated current of under 4 Amps to use, fortunately Amazon is full of quite decent automated chargers in “under $30” category

another few things to pay attention to

#1. your main battery discharge reason is Toyota’s proximity access system, the one letting you not to take your key out of pocket and operate locks by touching the door handle - it constantly draws power to provide that convenience

I’m not sure about your Gen3 Prius, but my older Gen2 Prius has a special button under the steering column to switch proximity system OFF specifically to save power draw - read your car user manual, find yours, use it appropriately

one more thing on this: if you keep your key on the counter next to a garage, it keeps “talking” to your car, killing both keyfob and car battery much faster

#2. eventually you will have to replace your 12V battery, normally it operates 7-9 years on Prii and doing so preemptively will let you cut the costs in half if you go to regular indie shop, not to the dealer

#3. if you do not drive your Prius much, your main concern is your high voltage battery, it is getting worn by “sitting idle” substantially more than from active usage. it must be recharged to full at least every few weeks and short trips might not do that properly. once again - open your user manual and read it. it has a whole section on that.