2013 Prius C had trouble starting due to cold weather…

That makes it sound as if your 12v battery (the much smaller and less expensive of the two) has had it.

Get that battery tested.

 

I think periodic starts might maybe help... but the benefit would be slim long shot; a lot depends on how long you stick around running/charging it etc. It rapidly becomes a complex exercise and I don't see any percentage in it to be honest.

The battery part alone should be about $230 at a toyota dealer, installing it might be another $85-100 There's no point in paying them to diagnose anything if you are already certain the battery has failed. Tell them to sell you a battery and install it. Don't let them add any other inspection or service. Just don't.

The battery in the 2013 model is common enough that you can get them other places, prices are likely to be a little lower away from Toyota. Wal-mart and O'Reilly would be good starting points. Good luck!

A dealer may test a battery for free but some of these other stores will definitely do it; they are more eager to sell batteries. Downside: not every retail battery test is done with stringent quality control- use your judgement.

 

if it is original, just replace it. shop around at auto parts stores for a new one. if you need the car to be reliable in the meantime, you need a battery tender.

 

I really think your 12v battery is dead. I've been wrong before. Life is risk acted out. Enjoy it!

 

This is true.
You should not start it unless you can run it for at least 30 minutes.
It is MUCH better to connect a battery tender to the 12 V when you won't be doing at least a couple of 30 minute runs a week.
Then..........if your 12 V battery is now more than 5 years old, start shopping for a new one.
Many auto parts stores will test them for free......figuring that they will be selling a new one if it fails.

 

Battery tender can help you squeeze out a little bit more time, but eventually won't help (maybe there already) and you can still get stranded.

If you want to spend money on something other than directly solving the problem, get a portable jump pack. Once you charge it and learn how to use it, they can help you start the car when the battery has failed for any reason, not just old age. You can buy one in your neighborhood on thanksgiving day, they are common enough.

I have an old AGM type unit and it works great. There is a whole new generation of smaller, lighter lithium powered models. I have not used one of those yet, no opinion to offer other than that I might not plan to store one in a hot car during summer.

It is worth looking for one that features reverse polarity protection- there's a lot to protect in a Prius.

 

Neither of us have the correct instrumentation on hand to make that call with useful accuracy.

It could work for months, and it might not see daybreak. That's how it is with old batteries. With experience and some testing you can refine that a lot, but it's going to be much cheaper and faster for you to get a new battery than to obtain meters and electrical skills.

 

It's a curve. It all depends on how much is already wrong with the battery. So it could get you an extra 6 months on one that has only started to fail, but also only give an extra 6 days to one that is mostly dead.

Again, it very much depends on information you don't have and can't easily get.

In your shoes I'd take one more shot at getting a proper replacement battery before the trip, and then take the el.