2012 Starter? Battery not holding charge after a few days?

you need a new 12volt battery, and keep it on a battery tender because of the short trips

 

If it's the original 12 V battery it already died - you just managed to jumpstart its heart momentarily. No battery tender will help.

All cars, including the Prius, constantly drain the battery at a very slow rate - this is called parasitic draw. A good battery can last four weeks without discharging enough to cause a no start. In your case the 12v battery is so old it can only hold one or two days of this parasitic draw. A new $200 battery and you are in business.

Honestly, this could be the first of more expensive problems to come. A high voltage battery costs around $2500 and there are other issues that cost more. If you're financially able, now would be a good time to look around for a newer car. You are good for the moment considering you only need a 12 V battery.

 

Many auto supplies have the battery which they look up based on your model and year. Often they will install it free. The Toyota dealer will also have it most of the time but the cost will be substantially more. Call around for cost and installation.

 

Yes, the rear right quarter panel. Easy to do yourself if you choose, plenty of helpful videos.
Dealer has a good one with good warranty, or auto parts stores are fine as well.
Expect to pay around $200. Or so.
I use this tender on the front jump point:

 

The install drill hasn't changed much over the years. Remove hold-down bracket, disconnect the neg cable clamp, then positive, lift battery out. Drop in new battery, connect clamps in reverse order. Have a few washers and shims on hand: the Toyota style of hold-down bracket has ZERO tolerance to any variation in height.

There's a few fine points as well:

Prius batteries inside the car have a vent tube you need to disconnect/connect.

It's good to clean the inside of the cable clamps, and the posts. Don't go nuts, the clamps may have a plating surface you don't want to remove. And the posts are lead, so be careful, it's somewhat toxic.

When tightening the clamps just one-hand it with a (10mm box wrench), snug it like you would a tap in need of a new washer, not too hard.

 

It's safe, as long as you do NOT let the positive cable for the battery to the frame.
You could put a rubber glove, or a plastic bag round the positive cable after you
remove it from the battery.... just to be safe...

 

You can do that, it’s called a memory saver. I’ve got a cable for my jump pack, has cig lighter plug at the pack, and an OBD plug at other end, with just the 12 volt pin. Has indicator light as well. It works. Still, whenever I do brakes I purposely disconnect 12 volt, to play it safe, and it’s no big deal after.