My 12v battery when its connected to car loses charge in 30min

How old is your battery?

 

Does the car work normally when you first install the battery?

Draining a 12v battery (even the Prius small 12v battery) in 30 minutes could be 2 things

1. a relatively high parasitic load on the 12v system.
2. a very weak 12v battery (how old is it? original from 2010? if so, just replace it)

Test the battery with a real load tester to determine if it has a reasonable capacity. A charger isn't really telling you anything about capacity.
If the battery tests good, install it and start looking for abnormal loads like lights, or pumps that are running when they shouldn't be. Would be relatively easy if you have an amp meter.

 

Well if you have SKS and it's not turned off under the steering wheel then you're 12 volt battery and remote and all of that every so often is hunting for each other The car wakes up to look for the remote goes back to sleep so on and so forth If the alarm get set off by varmint or whatever at night I have mine turned off it's completely disabled but apparently something can still set it off and make the lights flash It only does this for about 45 seconds and then turns off and Toyota software I have everything turned off at rest no door re-locking no automatic door unlocking nothing but yet still if I go outside and I walk by the car too hard or I don't know what exactly goes on the car will start flashing the headlights and the tail lights like the alarm is gone off there's nothing honking nothing making any noise I even have the beeper turned off under the left front fender or it's unplugged beeper sitting there it's unplugged so anytime those lights go to flashing some energies being used from the 12 volt battery anytime the car decides to wake up and hunt for something 5 hours after you shut the car off in a generation two the heat recovery pump is going to come on move some fluid check temperature and shut off It's in the manual so there are some things that happen they're not really that draining but if your battery is not really up to snuff I guess it could be because whatever you're doing with a trickle charger pretty much has no basis on what happens when it's off the charger in your car If you think things are on in your car hook up a good voltmeter or something that can read milliamps and see if you are in fact losing all this voltage you think you're losing You're 12 volt is probably just going south no matter how new it is or old it is or any of that It's just what happens to lead acid type 12 volt batteries a little better with AGM and gel but still I have very good batteries in my power sports equipment motorcycles four wheelers big bore scooters you'd be shocked how bad and how often the 12 volt batteries go bad in these devices It is unbelievable to imagine today this is still a thing but oh well here we are.

 

You created two threads earlier about the same issue with the 12v battery starting in October 2023. Now, you have created this third thread. It is time to buy a new 12v battery!!!!!

 

Have you answered the question on how old this battery is? If it’s old, a full charge won’t mean anything

 

It is a battery issue!!!

In a thread that you created in December 2023, you said the car sat idle for 2 years: https://priuschat.com/index.php?threads/245466/
That killed the 12v battery, and it needs to be replaced!!!

 

You need to actually pinpoint what is running. Use a mechanic's stethoscope if necessary. Also, reference lefts and rights from the perspective of sitting in the driver's seat looking forward. Somewhat the same principle as how the port and starboard side of a boat/ship never changes.

 

weird, sometimes my Gen 2s do that also...something runs when I first get to the car in the morning, then stops running.

 

A car that sits for two years will lose 90% or more of its capacity to store power. This is the most common issue. A 12v battery replacement fixes it. In the US any auto supply store can load test your battery with a special tester and tell you the remaining capacity.

All cars including the Prius use a very small amount of power at all times. This is known as Parasitic draw. Normally a Prius will draw 15-30 ma when off. ma is thousands of an amp. Very small. A good battery can take this kind of draw for three or four weeks and still start the car. A mechanic can measure the parasitic draw for you to ensure nothing is eating power fast. In reality you could leave the headlights on for thirty minutes with their very high current draw and still start the car. IF you have a battery with normal power storage capacity.

The brake master-cylinder will run a pump motor as you approach the car. It is in the front engine compartment behind the big fuse box. It is normal if it runs for perhaps a minute. They do go bad and run longer but will create engine codes a mechanic can read with a scanner. But even this running for thirty minutes would not totally drain a good 12v battery.

So everything points at your 12v battery needing replacement. Of all the possibilities, a new 12v battery is far less expensive than a Prius Master Cylinder. We are not talking about the 201v hybrid battery, we are suggesting the small 12v battery is old and has low storage capacity left.