All brake warning light came on when the 12v battery died - yellow line to wheels on when stopped

If you don't have a suitable code reader, you can get the codes by using a short wire to connect two positions of the diagnostic port, Tc and CG, as covered in many threads here on PriusChat, and counting the blinks of the dash lights.

If you get only a code 41 (a/k/a C1241, "hey, noticed the volts are kinda low here") and you know the battery was low at the time, then you could be fairly confident you knew why you had that code, and could go ahead and clear it. While you have those two terminals jumpered, there is an extra chicken dance you can do to clear the codes. That's also in a ton of threads here. In fact, it seems there are more people who know about jumpering those terminals as part of a code clear than who know they can read their codes that way, a sad irony that helps no one.

 

Battery Tender is a big brand.

Posted via the PriusChat mobile app.

 

I watched this video

and it says if your hybrid battery reaches a specific low voltage condition of charge your dc to dc converter (what charges your auxiliary 12 volt battery) may not function properly and the 12 volt battery don’t charge properly consequently upsetting the very sensitive electronics on the car generating random codes and lights.
I have all the brake and anti skid lights on at the moment. My breakdown service patrol scanned the car and code c0215 came up (left rear speed sensor fault) and I changed the speed sensor, no fix, then I change the wire harness and also no fix. I checked the anti skid module, brake pump module and done wiring checks and it was all satisfactory. Then the red triangle off death popped up a month later with a warning message on multi function display “ energy monitoring fault”.
The car is very sluggish and mpg reduced. Plus the hybrid battery indicator is fluctuating very erratic.
The car had a new 12 v battery fitted about 1 year ago but within the same period I have seen the hybrid battery capacity reducing a lot.
I will get some advice on the hybrid battery servicing and try to do it myself as I am not prepared to pay big bucks now and see if the problems disappears.
Soon as I get anything done I will feedback progress.
Please feel free to comment if anybody experience something similar.
Thank you everyone

 

That's one of John Kelly's videos and they are all very worth watching, and he is too knowledgeable to say something like that, so I watched it to see if he did, and he didn't.

There are a few places in the hour-long video where he mentions trouble codes, and in each case he is talking about specific conditions that get reported by specific codes being correctly set. Nothing about "upsetting" things or generating "random" anything.

Could it be that you watched some different videos, and that one wasn't the one you meant to link?

In a case of some different videos saying different stuff, if one of them is Prof. K's, I'll generally advise going with that.

 

It is possible that the code scanner the breakdown service patrol guy used was not of a level that it can only read basic engine and ABS codes. So, it could be that there are other codes in the HV (hybrid vehicle) ECU and battery ECU or other ECUs that you are not seeing. I'd be making sure you were confident you see all codes before just assuming you have a hybrid vehicle battery problem and rip into that.

Further, the aforementioned professor also has very good video presentations specifically about the Gen 2 (which your car is) that could be beneficial for you to watch.

 

Feedback on my previous post as i promised.
I gave up repairing the car myself. So after changing the rear left speed sensor and wire harness and no success I took the car to my closest Toyota Prius/Lexus garage (not Toyota dealer as I did not want to get bankrupt) and after a short troubleshooting it was fixed. The anti skid module was fault (not picking up/processing rear left speed sensor.
The mechanic told me there was a possibility that the original problem was only the speed sensor itself. He believes the anti skid module failed Because I fitted a non original (non Toyota ) speed sensor.
Just bear in mind what he said “it was a possibility.
He said some non original speed sensor brands quality are very low.
So at the end the speed sensor was changed (Toyota original) and the anti skid module.
But no anti skid module codes were generated.
Only rear left code c0315.
Hope this information is useful to anybody in the future.

 

many aftermarket parts are cheap black market ripoffs.

glad they got you sorted, thanks for the update!