Dead Prius 12V battery

Trying to troubleshoot a battery issue. I have had the battery on my prius get drained a few times for no discernible reason. Replaced the battery, had it happen again. It isn't time dependent. Will go a week without issue, but then die in a short evening.

Finally had a clue last night. My neighbor noticed that my taillights were on. He said "your red brake lights were on". A little stressed from trying to jumpstart the prius to no avail(still scratching my head on this one), and then climbing through my seat to disconnect the 12V from the car and placing the 12V on charger since my attempts to start the car had placed too much load on the car to allow charging from my little charger.

Any thoughts? Why would my taillights be on? My wife drove the car, but she didn't turn on the headlights. I checked that already. Even if she had turned on the headlights, they should have "auto-off" after a few seconds.

 

But do brake lights continue to operate when the car is turned off?

Nexus 5X

 

So, quick hypothesis:
I sometimes park my car on a flat surface and sometimes on an incline. I have only had the auxiliary battery die on an incline.I hadn't really paid attention to this fact, until now. I don't know that the e-brake was always applied when the battery died. I do, however, know that it was always on an incline.

I just did a fairly rigorous check to see if the brake lights would stay on when I pressed the brake. They never did. I don't think it is a relay. In my experience most relays either fuse or become corroded. I don't see a lot of relays(27s) get ocassionally sticky. I think it is something attributed to parking and stopping the car on an incline that is occasionally leaving the brakes stuck in a partially engaged mode. Stop me if I am wrong, but the brakes on this car are electric. Ergo, they would be drawing current from the 12V battery when the HV battery is disconnected. If the brakes somehow remained engaged, that would place a fairly significant load on the battery(a few amps). Which would explain why I had a hard time charging the battery with a 10A charger.

 

Yeah, I just verified. Should have done that before I responded. Brake lights will function without the car running. I don't know why I thought they didn't. E-brake does not operate brake lights.

As an aside. I tried to reproduce the "lights staying on". I tried killing the power with my foot still on brake. I tried tapping brake right after power was killed. I tried hard taps, light taps. A lot of different taps. The relay always responded pretty well. I will keep my eye on it though

 

I'll probably replace relay though. That is cheap and easy

Nexus 5X

 

Ok, so I did a bit of reading. I would have thought that this went through some kind of aggregate relay. I guess I am wrong? You guys were all talking about the pedal stroke sensor? That seems significantly more involved to replace on a hunch. I didn't really see a place to adjust this sensor, though I did see a place to adjust the actual pedal.

I wasn't implying that the parking brake caused it. I was saying that by applying the parking brake while the car was on the incline and might still have caused some feedback on the normal brake. Not sure how it would do that. I am 100% sure it is the brake, just trying to think of what would be causing the brake to stay engaged.

 

No, it does not. It is a "parking brake and not an emergency". Or, in the rare case that your brakes totally go out, it won't actually bring your car to an immediate stop so it doesn't matter that much anyway.

Ok, that is the max charge rate. My point was that I was having a hard time charging the battery with a 10A charger.
The max charge rate is how much the system allows to pass through to the 12V battery. I am saying that with a 10A charger on the 12V battery, it wasn't charging. This means that there was almost a 10A discharge on the battery(though it is a cheap charger, so probably closer to a 6A discharge). The maximum charge rate isnt really applicable.

An LED taillight isn't going to draw 6A. The headlights barely draw that much. Something much more significant had to be operating. My guess was the brakes. A motor load would make a lot more sense because most other systems in the Prius are going to have a cutoff threshold of voltage. In other words, they will simply stop working below a certain voltage. A motor load doesn't. It is essentially a giant resistor. It will keep drawing power as long as there is voltage.

Nope, bought her used. 130k miles