Mysterious 12V battery drain

This is the most posted subject on this forum. The mystery's of the 12 volt battery. Thousands of posts.

Most common drain is not driving the car. Letting it sit in a depleted state.That kills the hybrid battery too.

Second most common is leaving a light on in the car. Especially the cargo light in the trunk area. Its a good idea to look inside the car after arming the alarm make sure all the lights are off.

Lastly is the Back Up Battery supply in the trunk right next to the battery. Its the small metal box up on its end right next to the 12 volt battery. It has super caps in it that allow you some 12 volt power to stop the car if the 12 volt goes off line. When caps get old they sometimes turn into resistors this is called ESR.

Checking these caps requires an ESR METER which is very handy to have, There's hundred of those for sale on ebay.

The other issue is the main ground lead that goes from the battery to the chassis comes loose or is corroded. Good idea to unbolt that chassis connector sand all connection surfaces to raw metal lube it up bolt it back on.
Like in the picture.

Here's a grab of all Dead 12 Volt Battery posts using the search forums link:

https://priuschat.com/search/473415493/?q=dead+12+volt+battery&t=post&o=relevance

 

Get the clamp meter as suggested before, and take readings. I got a couple, not expensive now. No need to disconnect battery so the car has to reset the emissions monitors.

 

Resetting emissions monitors ?? Whats an emission monitor?

 

OBD2 Readiness Monitors Explained | OBD Auto Doctor

 

Ok so I've been having this issue. Bought brand new battery from Toyota 400 cad. Few months later dead again after a day of driving. No lights left on. Hearing beeping when I open the door occasionally and no. It's not the batteries in my key fobs. Both replaced. I know nothing about electrical systems but the abs first relay was clicking yesterday and drained the battery from full in a few minutes. Jumped it. Drove home and removed the abs relay (h1) I think. Anyways drove it around turned it off turned it back on and it worked. Seemed the relay was draining the battery. Is it okay to drive with the h1 relay out? Also side note: took battery back to Toyota and they tested and charged it. Said it was fine. Car dead two days later. I'm leaving it for a couple hours and am gonna to try and see if it starts up without the tender.

 

Have the dealership check for excessive battery demand. Or DIY:

Disconnect neg cable lead, and place multimeter in milliamps mode, in series between car metal and the battery. Specifically:

Use long leads, long enough you can close the hatch on them, clamp one to the neg post somehow, and the other to the car body (say via the disconnected from the neg post cable). Be very careful not to touch the other ends of the two leads together.

With the hatch closed, fobs well away, wait about 20 minutes, then connect the multimeter to the leads. Switch it first to amperage scale, then if not much reaction, switch to milliamps, highest scale (if not auto-scaling), then gradually step down the scale.

If it's similar to my experience (which I think was normal) draw will be around 18~20 milliamp's, with intermittent spikes to around 40, every 5 seconds or so. The spikes might be due to the blinking security icon on the dash.

 

Oddly phrased. I take it you mean that after the car was driven for a full day (normally more than enough to fully charge the battery), and then parked the car, the next day it was dead. Be sure the trunk area light is off. That light isn't very bright, so a person might miss it during the day if not looking for it, and it is possible to accidentally flip the switch while loading/unloading the trunk area.

While what you describe is almost certainly a parasitic load, draining the 12V when the car is off, it would also be a good idea to verify that the 12V battery itself isn't defective and can't hold charge for some reason. Park the car, turn on the headlights for 30s (drain surface charge), open the hatch, detach the ground lead at the negative post on the 12V, measure the voltage on it at the posts, come back the next morning and measure the voltage again. (Preferably at the same temperature evening and morning.) The two voltages should be very, very close like .02 or .03 difference max. Also, be sure to put something in so that the hatch doesn't close and latch, as it is pain opening the hatch if there is no 12V battery. If the battery passes the zero load test, carry it in to an auto parts shop and have them test it. It is pretty unlikely that the 12V itself is bad, but if it is all the steps needed to find the parasitic load are a waste of time.

 

Do you have the higher end Prius the Touring Model with leather seats and the factory JBL stereo?

There was an issue with the JBL stereo where the factory power amp under the seat its turn on circuit would fail in that amp and the amp would be on all the time even with the car off and would kill the 12 volt battery.

 

I see this advice a lot, mine turns off when the hatch is shut. Do NA Prii not do this?

 

They do. Unless the latch isn't closed completely, jammed on something??

 

What was the final outcome?

 

I am here to find out the same question. Same mystery happening with my 08 Pri2.

 

Measure parasitic draw as described several times in this thread and report back.