12V down to 6V - door ajar but dome lights off - what caused the drain?

Surprisingly although the OEM battery clearly states on it's label maximum charge rate 4.5 amps, the Prius charging system does not limit the charge rate to anywhere near this level.
Having done some testing in this area I have recorded charge rates of over 50 amps with a battery at a 30% discharge level and in continuing test am seeing even higher levels of charge . This sort of charge rate can boil off the acid from the plates making matters worse.
There have been many reported instances "not on PC only one to my knowledge" of the much vaunted Optima battery failing through such treatment and I am convinced one of the reasons why the OEM battery seems to become very week after being deeply discharged, even though in theory it is a deep cycle battery being an AGM.
I am convinced it is using the cars own charging system to recover a discharged battery that actually does the damage.
The reason for the discharge in the first instance could be the red lights mounted in the lower door panels as you cannot turn these off.

John (Britprius)

 

Maybe the puddle lights were on? Or the under dash? You can't really see them during the day...

 

^^^ Thats the word I was looking for "Puddle" lights.

John (Britprius)

 

No red lights (or any other kind) in my rear doors here. Those lights are only in the front doors.

Sometimes when I want to air out my Prius, I leave it all day with the rear doors and the hatch open. I've not had any issues draining the battery so far. I never leave the front doors open like this, because I know the foot-well lights can drain the small battery, but I've never had that problem with either the rear doors or the hatch.

2009Prius, it looks like your poor battery was so weak it was drained by the ECUs. Patrick Wong once mention here that the some ECU's come out of standby state whenever any door is opened, so the quiescent current drain increases, but I still thought it was a reasonably small current.

 

That must be a regional difference in models or something. I can swear that those lights are only on the front doors in my Prius.

 

Why didn't you connect the batteryminder to the car when it was sitting there?

And sadly the battery is probably now toast.

 

If your battery tender is happy with it, I'd speculate it'll be ok for a while, but events like this tend to shorten it's life.

 

Hi 2009. Try driving it as normal for a few weeks (no other charging) and then perform the MFD inspection mode test. It would be interesting to see the acc/ig-on/ready mode voltages after it's been sitting over night.

 

Thanks for the informative results and confirmation of the figures I have recorded. This gives extra data points from another vehicle other than my own helping to rule out the possibility of a problem with my charging system.

John (Britprius)

 

Have you the ability to monitor charging current on your trips to work? Perhaps even discharging the battery with the headlamps in ignition on mode for say ten minutes before making the car ready to give a partial discharge recharge test.

John (Britprius)

 

Thanks for the info 2009. Just one question about the graph, is the time scale accurate?

The reason I ask is that I notice that it clearly captures the two state nature of the Prius charging system, with the characteristic drop of about 0.6 volts (under certain conditions) after you've been driving for a while. In my case that 0.6V transition happens after about 20 to 25 minutes (1200 to 1500 seconds). By the looks of your graph it's happening after a mere 100 seconds in some cases?