SOC Dropping Over Night

SOC dropping overnight is totally OK. The SOC computation is an algorithm based on several factors, and temperature is one of them. In the heat it will be a bit different than in cool times. No worries.

Toyota has no method to test the battery. They can only wait until trouble codes are set, and then follow the recommendations in the service manual (sometimes known as trial and error). There will be few Toyota mechanics that know about NiMH battery life. All you can do is set yourself up with instrumentation (Torque, Priidash, mini VCI) and watch over time for individual cell variations of voltage and internal resistance.

 

Just wondering, what are you using to measure the SOC?

 

Check and make sure no interior lamps are on. Glove box or map lights. Something is pulling power on the chassis
12V side and the HV battery is keeping the 12V battery charged.

You need to have a parasitic load test performed if you can't find a light or some other load on. Map lights
are the most common cause of this condition.

 

Ok, so it's probably a real 10% then (some people read the bars on the standard energy monitor display and try to read it as percent).

So at 10% you should also notice a loss of bars on the display. For example, if you parked it at 60% SOC and by the next day it had dropped to 50% then that should correspond to a drop of displayed SOC from 6 bars down to 4 bars. Are you also noticing this happen?

When the Prius is off the HV battery doesn't keep the 12V battery charged. So a parasitic drain on the 12V may hasten the rate at which it loses HV SOC after it is started, but it shouldn't actually cause any discharge while it's switched off. It's important that the OP measures the SOC absolutely as soon as possible after starting the next day to avoid uncertainty about when the charge was actually lost.

 

Ive noticed this phenomenon before too via my ScangaugeII. However, my HV battery is still under warranty for another 30000 miles so Im not skeered

New Optima 12v here too.

A lot of times, before I get to my driveway the Prius is on all electric for a good mile or mile and half. So it runs down the SOC before I turn it off for the day. I chalked my low SOC the next morning to that.

 

Aaron, you should test the 12v battery after not driving it, like after you park it for the night, and test it tomorrow morning before you put the Prius into "Ready" mode.

After driving it, you arent going to get a true picture of the 12v battery health because you just spent the last xx minutes driving around with the HV battery charging it. And I wouldnt call those results "normal" because 12.8v resting volts is a true healthy battery with 100% SOC.

12.4v, especially after being charged by the car during your test drive, is not a battery thats a spring chicken.

 

Are you claiming a low/discharged 12V battery can't cause this problem ?

 

Yup me too. I had to return mine twice because the batteries they kept selling me were over 8 months old, and from sitting on the shelf with no maintenance (a charge), they sulfated. Wouldnt hold a charge over 12.4v. Overnight, it would dip to 11.9v. I finally got one that wasnt 8 months old (it was 4) and so far, so good - although the highest its ever metered was 12.6v.

 

I'm claiming that when the Prius is off that the DC-DC converter is not running and the HV battery will not provide any power to maintain the 12V battery/system.

If you don't believe me then try this experiment. Park your Prius with say 6 bars of SOC and then leave all your interior map lights and front doors open for a few days. When you come back your 12V battery will be as dead as a doornail. Put a new 12V battery in there and the HV SOC will still be at 6 bars, same as when you left it. Why? Because the HV system didn't do a thing to try and maintain the 12V system while the car was turned off. The OP is specifically talking about losing HV SOC while the car is off.

 

How would he know that if you turn the key on and the DC/DC converter starts to charge the 12V battery ???
I understand the 12V has no voltage input from the HV system with the key off.
As soon as the key is turned on it will send voltage to the 12V system. There's only one way to know.
Perform a parasitic load test on the 12V system

 

That's exactly what I said in the first instance here. If the 12V has a parasitic drain then it won't effect the HV SOC until after the car is next started.

Look, when the Prius is first started there are lots of things that can start draining the SOC. Then engine doesn't do any significant generating until after it's warmed for about a minute, so everything is draining SOC at this point. The 12V battery is just another on the list. If the driver is using headlights or aircond or has the car in gear, it's all draining the SOC, the battery is just potentially a few hundred watts additional there.

That's why I always tell anyone who's complaining of SOC loss overnight that they need to record the SOC the very instant (or as soon as possible) after the car is started, or otherwise SOC is certain to be lost anyway.