12v Battery Is Discharging, Re-Charge Now !

Recently I've been monitoring the 12v batteries in my different vehicles, what I've found is very interesting.

In a nutshell, my older Non-Computer cars have virtually no measurable 12v battery drain over a week or two. But when you apply the same exact experiment to both my Prius Prime and Prius v you get completely different results.

The Prime and the Prius v have an absolute measurable daily parasitic drain (Prime is the worse), because of this drain, your fully charged 12v battery will drain off to an unusable point within a somewhat short time.

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For those of you that would like to add your own testing results to this thread, here's what you need to do:

1) Fully charge your 12v battery using some type of external 12v battery charger.
2) Leave your car locked, BUT unlatch your hood so that you can access your 12v battery later without waking up the Primes computer. (Keep your key fob away)
3) Once each day after your initial full charge, check and measure your 12v battery voltage with a voltmeter and log your results.
4) Assuming you have other cars to drive, test for as many days as you conveniently can, the longer the better. For battery safety reasons, terminate your testing at roughly ~11.9 volts which is about ~40% for a 12v battery.
5) Re-Charge your 12v battery with your external 12v battery charger and bring it back up to a full charge.
6) Your testing is done, please post up your daily logged testing info.

** All of this assumes your Prime is not plugged into your OE EVSE unit because many of the Prime owners can't leave their Prime plugged in 24/7.


Rob43

 

^ That's an unexpected big Wow...but not in a good way.

Absolutely put your Prime on an external 12v charger, hopefully you'll get a better result*.


Rob43

* Your best most accurate result comes from a "Sleeping" computer that's been off for at least one hour.

 

That's very good news for pip owners ! Your pip seems to operate much more like the 2008 non-computer car I tested.

Like all of us, the pandemic/stay at home orders hit first....then I started to think about 12v battery testing. So out of my two tested Prius's, my Prius v is the most likely car to sit for a while. The Prius v had been sitting at least 2 weeks (maybe 3 weeks) before I thought to go out and test it. When I did I was shocked to find 11.81 volts* (~32%) on the 12v battery. I immediately installed the permanent ring adapter from one of my Battery Tenders so that I could easily check & charge when needed. Now almost a week later, it's sitting at ~72% charged.

My Prime does get driven on a quasi-regular basis, so testing my prime under the same testing guidelines is tougher. What I did test on my Prime starting about ~10 days ago was this:

1) I unplugged my 10 watt / 12v Solar battery charger.
2) I fully charged my Prime using a Battery Tender Jr. which floated the 12v battery at 13.10 volts. (Full Charge)
3) I would leave my hood unlatched so that I wouldn't wake up the computer or dome lights.
4) After several days of no driving, no usage, & no EVSE plugged in, I recorded a low voltage of 12.04 volts. This LOW voltage reading was not long after my full charge of 13.10 volts, 12.04 volts is roughly a ~49% charge.

Now imagine what some people are experiencing after possibly 3 plus weeks...



Rob43

* The Prius v was not charged up first, so there's no available record of beginning voltage.

 

The 13.10 volts is measured with the Battery Tender Jr completely finished (finished for hours) and still connected. Once the alligator clips were removed I still recorded 13.10 volts in the first minute.

Normally a healthy 12v car battery will fall back to roughly 12.6 to 12.7 volts once it has stabilized, the problem is the Prime keeps sucking down the voltage based on its parasitic draw.


Rob43

 

That's good luck.


Rob43

 

We look forward to your findings.

Even if you can't follow the exact daily testing that I outlined due to the need to drive your Prime, some testing is certainly helpful.


Rob43

 

Yep, like Bisco said, that's totally normal so far. I'm going to guess with no external 12v charger and no EVSE connected, you'll drop down more than you want.


Rob43