How do I check the 12 volt battery on my 2017 Prius Plug -In PHEV

11.8 volts on your Fluke is definitely low.

As a baseline, I want you to try this testing method to check your 12v battery:

1) Park your Prime with roughly a Depleted traction battery.
2) Then pop your hood so you don't wake up the car / computer with your key fob later.
3) Plug your EVSE in & fully charge up your Prime.
4) Unplug EVSE from car & wait ~1 hour.
5) Lift hood (no key fob) & test 12v battery with your Fluke meter, this should give you your best result.

No matter what, I'd tell you to put some type of low amperage charger or trickle charger on it until you get a full charge.

Here's what I did to help my 12v battery stay a little bit healthier, pics below.


Rob43

 

As described in my November 2017 posting, the Repair Manual (more info) gives a basic procedure for testing the auxiliary (12 volt) battery; you’d use the version from toyota-tech.eu, of course. That posting also describes the equipment used by Toyota dealers in the U.S. for more thorough battery tests.

As @Rob43 kindly notes, that reading is low. Was it measured with the car turned off? Are there any 12-volt accessories connected to the car, either permanently or at the power outlets?

See my February 2017 posting, but you may be disappointed: that mode is meant for troubleshooting the audio and navigation system, and it’s not a general diagnostic or indicating system for the car.

 

I've seen different multimeters showing different readings on battery. I say confirm the VDC reading again with another multimeter if avail.
Also, perform a load test after fully charging the battery instead of just relying on the terminal/surface voltage reading.
And, yes, attaching a trickle charger definitely prolongs the life of the battery. I had a (factory installed) battery that lasted 9 years in one of my previous cars that I bought new from a dealership by trickle charging it whenever not driving daily. I sold that car with the same battery after owning for 9 years. But some batteries lasted only 3 years even trickle charged.
If your battery already has started becoming weak/sulfated, usually it's too late to revive fully.
Sulfation in the battery is like a cancer in your body.
Once it has started to spread, consider it done.
It's best to prevent the sulfation when the battery is new by (1) avoiding deep discharge (2) and hooking up to a trickle charger.

 

(oops. duplicate post)

 

17’s had a lot of bad batteries

 

I have a charger that can provide either 10A or 2A, and is intelligent about the charging protocol, like stopping when full. Is 2A considered trickle charging for an automotive battery? Actually this question is for my wife's Honda Pilot, which isn't getting much use these days.

 

A "Smart" 2 amp (or less) car battery charger would be considered a trickle charger.


Rob43