How do I check the 12 volt battery on my 2017 Prius Plug -In PHEV [16000 miles European model]?. It shows 11.8 volts on my Fluke multimeter.How do I get into the built-in diagnosis in the multi display? When I originally charged the traction battery it showed a range of 40 miles, now it only shows 35 miles.
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.
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.
I got into the diagnostic page on the multi display and it showed the battery at 11.8 volts with the car switched to 'Accessories'. It went down to 10.8 volts with the seat and rear window heaters switched on. It shows 14.1volts when the car is driving on the traction battery, suggesting that it is trying to charge the low voltage battery. But it returns to 11.8 volts when the car is stopped and turned off.
Sounds like the charging system is working like it should--14.1 volts when in Ready mode. The 12 volt "accessory" mode should not be used, in general. Always go to Ready to power accessories. 12V battery is simply there to bring the electronics to Ready, connecting the main battery source.
All of those readings are waaaay too low. I would suggest: 1. Forget about reading the 12 volt battery voltage through the car's displays. Pop the hood, let it sit overnight, check it in the morning. A good reading would be 12.6 or higher. 12.4~12.5 is meh. Lower, it's time to intervene, either charge or replace. 2. Keep in mind: voltage does not tell the full story. The dealership (likely for some $'s) or a battery retailer (probably for free) can test your battery with an electronic load tester, testing both it's voltage and Cold Cranking Amps. Essentially it's "legs". 3. You can buy a similar, pro-sumer level tester, something like Solar BA9 for example.
There is some lower limit to the 12-volt battery voltage that will cause the car not to operate reliably. I don't know what the specific voltage is, but my 2010 Prius developed a shorted cell in the original battery after about three years of service. The 2-volt or so drop from the nominal voltage was enough to cause the start function to sporadically fail. You would have to wait for the brake pressure accumulator pump to stop, then give it 30 seconds or so for the battery voltage to come up before it would start reliably. The fix, naturally, was to replace the battery. The cost was slightly offset by the $11 I got for the old battery from a local recycler. Better to replace the battery before you get to that point.
It varies, but mostly the load and wiring losses are the problem If you could, a voltage check at the pertinent ecu’s would tell the story
Follow-up to my earlier post: Beginning in the Gen 4 (2016+) Prius added some smart charging circuitry that will reduce the 14.1 volts that charges the 12V battery when the 12V battery is full. I discovered this when driving with a 12V battery tester in the cigarette lighter socket. SO, if your 12V circuit shows 14.1 volts all the time while driving, it means your 12V battery is not getting charged to the level it should. Take your 12V battery to a shop and have it tested.
