Could be a bad 12V battery...

No, 12.4 is not acceptable for a battery that "normally" would be about 12.8.

There is no magic to resurrect a dying battery, long term.
Give your fancy charger a shot at it overnight.
But start shopping for a new battery.
Regardless of what the charger does, it won't last long.

P.S. Even though the main battery connections appear clean and tight, sometimes they are NOT. But 5 years is a pretty good run for an OEM battery.

 

Is there an additional connection for the ground on the negative cable?

The Gen3 has a ground connection to the body which can sometimes get loose.

If the C has one, I’d check that as well.

Keep us posted .

 

12.4 ain't bad. it is possible that something else is the problem. charge it up and see how she holds.

 

G'day AtkinsonCycle,

I'm inclined to agree with bisco that your battery's resting voltage of 12.4V is not bad and you should consider something else could be or was your problem.

I just went outside and turned on my 2016 c that's been in the driveway overnight. Not putting my foot on the brake, pressing 'Power', it started up in ACCESSORY mode and the battery read 12.3V on my cigarette-lighter-socket usb plug-in that has a built-in voltmeter and temperature display. Temperature display was 24C, felt about right.

Pressing the 'Power' button again (still no foot on the brake), more dashboard lights came on and the internal air-circulation fan started up, and the voltage dropped to 12.0V.
Turned the car off with another brakeless 'Power' push. I waited 5 minutes and repeated the exercise.
Same battery values - 12.3V on ACCESSORY start, and 12.0V with another foot-free 'Power' press to get a fan running, etc.

Now, with my foot on the brake, a 'Power' press brought up the ready-to-go display and the usb-plug battery voltage now was 14.4V. My belief is the traction battery is now charging the 12V battery as a pseudo-alternator, hence the 14V reading that is the 'alternator' input, not the output of the 12V battery. The ICE did not need to start up during this test.

These battery values match earlier tests I made when I first installed the $10 e-bay usb multi-function plug months ago.

I have not had any problems starting the 'c' in the 18 months I've had it.

Nonetheless, good luck with whatever you try.

David S.

 

no, i suggested charging it. if there aren't any dash lights, i don't think there will be trouble codes.
you are going to need a new battery within a few years. you can replace it now to eliminate it as a possibility, or see what happens after charging by checking the voltage every morning, if the car is starting.
it is possible that the amperage is bad, even though the volts are fair.
an auto parts store can test the amperage for free.

 

Yeah either a auto parts store with an electronic battery tester, or for DIY: Solar BA5, and you can assess the battery's health. Try entering your car data on Pep Boys, see what 12 volt batteries they're offering? For 3rd gen they seem to often have a Bosch on sale, with free install for around $145. They'd probably have the electronic battery tester available too, one stop shopping.

FWIW, my Optima Yellow Top is only reading 12.58 volts first thing this morning, just hooking up volt meter. I have Solar BA5 but it's a bit of a pain, you have to connect directly to posts. Anyway: only about 2.5 years old, and fairly religously kept on a maintainer when the car's idle, which is frequently, two or three days at a time. It had a good run yesterday, but 12.58 first thing the next morning, is not too impressive.

I'll likely go with a Canadian Tire Motomaster next time, apparently a rebadged Exide. I think Optimas are maybe overrated.

 

Any battery voltage above 12 Volts should start your car.

I don't know what battery the C takes but Pepper Boys sells one for a regular Prius.

 

While it is possible that the battery holder/space is different, it is highly doubtful.
I bet that the batteries listed for other models of the same year C would work fine.

 

It is NOT.
I've pointed that out to him several times but he leaves it there anyway.

A failing 12 V system, for whatever reason, can cause false trouble codes to be set.
Reading them out and making a record of them is not a bad idea necessarily but trying to USE them to try and fix the problem without first checking the 12 V system is just foolish; especially when there several set pointing to multiple failing sub-systems.

 

Sorry but not in the case of modern cars that rely on computers to get them going.

12.0 volts with no load indicates a battery that is about half discharged.
And when any load is applied to it, the voltage is likely to go down more; often a LOT more.

Bad advice.

 

While the state of charge is low Toyota disagrees with you.

 

The experience of literally hundreds of Prius owners......and THOUSANDS of the owners of other cars around the world pretty much proves that their statement is not right.

But you are free to believe whatever you please.

The sad part about that is: Many Toyota shops seem to believe that is true too. It simply is NOT.

 

Have to say, if I saw 12.0 I'd be alarmed. Anything lower than 12.5 doesn't look good to me.

 

I saw 12.31 on our 2010’s original 12v over New Years and swapped it out.

Gave us 8.5 years of service.

 

I saw 11.9 on our original Yuasa battery, after getting the grandkids out of the car, then forgetting to close one last door, for 18 hours. Immediately put it on charger, and it took extra long, but after that I figured it'd days were numbered.

Replaced it with an Optima Yellow Top September 2015, but that's turning out to be ok but no great shakes. I'm very careful, keep it on a smart charger any time the car is sitting more than a day, and still, if I check it first thing in the morning after a good drive the day before, it's around 12.58. Not that impressed.