Will a failing 12Volt Auxillary Battery adversely affect the hybrid EV battery?

Driving downhill - I can't see that it should affect the 12volt battery. And - as far as I know, it should have no effect on the Hybrid Battery.
@Elektroingenieur might be able to throw some better light.

 

Given the conflict of interest—one presumes the dealer would like to sell a replacement auxiliary (12-volt) battery and labor to install it—I’m not sure I’d accept their conclusion that the battery has failed, without knowing more about their test procedure and pass/fail criteria.

If they used a Midtronics battery diagnostic system, as Toyota requires mainland U.S. dealers to do when making warranty claims, and the battery’s rating was correctly entered into that system, then I’d be inclined to accept the conclusion. Instruments such as these can distinguish a battery that simply needs recharging from one that has failed, and in any case, using the air conditioner (with the car in READY) or driving downhill is unlikely to have had much effect on the auxiliary battery’s state of charge.

The printout, however, just says “Fail” and “240.0 CCA.” There are different definitions and test procedures for cold cranking amperes (CCA) in SAE, EN, DIN, and JIS standards, and without knowing which definition applies, or how the measurement was made—or if this was, in fact, a measurement and not a reference value—I wouldn’t rely on it. Consider that if the battery is the original one, it’s marked with a CCA (EN) rating of 295 A, and 240.0—if on the same scale—is more than 80% of that.

As for effects on the hybrid vehicle (HV) battery, I wouldn’t worry too much. The car has a sophisticated auxiliary battery management system, including protection for the DC-DC converter and HV battery. The real dangers are that the auxiliary battery fails without much warning, leaving you stranded, or leading to an expensive mistake while trying to jump-start the car. You wouldn’t believe how many reports we get on PriusChat that people have confused the + and – terminals, which can ruin your whole day.

 

Humm, the battery that came in my 2016 toyota prius was rated at 190 CCA. Just saying this out loud

 

This is a photo I took of mine back - it's still the original, 4¼ yrs old, which in Dog Years is ...:

 

Good to "see" you again, Jimmy. I'm looking forward to your 4-year review.

As always, @Elektroingenieur gave a great answer. When the dealer said the 12V could strand you, that's the important part. But just seeing 240CCA doesn't prove by itself that the aux battery is about to fail. A simple test you can do at home, if you have a multimeter, is to take it for a 30-minute drive or thereabouts and then check the battery voltage the next morning to see if it held its charge.

Your climate is similar to mine but without the chilly "winters" and I keep hearing that four years is about average in FL for a 12V battery. However, mine always seem to last longer.

 

The 12V battery is recharged from the Traction Battery when you are in READY mode, regardless of whether uphill, downhill, ICE or EV. A simple voltmeter plugged in the 12V outlet in the center console will let you monitor the charging operation. When you first enter READY mode, you'll see 14+ volts, meaning the 12V battery is charging. Then, after several minutes, the voltage will drop to 12-13 volts, meaning the battery is recharged. This is a smart DC charging system that is only energized when in READY mode and the 12V battery needs a top-off. By the way, if you press the Power button without pushing the brake pedal, you enter Accessory mode, and you will see the raw voltage of the 12V battery.

 

Back to the original question:
NO, a failed 12 V will not "damage" the HV battery.

It might cause it to not fully charge for a while.
It might cause some codes to come up that LOOK like the HV battery is failing when it really isn't.

4 years is on the low end of expected battery life so I suggest getting it tested at some other place.
Is there an auto parts store nearby ? They will often do it for free.

 

Yeah, more than likely the dealer is selling you a battery + install that you don't need, just yet. CCA is, technically, only important for straight up ICE only cars because the starter motor has to turn the motor over so it can start.

Because the starter motor is replaced with the motor-generator in the Prius, the power comes directly from the hybrid battery. So a simple measurement like CCA is not enough to diagnose a failing 12v aux battery. Does the maintenance report from the dealer have SoC numbers or internal resistance numbers on it?

Those are more indicative of a failing battery.

 

why are you asking question 1? i don't see where the dealer said it affected the hybrid battery