I want to alert everyone of an issue with the auxiliary 12 volts battery in the new Prius prime 2024

Somebody’s got to try a regular ammeter in series; the clamp meters are convenient, but milliamp readings dubious.

FWIW, if the at-rest milliamperes is 300, that’s BAD.

 

I think there are two main reasons these 12-V batteries are failing:

1. The batteries are not fully charged when installed at the factory. It is crucial for a new battery to be fully charged when first installed in a car.
2. The Gen 4 and Gen 5 charging system do not fully charge the battery for at least 20 driving hours to improve the fuel economy. This is stated in the Gen 5 new-car features manual.

So, what happens then is that the battery comes with a low SOC from the factory and stays at a low SOC for extended periods, as low SOC in a lead–acid battery results in sulfation and stratification. This results in quick permanent battery degradation/failure, as little as in days or months. Driving the car does not help because of the point 2 above.

Solution: Get a Noco Genius battery maintainer and use it immediately after you buy the car in order to fully charge the battery. Even the smallest one (1 A) should work if you have the patience to leave it connected for longer. You might then want to use it periodically as well.

 

Plus get a battery monitor real time voltmeter as there is no way to know the whole picture with a manual volt meter.

 

"The Gen 4 and Gen 5 charging system do not fully charge the battery for at least 20 driving hours to improve the fuel economy. This is stated in the Gen 5 new-car features manual."

Which isn't like Honda abusing the traction battery in IMA hybrids, because the 12V is a wear item, and the owner's problem.

 

The more I read the more questions. Looking at the graphs I supplied, when would one hook up the meter in series? What if the draw is from the design, and is normal? looking at my graph the purple is my standard Toyota truck, old one. The battery stays up a long time and no sudden things turning on and off. The green is my Prime. The right side is when I put the maintainer on last night. You apparently put a maintainer on most of the time, seems like a good way.

 

I couldn’t be more perplexed. I often let my Prime charge up in the driveway on returning and look at some news or something on the phone for a few minutes. Every time it’s in park, it will start charging, and it does charge.
What does this mean 20 driving hours, is this from the battery 20 hour rating has to be 45 ah?

 

It has nothing to do with that. 45 Ah @ 20 hr in that context means you get 45 Ah capacity from the battery at a C/20 discharge rate (battery uniformly and fully draining in 20 hours, which corresponds to a current of 45 Ah/20 h = 2.25 A in this case).

This is what the Gen 5 new-car features manual says:

3. Stable use the auxiliary battery

a. Constant voltage control is performed and the auxiliary battery is fully charged every 20 driving hours to minimize deterioration of the electrode plates due to sulfation by repeating charge/discharge of the auxiliary battery.

b. Constant voltage control is performed when the SOC of the auxiliary battery is low or the temperature of the auxiliary battery is high/low to minimize deterioration of the auxiliary battery.

Translation: The 12-V battery rarely gets fully charged in Gen 5 (perhaps every 20 driving hours at most).

There isn't as much info in the Gen 4 new-car features manual. Perhaps, it is less aggressive in trying to avoid charging the 12-V battery to improve the fuel economy.

Third day with new car, dead 12V battery, second day in a row | Page 11 | PriusChat

 

Sounds like the system doesn't bring the 12V to 100% SOC after every start event, but lets it drain down some before doing so. Supposedly to get better fuel economy, though it will be a small improvement. Lead-acid batteries last longer being kept at 100%, with shallow discharge cycles being best. If the battery was a deep cycle type, this may not be too bad, but it sounds like Toyota installs a starter type, and that is likely what replacement ones will be.

In short, Toyota abuses the 12V for a slightly better mpg rating. It is the same thinking Honda used with the IMA Civic hybrid, though they were abusing the traction battery.

 

I didn’t think it had to do with the battery rating, but still lost in the translation. “Stable use the auxiliary battery” what does that mean? The opposite of what we are seeing. Preventing sulfation means keeping the battery fully charged and at near constant voltage I think has been said. Like the purple line on my truck battery monitor. Like Trollbait said, they abuse the battery. But they also say they don’t want to cause abuse, or sulfation. It is translated from Japanese to English I presume. Constant voltage control would mean keeping the battery at 12.7 or above to be fully charged.

 

Not to get into arguing, but I think Toyota puts new fully charged batteries in the cars. They take a special fuse out for shipping just to not discharge the 12 v. Where is this coming from they put not fully charged batteries into new cars on purpose?

 

I'll answer the points one by one.

The 20-hr discharge rate (C/20 C-rate) when the Ah-capacity spec is measured is standard for any car battery. That way, you have a meaningful comparison of different car batteries.

Constant charging is also bad for the battery, as well as not fully charging it frequently. Ideally, the system needs to start the charging to bring the SOC back to 100% when it drops to below a certain level like 80% or perhaps 75%. However, in Toyota's case, at least in Gen 5 but also possibly in Gen 4, the battery seems to rarely see 100%, which is a problem that is reducing the battery life and causing very early failures as well, especially in cases where the parasitic drain is high or the battery is already coming with a low SOC.

Constant-voltage charging means charging at a constant 14.4 V or 14.5 V for a flooded-cell battery and 14.7 V or 14.8 V for an AGM battery, while setting a current limit to prevent battery damage. This is as opposed to constant-current charging, in which the current is kept constant instead of the voltage and a voltage limit is set to prevent overcharging.

Ideally, the battery needs to be at 100% SOC when it is first installed in the factory, not 80%, not 90%, not even 99%. Charging a battery to 100% SOC could take over 24 hours if not 48 hours the first time because the final part will be very, very slow, as you need to reduce the current to prevent overcharging while ensuring that the battery reaches its true capacity and it is truly optimized. I understand this is not practical in a factory, but you can do it at home using a battery charger when you replace an out-of-warranty battery. Even the 1-A Noco Genius 1 would be sufficient for that purpose.

 

First paragraph, I think it may be the translator was not up on things.

Last paragraph, Toyota is skilled enough to recharge the batteries they buy. They buy charged batteries from the makers I don’t think they manufacture them. I mean I am smart enough to recharge my car battery. Even if they buy dry cells to prepare, they are capable enough to handle it.

 

Are you talking about translation from Japanese to English? What do you think the translation error in the sentence “Constant voltage control is performed and the auxiliary battery is fully charged every 20 driving hours to minimize deterioration of the electrode plates due to sulfation by repeating charge/discharge of the auxiliary battery” is? The battery Ah capacity is not mentioned in that sentence at all.

 

You keep thinking I have the Japanese transcript, and I am fluent in Japanese. You do see 20 and hours is mentioned? You can’t see someone mixing up the translation? I can but it’s not that I know, as you present, it’s just a suggestion as I can’t make sense of the 20 hours at all.

 

It's a very meandering piece of text, possibly due to translation. In the attempt to parse the individual bits, it's easy to lose sight of the overall structure.

Here's an attempt to summarise it:

a) Basic control is power minimising, doesn't aim to fully charge, varies voltage aiming for target SoC. It uses this mode most of the time if it can.

b) The car instead uses constant voltage:

1) For a while after every switch to READY
2) If any loads might become unstable (detected how? Maybe just manual request, eg during reflashing?)
3) Every 20 hours.
4) If the SoC is low
5) If the battery temperature is high or low
​

So it's not trying to keep the battery fully charged generally, but does so periodically. And it also uses the constant voltage to top it up if low.

 

It is both bad technical writing and bad written English (could be AI translation), but it is not that difficult to decipher.

Third day with new car, dead 12V battery, second day in a row | Page 11 | PriusChat

The key sentences are the first and last ones (both with really bad written English/AI translation):

------------------------------------------------------------
(a) Basic Control: (1) Controls the output voltage of the hybrid vehicle converter assembly (DC-DC converter) in accordance with changes in the target SOC to change the SOC of the auxiliary battery to reduce the power consumption.

…

(a) Power Consumption Reduction
…
(3) Because the electricity is the product of the voltage and current, the power consumption is reduced so as to lower the output voltage.
------------------------------------------------------------

All this means is that the basic control tries to lower the output voltage of the DC–DC converter as much as possible in order to reduce the power loss in the 12-V system and improve the fuel economy. Make no mistake: The power loss in the 12-V system is huge!!! So, it is understandable that Toyota is going at great lengths to minimize it.

In the constant-voltage operation, the output of the DC–DC converter is 14.40 V as far as I can tell from @Mr.Vanvandenburg's data. This charges the battery faster and avoids unstable operation of the 12-V subsystems caused by the reduced DC–DC-converter output voltage when the basic control with a voltage much less than 14.40 V is being used.

And it is also obvious from the text that you cannot count on the system to charge the 12-V battery to 100% SOC more frequently than once in every 20 driving hours.

It took Noco Genius 5 about 42 hours to fully charge the new AGM battery I bought. The reason why it takes so long is that it finishes the last phase of the charging through constant-current charging and stops when 14.80 V is reached at the battery. Constant-current charging with the 14.80-V voltage limit ensures that the AGM battery is not overcharged but yet fully charged and optimized. I am also getting the BM2 battery monitor @Mr.Vanvandenburg suggested, and I will see what is going on. Ideally, you want a current sensor as well (as in the OEM battery sensor), but oh, well…

I will report the findings on the AGM battery in a separate thread; so, stay tuned.

 

That is much better than just every 20 hours as originally posted.
This behavior won't fully charge a battery already low to start, but that's been the case with car charging systems for some time.

 

From those rules, it presumably it would immediately charge a low battery up to its idea of a "reasonable level", but not all the way to full.

 

Assuming the trip is long enough to reach that reasonable level, while also running everything on the 12V grid.

 

I never said “charged only every 20 driving hours.” That would make no sense at all. I said “fully charged only every 20 driving hours,” which is correct.

The idea by Toyota is to minimize the power loss in the 12-V system, which is powered by the DC–DC converter. In order to do this, the DC–DC-converter output voltage is kept at a minimum—slightly over 13 V—and it is raised only enough to keep the battery SOC between, say, 75–85%.

Occasionally, the the DC–DC-converter output voltage is raised to 14.4 V, which is called the “constant-voltage operation and suspension of the basic (voltage) control.” This is done briefly at vehicle power-on, when the battery is too cold or too hot, when the 12-V-system load is high (potential instability), and when the 12-V-battery SOC is too low.

And the voltage is raised to 14.4 V (constant-voltage operation) and kept at 14.4 V only once in every 20 driving hours to fully charge the 12-V battery to 100% SOC.

Nevertheless, the 12-V-battery health takes a back seat to fuel efficiency, as the 12-V battery is significantly away from 100% SOC for most of the time. Running at a reduced charge such as 75% further increases the chances of 12-V battery failure from parasitic drain, as well as permanent battery degradation from such failures.

I hope this finally clears things once and for all.