Hopefully, ultimate 12v battery charging system discussion on PP

I am starting yet another new thread to discuss the behavior of PP on the aux battery (aka 12v battery) charging system. This topic has been covered in numerous threads, and we have a fairly good idea that PP and to some extent regular Gen4 Prius have some parasitic draw even when the car is off and cause the 12v battery to discharge which often requires an owner to replace the dead or weak 12v battery.

The most recent such thread is this: HELP - 12v battery on backorder
More extensive discussion on the same subject is found in this thread: 12v Battery Is Discharging, Re-Charge Now !
And, this thread is useful for examination of the draw on the 12v system: Parasitic draw & various other draws on 12v system...

In this write-up, I will try to summarise my finding on how PP charges the 12v battery.

The system I used:

• Car: 2021 PP Ltd for new data and 2020 PP LE for some previous historical data
• 12v Battery: OEM lead-acid Yuasa 345LN1-MF with 20HR 45Ah CCA 295A spec
• Monitor: Battery Monitor II (BM2) onboard BueTooth battery monitor accuracy checked with Fluke87
• App: BMII or JMP Battmon II. You can read about the monitor and app on this thread.

What makes the BM2 very useful is that I can monitor the voltage of the 12v battery at the post 24/7 without ever touching the car. As many of you know, just opening the door of the PP will activate the brake actuator which draws quite a bit of current. The monitor store the data read every 2min up to 31 days, and the app comes with a monitor that can store the data indefinitely. It also allows the exporting of a daily data file to a spreadsheet for further analysis.

My observations on changing the 12v battery:
READY MODE: The PP charges the 12v battery at ~14.4v while the car is in READY mode (no need to be driving or engine starting). The DC-to-DC converter charges the 12v battery as needed.

In a few experiments, it was shown that parking PP in READY mode for 3hrs continuously charged the 12v battery to full while 4.5 hours of highway drive did not fully charge the 12v battery.
See: 12v Battery Is Discharging, Re-Charge Now ! | Page 17 | PriusChat

EVSE Charging: While the PP is plugged in with EVSE, it seems to have two levels for charging. The first is ~13.5v charging which occurs only during the time the traction battery is actively being charged. I have never seen this active charging occurring outside of the period of the traction battery is being charged, even if the EVSE is kept plugged in after the traction battery charging has finished.



EVSE Maintaining: The second is ~12.7v maintainer mode charging that just keeps the 12v battery at near a full level without actively bringing the SoC any higher. This too only happens during the time the traction battery is actively being charged. I explained this observation in this thread.



EVSE Checking: There is one more mode that seems to check the voltage of the 12v battery and charging at 13.5v starts, but immediately stops and switches back to the maintainer mode at ~12.7v. This one is hard to catch and I don't know the reason why sometimes PP does this not all the time. @PiPLosAngeles observed a similar phenomenon and documented it with a video.



At moment, those observations are made only once or a few times at most. But there seem to be some underlying conditions that control what happens to the 12v charging system, but it is not a simple on or off at any given instance. One question I have not solved yet is that what causes the PP to change the behavior of the 12v battery charging system. With the same 12.38v resting voltage of the 12v battery, I have seen PP does either active charge at 13.5v or maintainer charge at 12.7v when the EVSE is connected and starts charging the traction battery.

Certainly, if the 12v battery is healthy and fully charged at above 12.6v, it seems PP preferentially keeps it in maintainer mode. But as the 12v battery loses its capacity, and drops below 12.4v, it sometimes triggers the active charging at 13.5v, but it is not always the case.

 

Adding another recent 12v battery discussion thread link: FOB is dead -- how do I roll up the windows? | PriusChat

NOTE: In this thread, OP thought the FOB had a problem, but it is most likely caused by a dead 12v battery causing the fob and other components to not work.

 

Don't know the real frequency on Gen4/PP, but still see the dead 12v battery posts often enough to think it is a real problem. However, this may not be just for Gen4/PP. I just saw a YouTube videos on a dead 12v battery causing NEW KiaEV6 and Ioniq 5 and Mach-E. So, it is a universal problem in today's modern cars with a lot and a lot of electronics?

 

I am sure many are COVID-related issues of not driving often enough on older cars. But for the newer cars like KiaEV6 and Ioniq 5 and Mach-E, I feel there are more reasons to believe excessive parasitic draw is causing the battery to drain soon after the purchase.

For example: Some Mach-E's are getting dead 12V batteries from bad software | Mach-E Forum | Ford Mustang Mach-E Forum and News
Disappointed | Kia EV6 Forum
Ioniq 5 - 12V Battery dead - Topic ... again | Hyundai IONIQ Forum

 

Point well taken. Certainly, I have experienced the "dead battery" issue many times in our cars. But they were all age and temperature-related issues. They were all "On one of those bitterly cold winter mornings, the car would not start the engine" issues. But those are fairly simple to know what is going on and how to fix. Today's modern cars have all sorts of electronics depending on the healthy 12v battery to function properly.

Incidentally, I had to replace an 8-year-old OEM 12v battery in our 2014 Nissan Pathfinder Hybrid a few months ago. It was a classical case of a cold morning (-26F to be precise) battery issue. This car, especially now with a very high gas price, tends to sit on our driveway undriven for a long time. I just checked how the battery is holding after replacing it with an aftermarket O'reilly's AGM battery.



The battery was new and fully charged and installed on Jan 24. The car was driven several days in Feb and had not been driven since then until 3/26 for a full month. The battery charge dropped only to 12.49v. Yeah, I am going to charge it again soon, but it is holding the charge far better than the OEM flooded lead-acid 12v battery in my newer PP. So, the model of the car and the make (or type) of the battery do matter.

 

I don't know how Tesla resolved their 12v issue, but Mach-E cases were fixed by a software update. I don't know about Kia and Hyundai. If the underlying cause of the 12v battery drain is obvious software or hardware issues, then manufacturers are likely to take an action. But if it is a common components working properly but drawing more than the old gold standard of 50mA, then the manufacturer is not going to do anything about it other than replacing the battery on-demand, if such a claim comes through warranty write-up.

 

The one I purchased for our PathHy is BCI Group 51R size. I am not sure if the CR rating is specific for the 65 or not. I bought AGM because it was available locally for a quick pick-up and the price difference was only ~$20 compared to another regular flooded lead-acid battery which was also available. It is generally stated that an AGM battery does last longer and is more suited for a deep cycle. But the OEM lead-acid also lasted for 8 years, so if it cost a lot more than a regular lead-acid battery, I probably did not buy it. So far, it is holding a charge very well, but then again, any new lead acid may have behaved similarly. I just don't know.

 

Are those OEM batteries for 14-16 Chevrolet Spark EV, and 17-22 Chevrolet Bolt EV? I don't think I will be needing my 12v battery on my PP anytime soon. I may not keep the car that long anyway. But, if I have to replace it in the future, I may give it a try.