AGM battery for Gen 4/Gen 5 Prius/Prius Prime and observations on the 12-V charging system

Regarding the aux port voltage, it agrees with the BM voltage and what the scan tool reads. What I was intending to say was that I would put the car in ACC mode by pressing the start button with foot off brake. The voltage I see on the aux port voltmeter is 12.2 to 12.4 volts. This is with battery under a load in ACC mode. Comparing this to the BM voltage gives me a sense of the battery health. Along with actual reading. I monitor the trend. I’m not OCD - I did this type condition monitoring CM in my work career and traing and habits carry into your personal life thinking

Drove the car yesterday but not today. Currently this evening, the battery voltage is 12.61 volts. Yesterday after driving with charge voltage 14.4 volts continuously, the BM voltage after car asleep for several hours was 12.80 volts. Most likely impacted by surface charge reading since BM voltage was 12.64 volts early this morning.

So for a recent 26.6 minute trip with charge voltage 14.4 volts continuously, what I recorded was initial aux battery current started at 3.6 amps and over trip time dropped to 2.4 amps. Using 2.8 amps as an estimated average charge current, only @ 1.25 AH is put back into the battery (2.8 amps x .44 hours). This is the longest trip time typical for me. Often trip times less than 15 minutes. Thus even at 14.4 volts with my trip times, I doubt the battery would ever reach a true 100% SOC. Not a concern for me personally with this AGM battery.

Now with the OEM battery, I would have a concern. Frankly, I don’t understand the Toyota statement of a full charge every 20 driving hours to prevent sulfation. How does that work if I drive a few short trips and not drive for a few days ? Maybe a clue as to why the OEM battery has reports of discharged state…..

I may decide to do a full charge of the AGM battery once a month to improve battery life. It’s not a big effort plus I do that already with my truck AGM battery since my trips are same pattern as the Prius. 3 years from a battery is OK for me and I understand others feel different. I still have the OEM, it’s tests good and I keep it charged so I do have a backup of sort.

Cheers

 

For the past several weeks, temperatures have been in the 30’s and low 40 range. During this period, the aux battery charging voltage has been a constant 14.4 volts. Morning battery voltage typically 12.6X volts per the BM.

Chinook winds today with warmer temperatures. On 4th trip this morning, temperature at 58, the charging voltage dropped down to 12.8 volts. Temperature hit 62 this afternoon. All additional trips initial charge 14.4 volts but decreased to 12.8 volts within a few minutes returning to pattern I have observed prior to the recent drop in temperature. So the takeaway would be the outside air temperature has an effect on the charging voltage.

Cheers

 

My car has no added accessories. I charged the traction battery yesterday evening and then unplugged the charge cable. This morning the aux battery voltage was 12.83 volts per the BM before first drive. Outside temperature 44F per car reading. 15 minute trip the charge voltage was 14.4 volts continuous. Return trip 13 minutes charge voltage 14.4 volts continuous.

Cheers

 

Battery voltage was 12.82 volts after drives this morning. Trip 3 this afternoon at 47F showed 14.4 volts for 15 of the 19 minute journey at which point it decreased to 12.8 volts. Return trip voltage went to 12.8 volts within a minute of starting car. Hour after arrival, current battery voltage 12.79 volts.

Tomorrow morning temperatures forecast in the 30’s. It will be interesting to see if the charging system stays at 14.4 volts as was the case for the 2 week period where temperatures struggled to hit 40 F.

Cheers

 

Made 2 trips Friday about 10 minutes each way. Temperature 44F. Charging voltage constant 14.4 volts both trips.

This morning voltage was 12.73 per BM prior to drive. Made 2 trips about 18 minutes each. Temperature 39F. Charging voltage 14.4 constant both trips.

Cheers

 

Well it's not happening from the head unit, that gets 0 power when the car is off.

So then it has to be through the DCM which stays powered and has its own small internal battery. There's a fuse to disable that if you don't use any features it gives.

 

I got my Gen 5 about a year ago, and I had a lot of trouble at first with my 12V battery going flat and being unable to start my car, sometimes this would occur over a few days, and sometimes within a few hours. I reported the problem to my dealer, and twice they said "it tested OK." I took it to a second dealer, and they replaced it under warranty, but the problem persisted. At that point, I suspected high parasitic drain, and I was able to measure 330 mA, but it varied considerably over time.

I purchased a 100Ah Li-ion auxiliary battery for it, and wired it in parallel with the standard battery. I installed a "smart battery isolator" which cuts out at 13.8V to prevent the auxiliary battery from draining along with the main 12V battery. They are connected by a heavy 10 gauge wire, with a heavy bypass switch to "boost" the main battery if needed (although I have never needed to do this).

I have a combination ammeter/voltmeter at each battery, which shows me the state of charge, and how much current it being transferred.

After almost a year, I can say that I haven't had a dead battery since.

 

2 batteries in parallel ? Is this message meant to convey that this's what it takes to stop / overcome parasitic loss? Far from encouraging.
That said - the same thing was necessary when I started running front & rear & side dash cams on our 2018 phev.
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Well, I'm encouraged by the fact that I haven't had a dead battery since I installed the aux battery.

 

I may have been concerned at first, but the way I have it installed, I've had no issues for at least 9 months. When I had the car in for service, the dealer had no issues with it either.

 

What's the basis of your claim that an aux battery is unsafe? I have wired it properly and protected the connection with a fuse. The dealer inspected my setup and had no issue with it. The parasitic drain I experienced was variable and unpredictable. The aux battery seems to have solved the problem of having a dead battery.

 

I've only owned my 2020 Prime for 3 months. It is 5.16 years old and has 56,000 miles. My wife was stranded shortly after we bought it because it wouldn't start without a jump. Since then it seems to have responded well to CTEK trickle charging and the problem has not reoccurred. I attached the voltage reading since December 15th as well as the reading from today. The voltage seems to jump at the beginning of a trip and drop quickly afterwards as shown in the second chart.

The lowest levels have been around 12.4 a few times and the garage is usually around 50 degrees. The state of charge table I have indicates about 75%. An AGM battery equivalent looks like it would be around 12.56 or so or not far from your 12.61 resting voltage on your new AGM.

I drive about 98% in EV mode. Perhaps I'm missing something on the battery monitor readouts but my plan is to keep trickle charging every week or so and keep an eye on it.