12v roundup

jetsam · Feb 13, 2025

chris johnson 2 said: ↑

It must be costing Toyota a fortune with all the call outs and replacements.
Click to expand...

I hope it is, because I had to take a brand new car and add $200 worth of aftermarket stuff to turn it into a vehicle that's likely to start and drive when I want it to. That's just silly.

How many car buyers will choose to charge the 12v themselves and carry a jump pack, and how many will instead choose to go buy a car that works when you turn it on?

Try explaining this issue to someone who owns a regular car to get a better feel for just how silly it all is. "My new car has a 13 kWh battery, but I can't start it because the motorcycle battery in the trunk is dead because Toyota needs to power the 4G modem in the car 24 hours so that the app which usually doesn't work anyway can have a slim chance at remote starting the car on command. Yes, the motorcycle battery can and does charge off of the big battery, but it can only do that if it's already charged."

frederw1701 · Feb 17, 2025 at 5:03 PM

jetsam said: ↑

Mine's under a little cover in the hatchback area, near the rear passenger side wheel. (That's a 2024 Prime.)

If anyone's curious to see some data on this...
Here's a workday with battery events annotated.

View attachment 267663

00:00: The car is parked and on charge. Voltage starts at 12.67v and creeps down to 12.65v.
00:27: The car charges the 12v at 13.59v for about 1h 50m. Then it draws on the battery to do something (??) for 20 minutes. Voltage bonks around in the neighborhood of 12.5v for 20 minutes. (Couldn't you have done this while the traction battery and/or L2 charger was supplying power to the 12v system??)
02:47: The post-charge load stops and voltage goes back up to 12.71v. It slowly drops to 12.69 by....
04:27! I remote start the car, which is barely even visible on the 12v voltmeter.
04:41: I unplug and start the car in EV mode. Car starts charging the battery at 14.4v. (It usually does this, but not always.)
04:55: Stop for coffee. Charging stops.
04:57: Back to driving. Charging resumes at 14.4v. Battery voltage sits at 12.3v, which is funny given that it was at 12.69 before we charged it.
05:00: Arrive at work. Charging stops. 12v is doing something for ~15 minutes and holds 12.3v, gets back up to a resting voltage of 12.6v.
05:51: ??? Unknown low amperage activity, 10 minutes. Spike down to 12.5, recovers back up to 12.62.
08:15: ??? Unknown activity, 10 minutes. Spike down to 12.39v. Sun comes up around this time and the solar starts charging, so we can't see where the battery levels out. From now until dark, it can be hard to tell if the car is applying load to the battery, or if the sun just went behind a cloud.
10:35: Solar charging has pushed the voltage up to 13.24v. I start the car to go to lunch; it charges at 14.4v.
10:41: Arrive at lunch. Car is doing that whirring servo thing it sometimes does after a drive; battery hits 12.16v with the solar panel providing charge.
11:29: Battery has slowly crept up from 12.16v to 12.94v with a few watts of charge being applied; clearly being driven and parked again is hard on the 12v. I start the car; it charges at 14.4v.
11:33: Arrive back at work. Car is doing that whirring servo thing again; battery hits 12.03v while charging. Being driven a short distance and parked again is clearly pretty hard on the 12v. This vehicle just spent 18 hours on L2 charge, was driven 3 times, and had a 20w solar charger providing a few watts of power directly to the 12v all morning, and it's at 12.03v with the car turned off.
12:05: Sitting quietly and under solar charge, the system voltage recovers to 13v. (Bear in mind, this isn't the voltage that the battery would be resting at if it wasn't being charged; the voltage being supplied by the solar charger is driving the voltage up.)
12:23: Unknown small power draw.
14:49: Unknown larger power draw.
16:37: Unknown small power draw.
17:23: 12.83v. Solar charging is little to none at this point. I start the car and drive home. Car charges the battery at 14.4v for the trip.
18:13: I park the car and plug it in. 12v charging is turned off. Battery starts sliding downhill from 13.08v.
20:00: Voltage has slid from 13.08 to 12.90. Unknown power draw spikes it down to 12.63; it recovers back up to 12.89.
21:15: Unknown power draw drags us down to 12.59; recovers to 12.79.
22:45: Unknown power draw drags us down to 12.62; recovers to 12.76.
23:59: Day ends at 12.75v.

...so clearly the car *does* charge the battery sometimes, including sometimes when we'd expect it to (for example, when the car is turned on and driving, it usually applies 14.4v to the 12v system).

Observations:
- The battery drains a little all the time, at a rate faster than you'd see from regular lead acid car battery discharge. It has a small load on it all the time.
- The car puts some decently large, long loads on the battery without bothering to charge it during the loading on the 12v system, and even without checking it afterwards to see if it's okay.
- The car usually charges the 12v at 14.4v when it's in READY, but sometimes not.
- The car infrequently charges the 12v at 13.59v when it's not in READY. It does this when it's in the mood; the 12v system being down in the 11.x volt range doesn't trigger a charge.
- Every problem observed here can be corrected in software. You don't need a team of engineers and a custom AI to analyze a thousand battery parameters. If the 12v system drops below whatever voltage threshold you want to set, charge it from the traction battery. If the traction battery's too low to charge the 12v, tell the driver that they need to charge one or both of their batteries before the car dies.
- Every problem observed here could ALSO be corrected by getting rid of all these loads that we apply to the 12v system when the car is off. Really, Toyota, do we need all that crap to be going on while the car's parked? (And even if we do, could you please put some juice on the 12v bus before you do it? Thanks.)
- As a driver, you should carry a jump pack until they fix the 12v charging logic. If you want your 12v battery to live a long life, you should also have a charging routine for it. (Although if you want the problem fixed faster, skip the charging routine and put in lots of warranty claims for new batteries. This will help bring attention to the problem. The rest of us appreciate your sacrifice. )
Click to expand...

Fascinating! Thank you.

12v roundup

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frederw1701 Junior Member

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12v roundup

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