12V battery - what circuits does it power?

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gwscheil

Member
Joined
Oct 10, 2022
Messages
15
Location
Kansas City, MO
Does anyone know exactly what the 12V battery powers? I assume climate control, headlights, would kill it fast. So essentially what is powered from the main battery via the DC converter vs the 12V one?
 
It would be easier to say what the high voltage system powers - everything else is the 12 volt system. So, drive motor and associated components, DC-DC converter, and the HVAC system but NOT the blower. Those are powered by the high voltage "traction" battery. When you plug in an EVSE (charging cable or station) nothing changes. The EVSE feeds power to the onboard charger (except in DC Fast Charge), and the traction battery still powers everything it usually powers. It's easy to think of the EVSE as "shore power" but it always just provides power to the built-in onboard charger. Actually the DC-DC Converter, which provides 12-14 volts for the low voltage system, probably uses low voltage as well.
 
Once the power is ON and system is ready to roll - does the 12V battery still supply power to anything while driving? Or does it need just enough to maintain idle standbypower, then unlock doors and power up the main computer and display? I suspect lazy engineers may have used the 12V battery as a large "capacitor" on the DC downconverter - thus a dead 12V becomes a dead Leaf? I remember my parents Prius had frequent problems with that tiny 12V battery it used.
 
gwscheil said:
...So essentially what is powered from the main battery via the DC converter vs the 12V one?

If I understand your question correctly, the answer is "nothing". The power for every 12V component is via the 12V battery terminals. Think of the DC/DC converter as the alternator in an ICE vehicle. Nothing is powered directly from the alternator (except charging the 12V).
 
gwscheil said:
Once the power is ON and system is ready to roll - does the 12V battery still supply power to anything while driving? Or does it need just enough to maintain idle standbypower, then unlock doors and power up the main computer and display? I suspect lazy engineers may have used the 12V battery as a large "capacitor" on the DC downconverter - thus a dead 12V becomes a dead Leaf? I remember my parents Prius had frequent problems with that tiny 12V battery it used.

The DC-DC converter does provide the 12+ volts used by most accessory systems while the car is in Ready Mode. The 12 volt battery is used more as a "battery" than as a "capacitor" because it is expected to power logic and other system drains while the car is Off.
 
Hmmm. Not sure I'm following here.

I thought the 12v battery is there to run the control electronics and accessories while the car is powered off. In Ready mode, I would expect the DC-DC converter to effectively power everything on the 12v system, just like the alternator does in a traditional car.

It's all tied together. While the DC-DC converter supplies power to the battery, it is also supplying 12v power to all of the systems tied to the battery, unless there are blocking diodes in those circuits somewhere. I can't imagine why there would be though. Which is to say, the battery is in parallel with the DC-DC converter on those circuits. If the battery become disconnected (say, a due to a lose connection) while in Ready mode, nothing should happen until you turn the car off.

No?

In a traditional car with a flat battery that won't take a charge, you can jump the car to get it running off the alternator and drive to a battery store to replace the battery. Likewise, with a dead alternator and a fully charged battery, you can drive to the auto parts store to buy an alternator, unless you run out of battery on the way.

Edit: One notable exception to the above is our Z4, which misbehaves badly with a bad battery. Misfires, won't run, loses power. I've always thought that a bit ridiculous. There's nothing wrong with the alternator. Charge the battery, and all is fine, and the battery stays charged. Go figure.
 
I'm with frontrangeleaf on this one. I would say the alternator (or DC-DC) converter is the main 12V power source while the car is running. The 12V battery is there just to start the car and to keep alive any circuits (locks etc) that need to be active while the car is 'off'.
 
I understood the original question to be asking what accessories were 12 volt, and which ones ran on high voltage. Looking at it again, though, I guess it was asking what the 12 volt battery powered instead of the DC-DC converter.
 
So, has the ultimate proof been done? Jump start a dead battery then drive to the battery store? I assume the Leaf can be powered up before disconnecting the jumper.... and not die as soon as the jumper is removed? Or does the battery sensing line force a shutdown?
 
If the battery will still absorb current, then the car will drive, even if the battery can't restart it. If the battery is so dead that it acts as a giant resistor, though, the car won't run.
 
LeftieBiker said:
If the battery will still absorb current, then the car will drive, even if the battery can't restart it. If the battery is so dead that it acts as a giant resistor, though, the car won't run.

That makes sense. I suppose that might be the case with our Z4 - perhaps the alternator can't keep up? Although the battery recovered after being recharged and has been fine since.

I also have that car on a battery maintainer now, along with our other ICE car, since we don't drive either that often.
 
If the Z4's battery acts anywhere near normal, it isn't dead as the proverbial doornail. It has to be completely, irretrievably* DEAD to prevent the car from running.

* I saw some videos on YouTube last week. Apparently, you can use an electric welder to revive and recondition really dead batteries. It's one of those "Are you feeling LUCKY, PUNK?" scenarios, though.
 
* I saw some videos on YouTube last week. Apparently, you can use an electric welder to revive and recondition really dead batteries. It's one of those "Are you feeling LUCKY, PUNK?" scenarios, though.

"Hold my beer" comes to mind.....Gee, it's only a few hundred amps going though a defunct battery full of acid. What could possibly go wrong?
 
Got LeafSpyLite to finally connect. With power on, my possibly OEM battery (original?) [Leaf bought in October this year} is showing 13.1V, less than 0.1A charging,with occasional boosts to 14.4V at 4A dropping fast to about 1A when voltage starts dropping toward 13.1V. Seems Ok.
after power off, voltage drops to 12.25V, with drain (negative sign) of about 1A! Can the OBD be drawing that much? Is 12.25V too low in power off state (no charging)? Pulled the OBD, manual VOM still shows 12.25V. With the L1 charger connected, voltage is always 13.1V. All these measurements done today with SOC of 80%, 55mv cell range difference, battery temp 43F. The range meter has definitely shifted from mild weather 4miles/kwh to winter 2.5 - 3. The battery has not yet been charged above 92% since I bought it.
 
gwscheil said:
after power off, voltage drops to 12.25V, with drain (negative sign) of about 1A! Can the OBD be drawing that much?
That won't be the OBD II dongle. There are many computers in the car that draw power, and after you power down and close the last door, the draw will be 1-5A for 5-15 minutes. The 5A doesn't last long, it should be under 2.5A after 10-30 seconds. After the 5-15 minutes, the drain should be of the order of 50mA, difficult to measure with a clamp meter. Especially with their offset and drift. These are all observations from my 2012 model. Later models may well be different.

So I suspect that you weren't waiting long enough, or the computers had some reason not to go to sleep, like a door was open or the charge plug is in. Or some other auxiliary battery load, like a camera.
 
coulomb said:
gwscheil said:
after power off, voltage drops to 12.25V, with drain (negative sign) of about 1A! Can the OBD be drawing that much?
That won't be the OBD II dongle. There are many computers in the car that draw power, and after you power down and close the last door, the draw will be 1-5A for 5-15 minutes. The 5A doesn't last long, it should be under 2.5A after 10-30 seconds. After the 5-15 minutes, the drain should be of the order of 50mA, difficult to measure with a clamp meter. Especially with their offset and drift. These are all observations from my 2012 model. Later models may well be different.

So I suspect that you weren't waiting long enough, or the computers had some reason not to go to sleep, like a door was open or the charge plug is in. Or some other auxiliary battery load, like a camera.

Not really! You need to spend a little extra and buy a good one, and learn how to properly use one,
e.g. zero the meter & attach it.
 
So what is drawing that 1A+ for a few minutes after power off? No phone home feature in my Leaf. Amazon will deliver a switchable OBD extension this weekend. The key fob system? As long as it detects a FOB in range, it must be ready to lock/unlock doors. With no FOB in range is it only checking intermittently?
Winter weather is limiting my outdoor test time and my clamp type meter is borrowed at present across town. Or do the computers just take that long to shut down like my PC?

I am in very slow test mode here with an early winter - only an occasional near normal temperature day.
 
gwscheil said:
So what is drawing that 1A+ for a few minutes after power off?
I don't know. Not the key fob system, that has to be on all the time. There is the courtesy light, but it can't be just that. Maybe it's checking for the need to power fans and/or cooling pumps in case the power delivery system, on-board charger, etc need cooling. After 5-10 minutes if there was no need, then cooling won't be needed and the car computers can properly sleep. Likely not needed in your present weather, but the car doesn't take chances.
 
gwscheil said:
So what is drawing that 1A+ for a few minutes after power off? No phone home feature in my Leaf. Amazon will deliver a switchable OBD extension this weekend. The key fob system? As long as it detects a FOB in range, it must be ready to lock/unlock doors. With no FOB in range is it only checking intermittently?
Winter weather is limiting my outdoor test time and my clamp type meter is borrowed at present across town. Or do the computers just take that long to shut down like my PC?

I am in very slow test mode here with an early winter - only an occasional near normal temperature day.

Some vehicles, e.g. BMW,Mercedes, can take as long 3-5 minutes before sleep mode is entered after the vehicle is locked.
Make sure nothing is plugged into the diagnostic port, and the hood latch is set if the hood is open.
 
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