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Discussion Starter · #1 ·
So, folks, help me out here - I'm just a simple-minded PhD in Electrical Engineering, but there must be something I'm missing.

The Volt never lets the main (Lithium) battery go below 15% or so (I don't know the precise number, so I'm making it up here). It's about 360V (nominal).

It's relatively easy to make a flea-power DC to DC converter that could supply the 12V needs of a parked Volt (the keyless remote circuitry). Once the door is opened, the car could easily switch to the existing 360V to 12V converter.

It's also easy to make that flea-powered converter disconnect if the main battery gets too low (to keep it safe). If this happens, then the car could be jumped by applying 12V to the terminal under the hood (and a jumping terminal could be put in the trunk).

So, why does the Volt use a "traditional" AGM 12V battery? It seems to me that's a way to save about 50 pounds and maybe $50. I bet it would only take a simple 360V to 12V flea-powered converter and some reprogramming of existing computers to turn on the main converter when needed.
 

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My guess is for the cold weather current capability of the lead acid, Li-ion doesn't work well at extremely low temperatures. I have thought the same myself though, there has got to be a way to get rid of that legacy baggage. Maybe the 12V system was the easiest from design perspective to isolate the ICE with using existing parts as I imagine different design teams designed each system. Come to think of it, doesn't the Li-ion provide the cranking for the motor anyway? So it really does seem unnecessary to have the lead acid.
 

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Because the automobile safety laws require that that there be a positive automatic disconnect for the high voltage battery for first responder safety. The biggest reason for the 12 V battery is to close the high voltage battery contactors. The Tesla also has a low voltage (12V) battery for the same purpose.
 

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Because the automobile safety laws require that that there be a positive automatic disconnect for the high voltage battery for first responder safety. The biggest reason for the 12 V battery is to close the high voltage battery contactors. The Tesla also has a low voltage (12V) battery for the same purpose.
Why does this safety requirement preclude the design suggested by the OP? What am I missing? The same signal that would trigger the opening of the contactors would work whether or not the relays were powered by the traction battery or the aux battery, as I see it.
 

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Why does this safety requirement preclude the design suggested by the OP? What am I missing? The same signal that would trigger the opening of the contactors would work whether or not the relays were powered by the traction battery or the aux battery, as I see it.
Ignition and other "switching" components are low voltage and will be for the foreseeable future.

Switching 360v via relay is well, not that good of an idea, likewise having 12v power is like having an emergency backup that allows basic operation of the car should something brick the HV side.

So why is LV left in the car?
1. Reliability
2. Emergency situations
3. Existing component selections that are standard between varients
4. Safety

We could go into this discussion but alas,

the don't worry about it they know better than you answer is probably as good as any.
 

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Discussion Starter · #6 ·
So why is LV left in the car?
1. Reliability
2. Emergency situations
3. Existing component selections that are standard between variants
4. Safety
I can see 1, 2 & 4 - 12V batteries fail gracefully - they almost never go from supplying 12V to supplying nothing. #3, well, the Volt has LOTS of parts that aren't common for the Chevy line-up.

I know other EVs and hybrids have 12V batteries also. Anyone know if the Tesla uses a 12V battery, or did Elon figure out how to drop it?
 

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I can see 1, 2 & 4 - 12V batteries fail gracefully - they almost never go from supplying 12V to supplying nothing. #3, well, the Volt has LOTS of parts that aren't common for the Chevy line-up.

I know other EVs and hybrids have 12V batteries also. Anyone know if the Tesla uses a 12V battery, or did Elon figure out how to drop it?
Tesla uses a 12 V battery - actually a small motorcycle sized one, in the passenger side front fender on RWD cars and just behind the frunk on AWD cars.

Problems with that battery and/or the contactors for the main pack are their most common stranding failure (there was a batch of bad batteries from the vendor...)
 

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Tesla uses a 12 V battery - actually a small motorcycle sized one, in the passenger side front fender on RWD cars and just behind the frunk on AWD cars.

Problems with that battery and/or the contactors for the main pack are their most common stranding failure (there was a batch of bad batteries from the vendor...)
Yup, since I'm out shopping for a CPO Tesla I needed to know if a Tesla has a small 12V battery, where it's located, failure rate, what happens when it fails and how to jump start it.

And yes, you'd think a car with an 85kW battery wouldn't need one but it does, but since it is fed/charged from the main battery how could it ever go dead needing a jump?
 

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Yup, since I'm out shopping for a CPO Tesla I needed to know if a Tesla has a small 12V battery, where it's located, failure rate, what happens when it fails and how to jump start it.

And yes, you'd think a car with an 85kW battery wouldn't need one but it does, but since it is fed/charged from the main battery how could it ever go dead needing a jump?
Usually that happens after the computers decide they need to open the contactors. I'm not sure of all the circumstances, but those pull over now cases seem to only have about fifteen minutes after the contactors open before the battery dies completely - so they have generally had to be jumped to get them out of gear to put them on the truck - but I think that is also tied to damaged 12 V batteries.
 

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I've always wondered this myself. It seems to me that a Dewalt 12v lithium battery would be sufficient for the 'keep 12v' reasons mentioned. That only weighs about 1/2 pound.

If I still have the ELR when the 12v finally croaks, I'll probably get one of those lithium-based replacements.
 

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Ignition and other "switching" components are low voltage and will be for the foreseeable future.

Switching 360v via relay is well, not that good of an idea, likewise having 12v power is like having an emergency backup that allows basic operation of the car should something brick the HV side.

So why is LV left in the car?
1. Reliability
2. Emergency situations
3. Existing component selections that are standard between varients
4. Safety

We could go into this discussion but alas,

the don't worry about it they know better than you answer is probably as good as any.
I was implying that the OP's circuitry would be connected to the traction battery, with it providing 12 volts to the contactor relays when signaled to do so, as opposed to having an aux 12 volt battery doing so.


I understand about switching high voltages - big, long sparks.

But the OP was posing reducing that high voltage to 12 volts as follows:

"It's relatively easy to make a flea-power DC to DC converter that could supply the 12V needs of a parked Volt (the keyless remote circuitry). Once the door is opened, the car could easily switch to the existing 360V to 12V converter.

It's also easy to make that flea-powered converter disconnect if the main battery gets too low (to keep it safe). If this happens, then the car could be jumped by applying 12V to the terminal under the hood (and a jumping terminal could be put in the trunk)."

The jumping would occur with a 12 volt battery in another car.

How would such an arrangement be in violation of good sense or law? It would save weight and reduce cost.
 

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The rules require that it be a separate low voltage source from the high voltage battery such that first responders just cut the cable leads from the low voltage source and are certain the high voltage battery is completely isolated. It does not have to a be a lead-acid or any other type of low voltage source. A large low voltage capacitor circuit could work for that single task. That a lead acid battery was chosen was because they are cheap, reliable and carry sufficient charge to operate a host of 12 volt accessories.

We'll see how soon Tesla changes over to li-ion or other solid chemistry for the low voltage source and then the others will start to follow.
 

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I was implying that the OP's circuitry would be connected to the traction battery, with it providing 12 volts to the contactor relays when signaled to do so, as opposed to having an aux 12 volt battery doing so.


I understand about switching high voltages - big, long sparks.

But the OP was posing reducing that high voltage to 12 volts as follows:

"It's relatively easy to make a flea-power DC to DC converter that could supply the 12V needs of a parked Volt (the keyless remote circuitry). Once the door is opened, the car could easily switch to the existing 360V to 12V converter.

It's also easy to make that flea-powered converter disconnect if the main battery gets too low (to keep it safe). If this happens, then the car could be jumped by applying 12V to the terminal under the hood (and a jumping terminal could be put in the trunk)."

The jumping would occur with a 12 volt battery in another car.

How would such an arrangement be in violation of good sense or law? It would save weight and reduce cost.
In this scheme, what is stabilizing the 12V circuit voltage? As things turn on and off, the power and current needed at 12V varies a great deal.

Most DC-DC converters won't be able to react fast enough (microseconds) - so without substantial capacitance in the circuit, the voltage will shoot up and down - bad for electronics and possibly enough to cause things to switch off and cascade the voltage drop.

The 12V battery provides this capacitance - and allows the car to sit with the main contactors closed when it's off, reducing the risks of something bad happening.
 

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My guess is for the cold weather current capability of the lead acid, Li-ion doesn't work well at extremely low temperatures. I have thought the same myself though, there has got to be a way to get rid of that legacy baggage. Maybe the 12V system was the easiest from design perspective to isolate the ICE with using existing parts as I imagine different design teams designed each system. Come to think of it, doesn't the Li-ion provide the cranking for the motor anyway? So it really does seem unnecessary to have the lead acid.
This is a joke, right?
 

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Important to note that the supplemental inflatable restraint (SIR) system equipped in the Volt is fed voltage from the traditional 12V battery exclusively and NOT the (A4) Hybrid/EV Battery Pack for safety/reliability reasons most likely!?:confused:
 

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I don't think you could eliminate it entirely.
But a better question is why is it still a large, heavy lead acid when it could be light lithium ion or ultracapacitors, the latter of which would never really wear out.
It's not doing any heavy lifting, just there to start up the computer and wake up the main battery.
 

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In this scheme, what is stabilizing the 12V circuit voltage? As things turn on and off, the power and current needed at 12V varies a great deal.

Most DC-DC converters won't be able to react fast enough (microseconds) - so without substantial capacitance in the circuit, the voltage will shoot up and down - bad for electronics and possibly enough to cause things to switch off and cascade the voltage drop.

The 12V battery provides this capacitance - and allows the car to sit with the main contactors closed when it's off, reducing the risks of something bad happening.
Read this post.
It has most all of the answers as to why there still needs to be a battery participating on the 12V rail.0
There's also various "keep alive" elements that require a constant power supply.

"Just smile and drive" comes to mind.
;)
lol

WOT
 
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