12 V battery - why?

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dunkinb

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Joined
Apr 10, 2015
Messages
11
Location
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It seems that the 12 V battery is one of the major sources of problems in the Leaf, and even if it is never discharged unexpectedly, it still has to be replaced every few years. I'm puzzled as to why this nexus of failure modes and maintenance hassles is necessary at all. Why can't the 12-V systems be served by a DC-DC converter? Design inertia? Some obscure regulatory issue?

I remember years ago being surprised when a friend told me it was time for a new 12 V battery in his Prius, and at the time I just considered it a curiosity to be filed away for later consideration. With a new-to-me Leaf in the driveway, this seems like a good time to consider it.

I imagine this has come up here from time to time, but the 12-V threads I've looked at have just taken it as a given and are more focused on practical aspects of avoiding discharge, etc. So if you know something about this, share it, and if not, feel free to speculate!
 
Why the 12v battery? The main reason is for keeping the car "responsive" when in "off" mode. It still needs to respond to Carwings requests. It needs to turn on interior lights when a door is opened. And most importantly, to run the computer that checks the safety of the main 400v battery before turning on that power source. They'd rather keep the main 400v battery disconnected when the car is off, for safety reasons.

There are other practical reasons for staying with a standard 12v battery & electrical system. They get to re-use a lot of the 12v electrical systems that were developed for other vehicle applications, resulting in lower development costs & faster time-to-market (remember, Nissan was really pushing hard to have the first mass-market EV). The batteries themselves are ubiquitous and replacements are easy to source.
 
garsh said:
Why the 12v battery? The main reason is for keeping the car "responsive" when in "off" mode. It still needs to respond to Carwings requests. It needs to turn on interior lights when a door is opened. And most importantly, to run the computer that checks the safety of the main 400v battery before turning on that power source. They'd rather keep the main 400v battery disconnected when the car is off, for safety reasons.

There are other practical reasons for staying with a standard 12v battery & electrical system. They get to re-use a lot of the 12v electrical systems that were developed for other vehicle applications, resulting in lower development costs & faster time-to-market (remember, Nissan was really pushing hard to have the first mass-market EV). The batteries themselves are ubiquitous and replacements are easy to source.
Click to expand...

The reason is for safety as you mention, not for backwards compatibility. The same reason why the Prius since the 90's has had to have a 12v battery as well. It flips the relays to connect the HV battery to the rest of the car. Just like in the Leaf, once the main battery is connected, it is powering everything in the car. Either way, all those 12v electrical systems can be re-used as normal.

There is a large push in the auto engineering world to go a 48v battery system instead of 12v for normal ICE vehicles to help with start/stop and things like that. The main ECUs and such will still run off of whatever voltage they need (0.8v, 1.0v, 1.8v, 3.3v, 5.0v, 12.0v, etc) just with a 48v nominal input vs. a 12v nominal input.
 
A number of reasons. You need a long-duration stored reserve of low-voltage power for onboard computers, telematics, alarm system, etc... Modern cars are never completely off. It would be possible to power these devices from the main pack via a DC-DC converter but probably not efficient at those low current draws (milliamps). It also would require that the main pack contactors be on full-time.

Also, sensitive electronics like a "clean" supply of DC. I.e., a steady voltage without any big spikes or ripples. A large 12V storage battery is great for providing steady flat voltage.

As for why 12V at all -- as already mentioned the entire world uses 12V automotive accessories (lamps, wipers, window motors, radios, etc...). It's impractical to design those low-power devices to run off of 400 volts and have live 400 volt circuits cris-crossing the vehicle and terminating inside the cabin space.
 
Thanks for the replies. I guess I need to clarify what I wrote because it looks like you think I'm objecting to 12 V systems in the car. I'm objecting to a 12 V battery as the source of power for the 12 V systems due to unreliabilty and what seems to me to be unneded complexity.

Wouldn't any of the considerations you bring up be addressed by a properly-sized 12 V DC-DC converter connected upstream of the contactors?
 
No, this part of the very first response gave you the answer: you can't have live 400 volts all the time. A DC/DC would require 400 volt input ALL the time, which would be very hazardous.

garsh said:
And most importantly, to run the computer that checks the safety of the main 400v battery before turning on that power source. They'd rather keep the main 400v battery disconnected when the car is off, for safety reasons.
Click to expand...
 
dunkinb said:
Thanks for the replies. I guess I need to clarify what I wrote because it looks like you think I'm objecting to 12 V systems in the car. I'm objecting to a 12 V battery as the source of power for the 12 V systems due to unreliabilty and what seems to me to be unneded complexity.
Click to expand...

To clarify, the 12v battery is only the source of power when the car is off. As soon as the car turns on, it is now being powered by the main traction pack.

dunkinb said:
Wouldn't any of the considerations you bring up be addressed by a properly-sized 12 V DC-DC converter connected upstream of the contactors?
Click to expand...

This is already done. But if you wanted to get rid of the 12v battery altogether, you would need a 400v to 12v SMPS and no safety disconnect. If isolated you are looking at a 15% or so efficiency hit. If not isolated, you are looking at a 1% to 2% efficiency loss. Personally I believe both are acceptable.

The only "downside" I see to having the HV battery as the only battery, is good luck getting a jump start. Since the 12v systems run the charger logic and 12v is the universal jump voltage, if you drain your leaf to death you couldn't get it back without a special external charger unless than 12v SMPS was a buck-boost (can do 400v -> 12v AND 12v -> 400v) which would make the efficiency go even lower. But it seems possible that the boost could be isolated from the buck so that if being boosted it only powers charging circuitry, but not the actual battery charge so that it can be jumped with a 12v charger and then plugged into the EVSE to actually charge back up.
 
keydiver said:
you can't have live 400 volts all the time.
Click to expand...
Depends what you mean by "live". There's 400 V at the battery terminals all the time, right? How does a converter connected to those terminals introduce an unacceptable hazard?
 
2k1Toaster said:
dunkinb said:
Thanks for the replies. I guess I need to clarify what I wrote because it looks like you think I'm objecting to 12 V systems in the car. I'm objecting to a 12 V battery as the source of power for the 12 V systems due to unreliabilty and what seems to me to be unneded complexity.
Click to expand...

To clarify, the 12v battery is only the source of power when the car is off. As soon as the car turns on, it is now being powered by the main traction pack.

dunkinb said:
Wouldn't any of the considerations you bring up be addressed by a properly-sized 12 V DC-DC converter connected upstream of the contactors?
Click to expand...

This is already done. But if you wanted to get rid of the 12v battery altogether, you would need a 400v to 12v SMPS and no safety disconnect. If isolated you are looking at a 15% or so efficiency hit. If not isolated, you are looking at a 1% to 2% efficiency loss. Personally I believe both are acceptable.

The only "downside" I see to having the HV battery as the only battery, is good luck getting a jump start. Since the 12v systems run the charger logic and 12v is the universal jump voltage, if you drain your leaf to death you couldn't get it back without a special external charger unless than 12v SMPS was a buck-boost (can do 400v -> 12v AND 12v -> 400v) which would make the efficiency go even lower. But it seems possible that the boost could be isolated from the buck so that if being boosted it only powers charging circuitry, but not the actual battery charge so that it can be jumped with a 12v charger and then plugged into the EVSE to actually charge back up.
Click to expand...

True- what you say is more accurate.

Many older EVs with low-load 12V systems had no 12V but suffered parasitic drains and of course the design is different as there is less need for larger load capacity. A battery can be a cheaper system and is more practical for a modern car since you would need two DC/DC systems one more efficient for low drain monitoring and contactor engaging the larger to run the main systems. A single larger "always on" DC/DC is going to come at a penalty. You can jump a battery free system with simple 12V posts to trip the main contactor with parallel charging. In all it is can be more complex, unfortunately the LEAF just suffers from a poorly designed 12V charge system. Also as mentioned the pack is not loaded to a DC/DC in any battery free-system. If you don't need car connectivity and remote functions a crank handle on the front would certainly work:)

A proper 12V battery system is not a bad option sans user abuse.
 
dunkinb said:
keydiver said:
you can't have live 400 volts all the time.
Click to expand...
Depends what you mean by "live". There's 400 V at the battery terminals all the time, right? How does a converter connected to those terminals introduce an unacceptable hazard?
Click to expand...
In the case of a significant accident or a malfunction, a contactor INSIDE the sealed pack is opened immediately isolating the car from the HV. If there was no low voltage system the car would immediately be dead, dead, dead. Instead the steering, brakes, lights and other systems keep working long enough to get to the side of the road.
 
dunkinb said:
keydiver said:
you can't have live 400 volts all the time.
Click to expand...
Depends what you mean by "live". There's 400 V at the battery terminals all the time, right? How does a converter connected to those terminals introduce an unacceptable hazard?
Click to expand...
You're in an accident. You design the car to turn off when it senses a collision. You design the car to disconnect the battery from everything else, and in a hard-to-reach location when it's turned off. First responders are somewhat more assured that they're not going to accidentally touch or cut through a cable at 400v.
 
None that I know of. Find under-hood pictures of all the EVs on the market and you'll find a 12V battery.

Yes, even the Tesla.
 
aarond12 said:
None that I know of. Find under-hood pictures of all the EVs on the market and you'll find a 12V battery.

Yes, even the Tesla.
Click to expand...

Who learned their lesson and switched to a deep-cycle AGM. ;)
 
one issue is the one that BMW ran in to many years when they planned to go to a 48 volt electrical system. Due to charging voltage, the dc level on the bus can exceed that for low voltage dc certification and then requires special protections and technician certifications... It became too much of an issue and BMW dropped it.
 
TomT said:
one issue is the one that BMW ran in to many years when they planned to go to a 48 volt electrical system. Due to charging voltage, the dc level on the bus can exceed that for low voltage dc certification and then requires special protections and technician certifications... It became too much of an issue and BMW dropped it.
Click to expand...
Looking around, it appears that it was a 42 volt system, just as current cars are a 14 volt system. This kept the charging voltage around 42 volts, and was a "36 volt" battery. The voltages were all 3 times current values for 12 volt systems.

See http://www.popularmechanics.com/cars/a2198/4226979/" onclick="window.open(this.href);return false; and http://en.wikipedia.org/wiki/42-volt_electrical_system" onclick="window.open(this.href);return false; for some more of the story, including some comments on why it died out.
 
Hello,
The main reason is isolation. At no time is the 400V traction battery connected to the chassis, EVER. All the 12 volt systems / chassis systems including the inverter logic controls have to be isolated for safety and stability reasons. The traction battery contains so much energy that if released by some fault, creates a plasma ball that melts the car down. Just ask John "Plasma Boy" Weyland (White Zombie) how he got his name.

It's also why the car checks for and shuts down when GFI isolation is lost. Like all those vehicle shut downs caused by leakage through the AC system.
 
desiv said:
Nubo said:
aarond12 said:
None that I know of. Find under-hood pictures of all the EVs on the market and you'll find a 12V battery.

Yes, even the Tesla.
Click to expand...

Who learned their lesson and switched to a deep-cycle AGM. ;)
Click to expand...
Which is still a 12V battery I believe.. ;-)
Click to expand...

Of course. My point being that Nissan should have spec'd a deep-cycle AGM for this application. I was kind of referencing the other threads where we hash this 19th-century technology to death :)
 
This isn't a deep-cycle issue. It's an improper charge level issue. Unless AGM doesn't care about being half charged, I don't see how this solves the problem. There must be some attribute I don't know about.
 

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