12v battery needed to drive?

[Rmasu]

Just wondering,
If your 12v battery is dead will the car still start and run? If not do you jump it like a ICE car?
I cant understand why nissan didnt put a battery elimination circuit in for the 12v power items in the car, and eliminate the need for a conventional 12v battery system.

 

[planet4ever]

If your 12v battery is dead will the car still start and run? If not do you jump it like a ICE car?

No. Yes.

I cant understand why nissan didnt put a battery elimination circuit in for the 12v power items in the car, and eliminate the need for a conventional 12v battery system.

I'm not sure I understand. What kind of "elimination" circuit would allow electronics to run off 400v DC? Incidentally, the DC/DC converter is liquid cooled.

Ray

 

[Herm]

The 12v battery is needed for safety.. its what switches on the 400v battery if everything is ok.

 

[Rmasu]

I guess I was thinking about a system where a 12v lithium system was included in the car and charged up when the main batts were being charged. Makes sense though that would be a pretty large drop in volts!
Thanks for the info

 

[ericsf]

I suspect the reason Nissan is using a 12 volts acid lead battery was to save time and money. The majority of car electrical systems are designed and more importantly tested to run from this kind of electrical power. It makes a lot of sense for them to reuse the same equipment (ABS, airbags, lights, power windows, etc...) they are already put in their other cars. It may seem like a very minor change to go from 12V from an acid lead to a 12V lithium ion pack but having worked in this industry a few years ago, I am quite sure it would have required Nissan to gets Bosh, Delphi or whoever supplies their equipment to requalify everything.

 

[sparky]

It is indeed needed to start the car and as mentioned earlier, it's the power source for the safety checks before the main contactors to the traction battery are enabled, among other things. The 12V went dead in our LEAF last week following a not too unlikely sequence of events I'll recount here:

1) I was out of town for work
2) Spouse drove LEAF home from work, parked, plugged in EVSE and went in for the night.
3) Next morning, the LEAF was stone-cold dead. Wouldn't respond to any button pushes or Carwings requests.

She called Nissan dealer, they said it needed to be towed in to the shop. I told her to leave it, I was pretty sure it was the 12V aux and I'd be home the next day. I didn't want her to attempt a jump (based on past experience).

So, when I got home, the 12V had 3V across the terminals. I hooked it to a charger, and gave it about 15 mins of charge before dis-connecting and re-connecting the EVSE. It now was charging via the EVSE.
So, what happened?

The most likely scenario, which I've re-enacted to verify, is she never shut the LEAF off when she got home. The parking brake was engaged, the interior light was on when she got out of the car and plugged in to charge. The LEAF signals when you exit with the key and the car is on but it makes a similar sound to locking the car, so I guess it went un-noticed.
Here's the thing, if the car is still on, it won't charge. If she had checked for the blue blinking dash lights she would have noticed I think. So, instead of charging, it just sat there with the lights on, discharging the 12 V. I'm kind of surprised that the LEAF lets that 12V get so discharged before shutting things down but maybe it's in no-man's land where if the EVSE is plugged in the traction battery won't keep the 12V charged but the EVSE won't supply any juice either.

Anyway, it'd all fine now but, I'm thinking I'll need a 12V aux battery sooner rather than later.

 

[TonyWilliams]

Makes me wonder if the 12v low voltage auto-disconnect wouldn't be a good investment for the LEAF. I walked away from my car with it on at the San Diego LEAF
meet yesterday.

You might need a relay with this one:

Power Planted LLC
(805)667-2142

Low voltage auto shut-off switch to protect batteries from over-discharge

40A switching capacity (Expandable)
Low Voltage Signal Output
Protect your battery investment!

Only $69.95

 

[kubel]

Doesn't the manual say no jump starting?

 

[planet4ever]

Doesn't the manual say no jump starting?

If that were true, why would they spend more than two full pages in the Owner's Manual describing how to do it? There are two special restrictions that apply specifically to a LEAF:

• Don't try to jump charge the 12v battery while the main battery is being charged.
• Don't try to jump start an ICE from a LEAF.

You might think the second one is just because the battery is too wimpy, but that's not the problem. When you jump start one ICE from another one, the "donor" car needs to be running, so its engine can be driving the alternator. The LEAF doesn't have an alternator, and its DC/DC converter, which charges the 12v battery, is usually not running even though the LEAF is powered up.

Ray

 

[DaveEV]

When you jump start one ICE from another one, the "donor" car needs to be running, so its engine can be driving the alternator. The LEAF doesn't have an alternator, and its DC/DC converter, which charges the 12v battery, is usually not running even though the LEAF is powered up.

The LEAF's DC/DC inverter is always on when the car is on. The highly respected Ingineer details proper car jumping procedure with the LEAF in this post which I've quoted below.

While Nissan doesn't condone it, you should have no issues jumping another car with the LEAF. Be absolutely sure to get the polarity correct, and connect the ground on the LEAF to the DC-DC converter, NOT THE BATTERY (-) TERMINAL!

Procedure: Connect the red/black jumpers directly to the Pos/Neg terminals of the dead car's battery first, then connect the red directly to the LEAF's Pos battery terminal, and then finally, connect the black jumper lead to the case of the DC-DC. The DC-DC is the large aluminum box behind the "engine looking" inverter. You can easily see the black negative terminal on top of it, which is a good place to grab. Once jumping is complete, disconnect in reverse order. Be very careful not to short the jumpers together when removing them from the LEAF.

The LEAF's DC-DC converter is about as powerful as an alternator on any normal car, and should charge a dead ICE battery enough to start it in a few minutes. No need to "rev" the LEAF! =)

-Phil

 

[NissanTech375]

I cant understand why Nissan didn't put a battery elimination circuit in for the 12v power items in the car, and eliminate the need for a conventional 12v battery system.

Nissan uses a battery saver function, that will cut off circuits controlled by the modules, left unintentionally on.

Although if a human does an "end run" around module control, IE: turn on map light (provide a dedicated ground), the system doesn't see the draw and does not save the 12V house keeping battery.

Just this past week I had one in the shop w/HK battery dead. I discussed this w/Nissan and inquired about the possibility in the future of adding a amp sensing field to the positive side of the HK battery to sense an unintended event and interrupt the B+ feed, avoiding an unintended discharge.

So now the mother ship is aware.

 

[TomT]

There is already a hall effect sensor on the 12V battery that monitors current in and out (columb counting). It should sense a map light left on (and, in fact, does on my car and turns it off after a period of time). Why would this not be sufficient?

Just this past week I had one in the shop w/HK battery dead. I discussed this w/Nissan and inquired about the possibility in the future of adding a amp sensing field to the positive side of the HK battery to sense an unintended event and interrupt the B+ feed, avoiding an unintended discharge.

 

[EVDRIVER]

It's more perplexing that you can't get a text if the car is left on for an extended time with no fob present, and easy feature to turn on. If you use the car normally there should not be an issue, in alms all cases battery issues are related to careless drivers leaving the car on or walking away and ignoring the beeps.