Speak EV - Electric Car Forums banner
1 - 17 of 17 Posts

·
Registered
Joined
·
475 Posts
Discussion Starter · #1 ·
Just curious, after a discussion with a friend, do all EV's have a 12V battery like an ICE?
I-Pace, eTron, Tesla 3, Y etc?
I know the 2016 Tesla Model S does, though he didn't think it did, unless they've changed?

From my understanding the 12v battery has given various vehicles issues,
Though I also didn't think it was simple to switch to just using the traction battery?
It's probably been discussed before, i just didn't find anything, 12v brought up lots... :oops:
 

·
Registered
Joined
·
180 Posts
Yes, a 12V battery is a necessity for operating the car, I don't know of any EV without one. As well as for safety (i.e. operating the hazard warning lights in the event of a breakdown), it's need to 'power on' the car systems and operate the contactors to connect the HV traction battery to the HV electronics during start-up.
It's also true that many EVs from various brands suffer from poor charging regime of the 12V battery from the DC-DC converter, especially if parked up for a period or little used, so shortening the life of the 12V battery and giving rise to some of the bizarre errors folks report if the 12V battery voltage is low. You'll often see reference on here for charging or replacing the 12V battery as the first step to fault-finding some other more serious sounding problem.
 
  • Like
Reactions: Mofo

·
Registered
MG EZS 2020
Joined
·
4,972 Posts
At present all EV need a 12v as said above. There are some manufacturers that are working on removing the 12v and just having the traction battery. I think Tesla is one of these.
 
  • Like
Reactions: Mofo

·
Registered
Joined
·
475 Posts
Discussion Starter · #4 ·
Thanks gents :)
Pretty much as i thought, one of the main things is as a safety feature to 'engage' the HV battery
Not to mention the usual powering of alarm systems/emergency lights 👍
 

·
Registered
Kia e-Niro 4 MY20, Zoe Z.E.50 GT Line
Joined
·
1,363 Posts
Tesla seem to use the traction battery to keep a lot of electronics active, such as for sentry mode, which seems to result in the car using an awful lot of power and generally having terrible overall efficiency.
 

·
Registered
Kona PremSe64k 2020+bluelink +ohme
Joined
·
538 Posts
It's a safety thing. The 12V battery is there so that the high voltage system is not deployed throughout (and I mean throughout) the car all the time. I'm not saying it's the best solution, but it is cheap to deploy, partly because ever ICE has the same system.
 

·
Registered
Joined
·
844 Posts
There is a dc/dc converter to charge the 12 volt battery once the main traction battery is active so in theory no auxiliary battery is needed. I guess the safety issue of having the 400 volt traction battery always active may be difficult to overcome but not insoluble one would think.
 

·
Registered
Joined
·
5,740 Posts
There is a dc/dc converter to charge the 12 volt battery once the main traction battery is active so in theory no auxiliary battery is needed. I guess the safety issue of having the 400 volt traction battery always active may be difficult to overcome but not insoluble one would think.
At present I suspect efficiency is a big driver. A battery maintaining a few low power circuits only uses a tiny amount of power. A 400-12 converter that can supply a few hundred W at full blast will likely use quite a few watts just idling in standby which will produce a significant drain on the system over time. Bear in mind that the law now requires things like TVs to use less than 0.5W in standby, having an EV that uses several times that 24/7 is going to raise some green eyebrows.
That size problem could be reduced by having a separate very small converter for standby, but that will probably cost as much or more than a 12V battery - and be more complex and less safe.
I'm sure there is a solution being worked on, but I'm not sure what it will look like.
 

·
Registered
Joined
·
1,560 Posts
At present I suspect efficiency is a big driver. A battery maintaining a few low power circuits only uses a tiny amount of power. A 400-12 converter that can supply a few hundred W at full blast will likely use quite a few watts just idling in standby which will produce a significant drain on the system over time. Bear in mind that the law now requires things like TVs to use less than 0.5W in standby, having an EV that uses several times that 24/7 is going to raise some green eyebrows.
That size problem could be reduced by having a separate very small converter for standby, but that will probably cost as much or more than a 12V battery - and be more complex and less safe.
I'm sure there is a solution being worked on, but I'm not sure what it will look like.
This is a completely solved problem in the world of AC mains PSUs, and the DC voltage after the rectifiers in those is very similar to an EV battery.
All that an in-pack converter would need to do is provide enough 12V power for entry systems, LED hazard lights etc. and to close the HV contactors, the main car's DC-DC could then take over.
For the case where the HV battery runs very flat, a "jumpstart" 12V connection could be used to wake it up for recharging - this could come from the OBC when plugged into AC.
Maybe also a manual wake-up, e.g. via the mechanical key that's often in keyless keyfobs.
 

·
Registered
Joined
·
5,099 Posts
At present all EV need a 12v as said above. There are some manufacturers that are working on removing the 12v and just having the traction battery. I think Tesla is one of these.
Well, they were working on it in as far as it was rumoured that the Model 3 would not have a separate 12v battery but that didn't come to pass when it actually released and I haven't heard any more since then. The Model Y also has a 12v battery.

Eliminating the 12v battery is actually a really challenging problem on many fronts when you get down to the nuts and bolts of it, and the complexity is such that it's probably not warranted to be honest as the possible solutions are a bit kludgy. The simpler solution is just to perform more intelligent "maintenance" and top ups of the 12v battery than most EV's currently do, and make it fool proof so that "user error" can't cause the 12v battery to be excessively discharged. It's a solvable problem when the car has the ability to do autonomous top ups from the traction battery when the car is parked.
 

·
Registered
Joined
·
5,099 Posts
For the case where the HV battery runs very flat, a "jumpstart" 12V connection could be used to wake it up for recharging - this could come from the OBC when plugged into AC.
Except that there is no AC from an EVSE until after the OBC has appropriately signalled the EVSE to activate. Chicken and egg.. No 12v power to the OBC ECU means you can't activate the EVSE to start charging...your 12v "jumpstart" would have to come from somewhere else.

Not being able to bootstrap traction battery charging of a completely discharged traction battery is one of the problems eliminating the 12v battery introduces.
 

·
Registered
Joined
·
1,560 Posts
Except that there is no AC from an EVSE until after the OBC has appropriately signalled the EVSE to activate. Chicken and egg.. No 12v power to the OBC ECU means you can't activate the EVSE to start charging...your 12v "jumpstart" would have to come from somewhere else.

Not being able to bootstrap traction battery charging of a completely discharged traction battery is one of the problems eliminating the 12v battery introduces.
True, forgot about that, but as "completely discharged" would actually mean that the BMS in the pack decided that the voltage is too low and disables the 12V DC-DC.
There would still be plenty of energy available to start things up, so there could be a contact-closure input to re-enable it for long enough to allow charging to start, or you could just make the CP signal available to the pack BMS, so it would force a wake-up when an EVSE is connected
 

·
Registered
Joined
·
4,795 Posts
...Not being able to bootstrap traction battery charging of a completely discharged traction battery is one of the problems eliminating the 12v battery introduces.
Here's how I'd do it. I'd have a very small NiMh rechargeable long-life battery, say 6V worth, about the capacity of a couple of AA batteries, that can store charge for let's say a year without going flat, with an internal drain current of a few of microamps or whatever.

A separate button switch looking like that Kona Battery-Emergency-Refill button, connected to this 6V, when pressed and held down, powers up a v small & cheap Microchips 8-bit CPU running off 3.3V fed from that 6V by buck down-converter (so if only 5V from the Nimh should still work hopefully) that in turn closes a small 2-pole relay with say 5A rating. That's it for the small CPU, the 6V is in turn trickle-charged from a switched 12V line via some efficient buck down-converter, so gets auto-topped up whenever the car's properly on later on.

On the other side of the 2 poles of the relay, one goes to the 0V end of the 400V HV battery, by-passing the main contactors (which are open at the moment). The other goes to about 5 cells up from that end, so we're at around 16V to 21V taken from the HV cells. Stick a fuse in here, 10A/whatever. What! you say, you're not allowed to do anything with the HV battery when the contactors are open? Well, even when they are, there's still a lot of BMS stuff connected to all the cells in there, so why can't we add a tiny bit more in? The 16-21V tapping in turn feeds a modest buck down-converter to supply 12V at 5A, and this is in turn fed through the relay & a diode into the actual 12V battery to recharge it. You would probably add in a separate, special-purpose contactor, a beefy one, that connects the 0V of the HV battery to the car's Ground. So maybe we need a slightly bigger 6V supply, whatever. You have now successfully bootstrapped the 12V system into life, the 12V tapped from the bottom 5 cells will have enough oomph to operate the HV 400V contactors, and the car's then alive. All this 6V stuff & the relay needs to be very well double/triple insulated just in case something goes belly-up, but at worst it should never see more than about 21V, surely?
 

·
Registered
Renault Zoe 50
Joined
·
21,644 Posts
Anyone thinking about investing in a 12v battery maintainer/charger... The NocoGenius5UK is on Amazon today for £49. I paid £65 when I bought the same earlier in the year.
You can, but it really needs to be in a garage setting, unless you wire in a permanent port.

This can be a hassle, for what, extending the life of the battery by a year or so?

You’ve already spent half the cost of a battery and introduced another plug-in ritual. Not to mention the chance of driving off with the maintainer still plugged in!
 

·
Registered
Joined
·
4,795 Posts
Hmm, may not be such a bad idea, particularly of it can recover a hammered battery. My not-so-old 12V Bosch somehow ran itself flat a few weeks back, I have absolutely no idea why; must have been something stayed on inside, but am baffled. Anyway, was down to 4V ! Thought it would be knackered, but stuck it on my Ctek MXS 5.0 on the AGM & Recovery-desulphatine/whatever settings, left it a couple of days, and it's come back to life perfectly, the Ctek's happy with it, and car's been fine since. I'm glad I got that kit now, and would hope the Noco one gets similar good reviews to those eth Ctek's getting generally. Honest John in D.Tel keeps recommending the Cteks, I can see why.
 
1 - 17 of 17 Posts
Top