Do EV have a smaller 12v battery?

Do EV have a smaller 12v battery?

is there a smaller battery to run accesories and start the car and if so what would happen if that battery died.

could the big traction batteries start the car and recharge the smaller one or would it be completely dead and need a jump etc?

Tâm | 24 september 2017

Since the later version of Roadster, to all other comes after which include Model S, then X, then of course, Model 3, 12V battery for accessory functions has been the design. (Elon talks about getting rid of 12V system in future but it is unclear of how the design will be done).

The high voltage/main battery does normally recharge the accessory 12V battery when things are optimal.

It will stop recharge the 12V battery when its own state of charge is down to a reserve level.

It will let the 12V dies first for its own high voltage/main battery survival.

It's cheaper to replace a dead 12V battery than do that for a high voltage/main battery.

When 12V battery level is low enough to function, you can't electronically open doors, turn on lights and of course you can't charge your high voltage/main battery.

To charge your high voltage/main battery, you need a functional BMS battery manage system that is alive with 12V battery.

You can jumpstart the 12V battery to wake up the normal 12V accessory system from doors to of course the BMS to charge your high voltage/main battery.

If that can't be done, you'll just swap the 12V battery out with a new one.

Tâm | 24 september 2017

For reference, Tesla Roadster up to 1.5 did not have a separate 12V battery. It uses 12V from the high voltage main battery pack.

It sounds intuitive: You've got one big battery there already, why do you need a separate 12V battery?

And you guess what: When someone neglected to plug the roadster in, according to the article below, it would cost about $40,000 to fix it.

That's why, since Roadster 2.0 and up to Model 3, now have a SEPARATE 12V battery as a design.

Iwantmy3 | 25 september 2017

The model 3 has a 12V battery. It is shown in the document below.

jordanrichard | 25 september 2017

The 12v runs the dash/computers and is recharged via the main battery pack.

Also a heads up, you will not be bale to jump start your neighbor's car with your Tesla. The 12v battery in our cars is not designed, meaning has the CCA (cold crank amps) to turn over an ICE. | 25 september 2017

OP - The battery is about 20% smaller than a typical ICE battery. The model S/X uses a 33 Ah sealed absorbed glass mat deep cycle lead-acid battery. I suspect the 3 will use the same battery.

@Tam - I didn't know the first Roadster didn't have a 12v battery! Glad they changed the design.

Haggy | 25 september 2017

The battery is smaller because it doesn't need the cranking amps to turn over an engine.

What Tâm said is true, but keep in mind that it's an extreme situation that there's about a 0% chance that you will encounter when it comes to the car not charging the 12v to save the main battery.

RedPillSucks | 25 september 2017

@Tâm, jordanrichard
What about the reverse? If your tesla 12v battery is dead, can you use an ICE battery to charge up the tesla 12v so you can at least unlock your car?

Carl Thompson | 25 september 2017

The weird thing is I've experienced a dead 12v battery more frequently in my EVs than my ICE cars. Go figure.


deemo | 25 september 2017

You can jump the car to unlock the doors, need to get into frunk to do so. There is a jumper point under the tow hook cover to open the frunk if the battery is dead.

Tâm | 25 september 2017


Of course, you can use an ICE as a 12V source to wake up Model 3's accessory functions.

However, in a depleted Tesla 12V battery, regular ICE 12V supply won't be strong enough to do it. You then will need to call AAA because they can deliver high current to wake up your Tesla's accessory functions.

Carl Thompson | 25 september 2017

"However, in a depleted Tesla 12V battery, regular ICE 12V supply won't be strong enough to do it. You then will need to call AAA because they can deliver high current to wake up your Tesla's accessory functions."

Are you sure about this? I've never had an issue jumping the 12v batteries in my EVs. If a standard ICE car's 12v is more powerful (more CCA) than a Tesla's then why would it have an issue?


Tâm | 25 september 2017

@Carl Thompson

Here's the case study:

"Thanks for all of your help. Got the frunk open thanks to you all. Tried to charge the 12V by jumping it but it did not work. Tesla said I needed to contact AAA and ask for a Charge Pack which I did. All good now."

deemo | 25 september 2017

I highly doubt that could not jump from another vehicle, An ICE vehicle alternator is putting out a 14 volts + and more than a few amps. More than I would expect is needed to start the car.

Speculation - I believe you may need to charge the battery much longer than may think because need to jump until car starts the charging cycle, not just jump start like an ICE. You need to have car turn "on" since it was sitting in a very low state of charge (25 miles) when left for 3 weeks, it stopped charging the 12v trying to protect main battery.

Tâm | 25 september 2017


My speculation in the case of AAA is: If the owner just disconnect the depleted 12V from the system and let the ICE to power Tesla accessory electronics, it would be fine.

However, because the whole system is connected to the depleted battery, any injected power will be diverted to the depleted battery and thus bring the whole effective power lower.

As you suggested, if the owner let the ICE charge the depleted battery long enough, (but who knows how long, few hours?) the Tesla power system would be raised high enough eventually to power its accessory which would make calling AAA unnecessary.

deemo | 25 september 2017

Should only take a minute or so to get the car to boot up and take over. Without disconnecting the battery, the battery is a sink but only need to provide enough power to the car to finish booting to have the on board DC-DC charger take over and supply power to the system and start to charge the 12v battery. An ICE requires much more Amps to jump start but only the relatively short while cranking over and you don't disconnect the battery or need it to charge it for a time before can keep running, the amperage put out is sufficient and would be certainly for an EV with lower amp requirement. It should only take a min or so (maybe less) before the car boots and the internal charger can take over (definitely not hrs) but also not the touch and go like on an ICE jump start.

Again this is speculation, but what I would expect. I think AAA was called because they did not jump long enough, not because not enough power available from the jump start.