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12 Volt Battery Compendium

Some owners may be surprised to learn their car includes a 12v lead-acid battery. We’ll explain why and delve into everything about the 12v battery used in Tesla vehicles today.

History and Use

The first Tesla roadsters didn’t include a 12v battery but used a portion of the main lithium-ion pack to supply 12v for accessories and lights.  This was not ideal, and in 2010 Tesla switched to a separate 12V battery in the Roadster 2.0. All Tesla vehicles since that time also include a separate 12v battery.

Roadster battery

2010 Roadster 2.0 12v battery position

The reason for a separate 12v battery is to power critical systems in the event of a high-voltage battery disconnect. This keeps key systems operational such as emergency blinkers, airbags, seatbelt pre-tensioners, brake booster and a host of other systems. The high-voltage battery might be disconnected due to several rare events – such as a high-impact crash, internal main battery pack problems such as some types of cell failures, a contactor failure and a few other unlikely situations.

Most of the S/X/3 is powered by the 12v battery, including:

  • All lights, such as headlights, turn lights, fog lights, taillights, backup lights, license plate and interior lights
  • LCD Display –Instrument Cluster (S/X) and main display
  • The MCU and many modules with processors (50+ in the Model S/X)
  • Non-traction motors, such as windows, liftgate (S/X), wipers, washer fluid, seat motors, side mirror adjusts, sunroof (if equipped) charge port door, steering assist, fans, valves and coolant pumps
  • Audio system including tuners, antenna amplifiers and audio amplifiers
  • Safety systems, such as airbags, brake booster and seatbelt pretensioners
  • Autopilot systems including cameras, sensors and radar
  • Heated items, if equipped such as seats, steering wheel, washer nozzle, side mirrors, rear defrost and camera heaters.
  • Other items like the alarm, rearview mirror dimmer, USB ports, horn, Homelink, air suspension (if equipped) and various latches (doors, trunk, frunk, glovebox)
  • High-voltage battery pack contactors

Only a few items are directly powered by the high-voltage battery pack. This includes the traction motor(s), the a/c compressor, cabin air heater, battery coolant heater (S/X only) and the DC to DC converter for 12v power and charging the 12v battery.

Tesla ran into a major snag with the introduction of the Model S in 2012.  The 12v battery was sourced from an American firm, who unbeknownst to Tesla, outsourced the battery to China, and the Chinese company outsourced it to Vietnam! The quality was sub-standard and rarely lasted more than a year. Tesla switched vendors in 2013 and replaced all the marginal batteries in early build cars with a far more reliable battery under warranty.

 

Battery Types

The Model S and X use a 33 Ah sealed AGM (Absorbed Glass Mat) deep cycle lead acid battery. One recent supplier is the C&D Technologies DCS-33IT.

C & D Battery

Model S/X C&D Technologies Battery

The Model 3 uses a 45 Ah, AGM Battery. One supplier is ATLASBX with the Model 85B24LS.

Model 3 Atlasbx battery

Model 3 ATLASBX battery

AGM batteries provide more power for the size and weight than traditional lead-acid batteries. Deep cycle versions, like those used in Tesla vehicles are designed to be repetitively charged and discharged. If you were to use a typically lead acid battery in this application, it may only last a few months!  AGM batteries are also sealed, which eliminates acid spilling accidents and the need to periodically add water.

Battery Longevity

In new Tesla cars, the 12V battery lasts about 3-4 years for most owners, although paradoxically, if you put few miles on the car, it may last as little as 1-2 years. The difference is how many discharge/charge cycles the battery goes through and how deeply the battery is discharged. When driving, the DC-DC converter helps power the 12v system, and fewer charge cycles occur. A car left sitting, will need more charge cycles, and will age the 12v battery faster.  Some Model S owners still have the original battery after 4 years and over 170,000 miles of use!  The Model 3 is too new to evaluate its battery longevity, but Tesla made considerable changes to the electronics, which may increase battery life as well.

AGM life cycle

AGM life cycle – deeper discharges results in fewer cycles

Your Tesla maintains power for some key computer systems all the time, so you can disarm the alarm, unlock the car with your FOB (or phone for the Model 3), and when you enter the car, everything starts up without any pause. In addition, the cellular connection for remote access needs power. All these systems require power even when the car is not in use. This is often called the car’s vampire drain. Tesla, via software, has significantly reduced the vampire drain of all cars over the years, but it can’t be eliminated.

Newer cars also use modules that take a bit less power when the car is parked, so that helps. Even the oldest cars should easily get 2-3 years of 12v battery life, although there can be the rare exceptions. Just like ICE cars, sometimes the battery fails prematurely.

What is Different About How Tesla Uses the Battery?

Often an ICE car goes 3-5 years before replacement.  The way the 12v battery is used in ICE cars is quite different than an EV. When the motor is running, normally the alternator powers all the 12v items in the car and charges the battery. The primary use of the 12v ICE battery is to start the car, which takes 2-3 seconds with very high currents.  After starting the car, the battery is not really used. When the car is off, very little power is used, otherwise the battery would die in short order.

A few BMW ICE models, and perhaps other makers, use a far larger battery and intermittently use the alternator. This reduces gas use with less alternator drag on the motor. The downside is the far more charge/discharge cycles means the battery is replaced about once a year, a $700 hit in the BMW!

Improving Longevity

Tesla provides one option that may slightly improve the 12v battery longevity, if that is important to you. Turn off Mobile Access. This means you cannot access the car via your phone app.

Mobile access on the Model 3

Mobile access on the Model 3, and is similar on the S/X (v9+)

For most owners, the battery replacement cost a negligible part of the ownership costs. Having remote access is great feature, usually worth trading off for a slightly shorter 12v battery life.

Charging the 12v Battery

The 12v battery is always charged via a DC-DC converter from the main battery pack. This takes the packs high voltage and converts it to about 14v to charge the 12v battery. It’s a bit more complex, as the voltage to charge the AGM battery is tightly regulated and is compensated due to temperature.

Supercharging

Even when you are charging the car, only the main pack is connected to external power.  While quite rare, if the DC-DC converter fails, the 12v battery can no longer be charged.

The Gen 2 DC-DC converter in the refreshed Model S accepts 220 to 430 VDC at 15 amps, and outputs 9 to 16 VDC. When outputting 12 VDC, it can deliver about 200 amps.

DC-DC inverter (Model S, Gen 2)

DC-DC converter (Model S, Gen 2)

The Model 3 integrates the charger and the DC-DC converter into a single package, the PCS (Power Conversion System).

Battery Failure Warning

Tesla included additional hardware at the battery to help detect when the battery is nearing its end of life. Should the battery get within 3-4 weeks of the end, Tesla warns you with a clear message that the 12v battery needs replacement. This is dramatically better than most ICE cars, where the battery dies without much, if any, warning.

Warning message

Warning message for the Model S/X (left) and for the Model 3 (right)

Part of the reason ICE car batteries often die unexpectedly is the type of battery used. A standard lead acid battery will degrade over time, but the lead sheds during use and can short out plates at some point, causing the immediate cell death. An AGM battery, like the ones Tesla uses, should never fail this way.

Battery Location

In all Tesla, the 12v battery is located under the hood. Over the years and models, it has moved around somewhat. The classic Model S RWD had the battery deep on the passenger side in front of the firewall. This location is tricky to get access too, but a trained Tesla tech can often replace this battery in less than 20 minutes. For someone not familiar with the process, it will take quite a bit longer.

Starting with the AWD Model S, the battery was relocated to be easier to access and replace – on par with most ICE cars. The location remains about the same for the S, X and the Model 3. There are several easy to remove plastic panels to gain access to the battery. No tools are required to gain access.

Battery Location

Refreshed Model S Battery Location (with cabin air inlet panel removed)

What Kills an AGM Battery?
Over time the ability to charge and discharge effectively slows due to sulfation. Lead sulfate accumulates on the negative and positive parts within the battery during discharge. Over many charge/discharge cycles this process reduces the amount of energy that can be stored and extracted.  At some point the battery is considered bad, even though it continues to work at some level. With proper instrumentation, such as done in the Tesla, a degraded battery can be detected well before the useful end of life.

Fully discharging an AGM battery is bad, but not normally fatal. Fully discharging an AGM battery does reduce the batteries longevity. Normally this never occurs in a Tesla, but if vehicle is not connected to a charger, and the main battery is drained to 0% SOC (which has a hidden remaining charge), the 12v battery will stop being charged to protect the main battery. Within a few days, the 12v battery will be drained.

Dead battery

One rare failure mode is an open between two cells.  This is usually caused by improper manufacture of the battery or an external short.  The battery voltage goes from normal to zero as soon as the open occurs, usually early in the life of the battery.

Heat is also bad for an AGM battery, but the heat conditions encountered in a Tesla are fine. AGM batteries are not suitable in an ICE engine compartment that exceed the AGM’s maximum charging temperature of 140°F (60°C).

Why Lithium-Ion 12v Battery is a Bad Idea

There are 12v lithium-ion drop-in replacements for ICE car batteries. These are quite a bit more expensive but do weigh less – so you’d think they would be great in an EV. Not so fast. Turns out for Tesla’s application the frequent deep charge/discharge cycles would greatly shorten the life of a lithium-ion battery, perhaps only lasting 6 months to a year. Since there is no climate control on the 12v battery, it’s life would also be shortened if you encounter climate extremes. Lithium-ion 12v battery makes little economic sense in a Tesla.