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Understanding Tesla 12V Battery

Aug 11, 2021

Electric vehicles are powered by batteries in a battery pack, but did you know there’s a second battery? Just like internal combustion vehicles, electric vehicles also have a 12-volt battery.

However, typically, the Tesla 12V battery tends to go unnoticed until you can’t start your Tesla.

Tesla battery range and charging, this article was backed by Tesla from their website and owner’s manual. Unfortunately, there isn’t much coming from Tesla about the 12V battery. We’ll do our best to make sure the information this article elaborates is accurate and reliable.

In this article, we’ll help you understand why Tesla vehicles need a 12V battery, Tesla Model 3 / Y 12V battery replacement options, and how to replace the 12V battery.

Why Does Your Tesla Have A 12V Battery?

Why does your Tesla have a 12V battery when it has access to a gigantic battery under the floorboard of the car? Let’s get into it.

If you’re like a lot of car owners, you probably don’t pay much attention to what’s under the hood. Obviously, the big difference between an EV and a gas-powered car is the lack of an engine, but when you pop the frunk of your Tesla, you still have access to things, like brake fluid, washer fluid and even a 12V battery.

 

tesla-model-3-12-volt-battery

 

For some people, it may have come as a surprise that their Tesla even has a 12V battery, especially since there is a high volt battery that could easily provide all the energy they need for their 12V systems. There is a reason Tesla doesn’t do this. When the first-gen Roadster was designed, it actually didn’t have a 12V battery. Instead, a DC to DC converter was used to take energy from the main battery and power the 12V system, such as the headlights, turn signals, media controls, etc.

The problem with that is if the high volt battery needed to be powered down, such as after an accident, all of the 12V systems shut down as well as safety features, like the hazard lights, no longer worked. This is one of the reasons why a 12V battery was added to the gen 2 Roadster and every other Tesla up to this point.

So the 12V battery in my Tesla must be lithium-ion like my high volt battery, right? NOPE! Much like that of a gas-powered car, your Tesla has a lead-acid 12V battery (minus the refreshed 2021 S & X). What’s the difference between these two?

For starters, lead-acid batteries are far cheaper than lithium-ion batteries up front, but over the course of the batteries’ lifetime, lithium-ion batteries make far more sense.

Not only that, but lead batteries have a limited cycle life. Lead batteries are good for about 500 to 1000 cycles as the best deep cycle. If you’re frequently tapping into your battery bank, that could mean that your batteries may need a replacement in less than two years.

In fact, Tesla had a reoccurring issue with its 12-volt batteries. Many in the community have seen their 12-volt batteries die on them with no warning sign whatsoever which could cause you to be locked out of your vehicle and essentially stranded.

Elon did address that they would be solving this by giving customers a heads up when their 12-volt battery is going to die, but that doesn’t address the problem and how to resolve it until their most recent 2021 Model S and X refresh. In fact, in an interview with Sandy Monroe, Elon did acknowledge that in their new refreshed vehicles for Model S and X 2021, and probably later, they’re going to be changing their 12-volt battery to a lithium-ion battery.

This means more capacity, more cycles. Essentially, your battery shouldn’t need to be changed quite nearly as often and should essentially last the same amount of time as your main battery pack. So if you drive a Tesla that is older than the new 2021 Model S or X, most likely, your 12-old battery is going to need to be replaced soon.

The 12V battery in a gas car is used primarily to give a jolt of high current energy to turn over the engine for a few seconds. Once it’s started, the 12V battery really isn’t used that much since the alternator powers the 12V accessories in the car. Even when the gas car is powered off, it doesn’t need much energy from the battery.

If you were to use this type of battery in a Tesla, it wouldn’t last very long, at best, maybe a few months. So Tesla uses what’s known as Absorbent Glass Mat or AGM since Tesla doesn’t need to turn over a gas engine and doesn’t have an alternator and The design of the AGM battery is better suited for Tesla vehicles. It needs to use a 12V battery to power the 12V systems and not to mention the fact that the car still needs to use the battery even when it’s not in use, like Sentry Mode, unlocking the car and being able to turn everything on quickly once you enter the car.

This will mean more charging cycles, and the particular AGM battery used in your Tesla is specifically designed to be repetitively charged and discharged on a regular basis. AGM batteries are also sealed, meaning there won’t be any liquid spills or corrosion.

You might be asking what parts of Tesla use the 12V battery. Rather than bore you with a long list, let’s take a look at what doesn’t use it and uses the high volt battery pack instead.

Obviously, the motors, the air conditioning and cabin heater, the battery coolant heater used in the model S and X, and the DC to DC converter for charging the 12V battery use the main battery.

Everything else is using the 12V battery. When the 12V battery starts getting low, the main battery pack will charge it by using the DC to DC converter, which is the only way it will be charged even when the car is plugged in. If the DC to DC converter fails, the 12V battery won’t be able to be charged. This also helps to explain the phantom drain of the main battery pack.

Tesla 12V Battery Options

The biggest topic of discussion in the Tesla community is the battery, in particular, concerns about degradation, but what tends to go under the radar is the 12V battery which is noticed until people start getting the alert that their 12V battery needs to be replaced. For many people, it is actually the first time they realize the car even has a 12V battery.

For gas car owners, it’s easy to replace the 12V battery. You can stop by your local auto parts store, pick up a new 12V battery and have it installed in minutes. For those that aren’t comfortable swapping out the 12V battery themselves, sometimes the local auto parts store might actually do it for you or you could have a shop do it for you within 12 to 24 hours. In some cases, it may take longer, but most likely, there will be a local shop in town somewhere that will have an opening available.

However, it’s a different story if you have a Tesla. Until auto parts stores start keeping 12V batteries for Tesla vehicles in stock, you’re pretty much at the mercy of Tesla, but many of us came to terms with that long ago.

The best-case scenario is you live close to a service center, and you can pick up a 12V battery there and swap it out on your own in less than 30 minutes. This is, of course, only if you can actually drive your car. You may either need to have your car towed to a service center or make an appointment with mobile service, whichever one will get you your new 12-volt battery faster.

The worst-case scenario is you don’t live close to a service center and you will either need to

1. drive a long distance to get the battery

2. have it towed to the nearest service center

3. schedule an appointment with mobile service

How long it will take with numbers two and three is all a matter of luck.

There is a third-party option that can get you your new 12-volt battery in 24 to 48 hours via overnight shipping, but it won’t be a Tesla OEM 12-volt battery. Instead, it will be a lithium battery manufactured by a company called Ohmmu Batteries. So let’s take a closer look at these two options more closely.

 

tesla-oem-vs-ohmmu-12v-battery

Performance

The OEM 12V battery is expected to last on average between two to four years, give or take, depending on how many charge cycles your battery has gone through.

According to Ohmmu, their batteries are expected to last up to four times longer than the Tesla OEM battery, six to eight years.

 

ohmmu-battery-is-more-efficient

source: Ohmmu

The Ohmmu battery is expected to be more efficient for two reasons:

  1. It weighs about 60% less than that of the Tesla OEM battery. The Ohmmu battery comes in at a little over 11 pounds, while the OEM battery weighs about 28 pounds.
  2. The Ohmmu battery is also more efficient because overall, it loses less energy during use. Most Tesla owners are aware of the efficiency loss of the main lithium battery, particularly during colder winter months. That being said, the Ohmmu battery has been tested under extreme conditions from temperatures as low as minus 94 degrees Fahrenheit and as high as 365 degrees Fahrenheit. Based on their testing, the Ohmmu battery passed the test in extreme conditions.

Price

There is a big difference in price between the two batteries. One quote from a local Tesla service center was $122 for parts and labor with mobile service and a one-year warranty on the battery. The battery alone is approximately $85. Both of those prices will most likely vary depending on where you live.

The Ohmmu battery will set you back a whopping $439 with a four-year warranty. If you’re still covered under your Tesla warranty, the decision is a no-brainer stick with the OEM battery at no charge, but if you need to pay for it out of your own pocket, it actually may still be a no-brainer for many. But let’s take a look at the math.

When comparing the price, the Ohmmu battery actually pays itself off in 5.2 years. Anything you get out of it after that will actually be a bonus. That being said, on average, most people only keep their cars for about six years, so for some people, this may be a moot point. There may also be some hesitation to getting anything other than OEM, but considering the Model S and X refresh has a lithium 12V battery, it’s most likely only a matter of time before the 3 / Y follows suit.

In the end, it all comes down to what you can afford and what you’re most comfortable with. Understandably, $439 is going to be way too much for many and on top of that, it’s not OEM, which can also make some people uncomfortable. For those of you, considering the Ohmmu battery but have it needed to replace your battery just yet, it actually might not be a bad idea to purchase it now and store it and keep it charged as Ohmmu recommends which is about every six months. That way, when you get the warning or your car won’t start, you have the battery ready to go.

How To Install A 12V battery

It’s time to change out your 12-volt battery in your Tesla model 3 or Y. You might be asking yourself, “Do I change it out myself or do I have a service technician do it for me?” Before you have someone else do it for you, we’re going to give you step-by-step directions on how to swap it out yourself. Believe it or not, it’s easier than you think.

 

how-to-install-a-12v-battery

1. Open the frunk, windows and doors

2. Power off the car

3. Unlatch the back seat

4. Disconnect high voltage interlock loop (you will hear a loud” popping” noise. Don’t be alarmed)

5. Disconnect Negative (Black) terminal First (Make sure it doesn’t make contact with metal);

disconnect Positive (Red) terminal Second

6. Push to release bracket

7. Disconnect gas vent tube (you won’t need this if replacing with Ohmmu Battery)

8. Remove plastic covers

9. Lead lug spacers Provided by Ohmmu (Larger one goes on the positive terminal, smaller one goes on the negative terminal)

10. Spacers are tapered. The larger opening faces down. Press down to secure

11. Connect Positive terminal first (Don’t forget to put the red rubber cap back on);

connect Negative terminal second

12. Connect high voltage interlock loop

13. Snap the back seat into place

In conclusion

Tesla is constantly innovating and improving their production on their vehicles. It’s a no-brainer that they are now starting to move from a lead battery to a lithium-ion battery that gives better performance, better longevity in their S and X line. Maybe the margins are a little bit higher, so they’re able to do that. Hopefully, we see that reflected on future 3 and Y models.

Hopefully, you learned a little bit about the batteries in your electric vehicles. You left a little bit more educated and a little bit more knowledgeable on if your 12-volt should fail, you know exactly what needs to be done to replace that and why.

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