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When an EV is “off”, the 12V load is much higher due to the advanced computer systems that are constantly running to maintain the high-voltage battery pack, keep the vehicle “connected” via remote access features, maintain charging and BMS communications, etc. It can put up to 50 Watts of load on the 12V system when the vehicle is off. This cycle ends up draining the 12V pretty quickly and requires it to be recharged by the high-voltage battery pack regularly.

When turning on an EV, the 12V uses no additional load. While driving, the 12V battery is used for non-driving tasks (powers the lights, media unit, windows, locks, etc.), while the high-voltage battery pack focuses on the driving part, which extends the driving range. The 12V battery is subjected to relatively low discharge and recharge currents due to the separation of tasks. 

**Tesla recommends you leave your car plugged in when not in use. This will further the life of both of your batteries.  

The problem: 

An EV needs a 12V battery that can handle a high number of cycles and low current requirements, and an ICE vehicle needs the exact opposite: a battery for high discharge and low cycle counts. So, while these lead-acid batteries are doing their job just fine in an ICE vehicle, what does a Tesla need? 

The Solution: Lithium batteries

Lithium batteries are typically used for high-voltage battery packs, cell-phones, laptops, medical equipment, and cars where batteries are being cycled frequently, and longevity is important.

For your EV, lithium batteries are capable of tremendous cycle-life and still provide minimal capacity loss and degradation. They also weigh less. 

Teslas are programmed to protect the main battery pack as much as possible, and if it’s running low, it may put the battery into a deep sleep/low energy mode. If low-power consumption mode is active due to a low charge on the main battery pack, immediately plug in your tesla to prevent the 12V battery from dying and having to do a jumpstart and/or 12V battery replacement. The 12v battery can run flat within 24 hours once the main battery pack has stopped supporting it.


How to replace 12V battery:

To gain access to the 12v battery, the steps are in the user and first responder manuals because every model and year is slightly different. This is how we did it:

What you need: 10mm ratchet & 12V battery, gloves are recommended

  1. Open the frunk.
  2. Open your windows to prevent being completely locked out of your car while it’s off. You can even keep a door or two open.
  3. Turn off your car completely using the touchscreen. (menu > safety & security >power off)
  4. Unclip the backseat on both sides (two clips) and raise it up on the passenger side. You’ll see a black cover. Take that out (- no tools involved). Underneath that, there is a rectangular clip that you’ll release and let it hang off to the side.
  5. Go back to the frunk and take out the top cover. It pops in and out with clips so be careful with them as they break easily.
  6. Take out the vent cover by also popping it out with the clips. You should now be able to access the 12V battery.
  7. There is a bolt right on top that you’ll take off along with the two support bars.
  8. Negative side comes off first (black), then positive (red).
  9. Once all is disconnected, remove the battery.
  10. With the new Ohmmu battery, put the adapters on the positive and negative sides (they come with it). It’ll fit like a cone where the bottom is slightly bigger than the top and one is also a little bigger than the other, so pay attention to those two key things when applying.
  11. Put the new battery in place.
  12. Secure with bolt and support bars.
  13. Put on positive first (red), then negative (black).
  14. You should hear the car come on with a whirring noise after this part.
  15. Put vent cover back on.
  16. Put top cover back on – line it up before pushing in as the clips will break easily.
  17. Go back to your back seat and reattach the high voltage plug and cover. The seats should snap back into place.