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What to expect and do with 12V battery failures - pre-MY21

10000 Views 38 Replies 22 Participants Last post by  Untdrums
In vehicles made before the 2021 model year, there are two 12V batteries. These are located in the frunk under the center removable panel and easily located by the red caps on their positive posts.

The larger battery is referred to as the starter battery. This one is oriented differently from model year 2021 onward.
The smaller battery is referred to as the auxiliary battery. This one is not present from model year 2021 onward.

They supply 12V power, or act as a buffer, for the low voltage (non-driving power) circuits in the car. These include the instrumentation, infotainment, sensors, door locks, power brake booster, climate, power steering, mirrors, electric parking brakes, etc.

During normal operation of the car fully powered up the 12V is supplied by the DC-DC converter that draws power from the 400V traction battery and steps it down to 12V to supplies the needs of the low voltage devices, but they still need the batteries as a buffer, or to operate when the car is powered down.

The auxiliary battery powers the (1) power brake booster, (2) the front EPIC (electronics for the front motor), and (3) the parking pawl in the front motor/axle/transmission. When this battery fails, it can cause messages to appear on the instrument display that are related to reduced power, reduced braking power, and gearbox faults, as well as not releasing the car from park. The messages are due to the fact that the EPIC is not getting its 12V to operate properly, and the brake booster is not getting its 12V to operate properly, and there isn't 12V power to operate the parking pawl to release it.
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The starter battery is supplying 12V for all the other low voltage devices. When it fails you may not be able to unlock the doors, you won't be able to release the bonnet (hood), you may have a very quick shutdown of the car, the displays won't light up, more nasty messages appear on the instrument display (before it quits), the alarm system won't work, etc. It is not a driver friendly experience.

DO NOT IGNORE MESSAGES AND CONTINUE TO DRIVE. You will risk a complete shutdown in traffic and that could be rather hazardous.

Known causes of battery failures are: normal aging, missing software updates, power supply distribution box failure. DC-DC converter failure, BSM/GWM failure, wiring harness failure, loose cable connections, BCM failure, BECM failure.

Get the vehicle to a dealer as soon as possible. It may require being towed on a flatbed tow truck. If out-of-warranty, the starter battery can be replaced by a 99R/T4 size battery. Unfortunately, the auxiliary battery is an odd size and can only be sourced (as far as we can find) from a JLR dealer in the US and Canada, or ordered from European sources that can supply an Exide EK151 battery. Replacing a battery won't do any good if the fault is one of the other causes. Charge and test a battery before replacing it. A good battery would indicate the fault is something else. Update Jan 2023: See 2020 MY and before Auxiliary battery replacement for alternative battery installation.

If one is lucky enough to be where a 12V battery charger, or booster battery, is available, you may be able to power up the car using these on the affected battery/batteries. This should be done with the expectation of getting the vehicle on a tow truck or to a dealer (if very nearby). Once powered up, the DC-DC converter may be able to supply sufficient power to keep the vehicle going. However, dead batteries sometimes act as a big power sink and prevent cars from being powered up or continuing to run. Be sure that the boosting source is connected to the positive post of the battery and grounded to the body of the car (not directly to the negative post of the battery).

If you need to get into a locked car, try using the fob or Jaguar app (or equivalent) to unlock it. If that fails, the fob contains a mechanical key. It can be inserted to a mechanical lock located behind the driver's door handle. Push the front of the handle (near the small button) inwards to pivot out the back edge and pull out the handle as far a it will come. This will expose the door lock and you can operate it with the key from the fob. Turn the key clockwise in the lock to unlock the door. Once in the car, you can open other doors by pulling and releasing the interior door handles twice.
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If you need to get into the frunk, there is a manual release cable behind a black plug in the right side footwell. Unscrew it (yes it is a tedious operation) and pull the release cable.

Once you have the frunk open you can remove the center panel and get to the batteries and manual releases for the parking pawl and charging cable.

If you have a charging cable connected to the car and it needs to be detached, there is a manual release cable that can be pulled. It is near the left hinge (next to the charge port). Pull the red ring up to unlock the charging cable.

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Release the parking pawl by lifting the lever up 90 degrees. This will allow the front wheels to roll. The electric parking brakes in the rear will still keep the car from rolling if they are applied. You should chock the wheels just in case they are not before releasing the parking pawl.

To release the electric parking brakes, you need to power up the car (one press of the start button with foot off of the brake). You will need 12V from the starter battery circuit to do this. Supply a 12V booster source if needed. Then follow this procedure:
  • Block the car from rolling by using something as a wheel chock on a front wheel and a back wheel
  • Press the start button once without foot on brake and if the car doesn't power up you can stop here
  • Press the brake pedal all the way down and hold it there
  • Lift up and hold the parking brake switch (down by the driver's knee) for at least 1 second and you should hear the parking brakes release as well as the brake symbol on the left of the instrument display go off
  • Press the start/stop button once or twice (as needed) to power off the car and you should not hear the parking brake activate
  • Release the brake pedal and parking brake switch
** Caution at this point since the car may roll without warning if the parking pawl has been released and wheel chocks are not used. **

If this procedure does not succeed, then "skates" should be used under the rear wheels to pull the car onto a flatbed tow truck.

If pulling the car onto a flatbed tow truck, retrieve the tow "hook" from under the loadspace floor and screw it into a position on the front or rear of the vehicle as appropriate (behind one of those small caps in the bumper covers). It is left-hand thread so turn it counterclockwise to screw it into the car. If you are not able to power release the rear hatch, lower the rear seat back sections to get to the loadspace area and lift up the floor.

Additional notes:

I recommend using a charger or other source to supply at least 15A as a alternate 12V source for "jumping" either or both batteries. Lower amperage chargers can be used if you want to try recharging the batteries but they will take a lot longer. They do not have the power required to operate the parking brakes.

There are Bluetooth battery monitoring devices available that connect across the battery terminals and allow monitoring of battery voltage via a smartphone. You might consider adding these and checking that voltages are dropping below 10V at any time. A low voltage of less than 10V indicates a dying battery or a charging system issue. Search your favorite shopping website for "12V battery monitor Bluetooth".
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Awesome write-up, thanks a lot.
One point from personal experience: when the car gives a warning about failing 12V battery, do not turn it off, as most likely it won't be able to turn on again until the battery is replaced. Proceed with caution to the location where you can comfortably wait for the tow truck.
What's happening, is this. When the battery degrades over time, it looses usable capacity, and at the same time increases internal leakage. The car's charger is continuously monitoring current, and when it's reaching certain level due to leakage, it generates that error. But while the charger is active, the battery can still sustain usable voltage. Things go south when you power off - then high internal leakage quickly drains already low capacity, and the car turns into a real estate.
In my case, wife pulled over and powered off hoping that it will clear the error, and the car got stuck. The only way I could recover it - bring the truck and kick the battery with jumper cables (which is a risky arrangement I admit). As the current in this case is limited by internal resistance of the batteries only, it quickly gains enough capacity to start the car and drive to safe place, but usually damages the battery even more. Predictably, attempt of charging it back home overnight at 6A setting gained nothing, so still flatbed to the SC.
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