No one "forgets" to get collision coverage, you have to actually decline it. This OP declined it on purpose (they are side stepping that by calling themselves "brain dead" as if they forgot somehow but they didnt forget, this was a conscious choice in order to save money on insurance because collision is the most expensive portion of coverage.
They dont say what year this vehicle is, but its likely the declined collision to save money thinking they didnt need it anymore due to the relative cost of the vehicle.
There are no pictures in this thread anymore, but based on responses I can tell some of you have seen them. Based on the responses it sounds like the damage was visible. Its impossible to run over something and have it hit the car and not know it, that would cause that much damage. Its possible a person didnt see it because they didnt want to look, but not possible to hit something like that and not know you hit it if you were driving.
So, OP knows they hit something, knew it was leaking (may not have known what they hit, but knew they hit "something") probably continued home and parked. The "didnt realize it" can be partially true as maybe they didnt know what they hit, but they know they hit something.
They then took the vehicle in, hoping and praying it would be either covered under warranty, or be able to be billed under comprehensive coverage, but neither of those is the case.
Now they need options.
For those saying "maybe it was covered under the old policy", that might be the case if this happened before the insurance swap over. Its not legal in california to be covered by two different insurance policies on a vehicle at the same time (not sure about other states). Policies always end at midnight and start at like 12:01am. There would not be a time when the vehicle was covered by both policies, it could be covered by the old one if the accident happened then, but if that was the case, OP would know this already.
So, as
@ucmndd says, this is likely squarely on OPs dime, through the choice of neglecting collision coverage on a car which has a part which is still north of 20K to repair in it. The choices
@ucmndd laid out seem to be OPs options to me.
I would either reach out to one of the aftermarket repair companies or just sell for scrap and start over. I am going to guess there isnt a loan on the vehicle because the OP didnt elect collision coverage, so the choice is to decide if the car "is worth" the amount it costs to repair the battery, or would that money be best put into a different car.
Since we dont know year of vehicle, just taking a guess because OP didnt elect collision coverage on purpose (you have to uncheck that selection, so it cant be an accident, its a choice that happens to have backfired, it happens), I am going to guess the answer to "is the vehicle worth repairing for a cost of 20k?" to be "no".