Here is Why you should not keep your electric car battery in Electric car.

Can you have  pre-charged electric car battery in your electric car?

You can have a pre-charged electric car battery in your car, but it’s not a good idea

Can You  have a pre-charged electric car battery in Your car?

There is no absolute need for an extra pre-charged battery if you have an electric car. They can last up to 10-20 years without replacement.

Need

Electric car batteries are not cheaper. They can cost anywhere from $0 to $20000. So spending this amount for thing that mostly not going to happen, is not a good idea.

Cost

If your car exposes to different climates then there is a risk of catching fire in the car. Keeping substances that can catch fire is always hazardous.

Safety

EV batteries weigh more, so they will require lifting equipment. You need to lift the car to unscrew the previous battery and replace it with another one.

Hard to Carry

Instead, You can always charge your battery at nearest charging station. Or battery swapping is a good option. 

Alternatives

In emergency system, you can call for SOS support. Have look at alerts when batteries are low. So, you can better plan for help. 

Emergency