Well you can try out these fix:

1. Check The Screen Brightness.
2. Reduce The Number Of Network Connections.
3. Turn Off Keyboard Backlight. 
4. Adjust Laptop Power Settings. 
5. Remove Unwanted Peripherals. 
6. End Unnecessary Backgrounds Tasks/Programs. 
7. Close Unwanted Processes.
8. May be you need a new battery for you laoptop.

Start a command prompt with admin privilege. 

Type powercfg /batteryreport

What are the current and design capacity values?

Design is  - 60,002 mWh

Full Charge is - 52,220 mWh

I have 02 units of Dell Latitude 7490 with me & both of them have same similar issues

Please disable any power options if its set.

Update the BIOS.

Also once the machine is shut down with battery at 100%, take out the battery. Leave it out for couple hours and when you wish to power on the system put back the battery , let us know if it s till drains.

Ensure you disable any startup programs. How long does the machine takes to boot??

BIOS uppdated

Power Options are default its on Balanced 

Laptop Just take around 60secs to boot hardly 4-5 programs on startup 

The draining of the battery starts when its nearby by 40% from 100% to 40% it works all good but once the battery level reaches 40% i see a sudden drop to 6% within few seconds