My bmw 730li battery has gone flat and the problem is that i cant open the trunk either by the remote or the button inside the car. how can i open the trunk for changing the battery?

The first issue is that you purchased a BMW. Enough said.

Check coolant level may need t stat

The remote isn't working? Perhaps a new battery can fix this, do the doors unlock? If not the remote, does the button on the dash open the trunk? If you are using the key, have you tried flipping the key over, it may not work the same each way even though the ignition does. Do you have folding rear seats? If so there is an emergency latch for the trunk to open it in case somebody gets trapped inside.

if the doors and trunk are vacuumed operated, make sure all the doors are unlocked..this might make it easier to open the trunk..or get the car running and then try to open the trunk with the car running..turn key all the way to the left and push slowly in..I think it is vacuum operated like the mercedes

Try to remove the back of the back seat or try to remove the radio speaker in the back and if you can do either one of those you will be able to get to your keys in the trunk.

you can charge the battery from the engine compartment. check your owners manual that is the correct place to jump and to charge the battery.

check inside the center console i think there is a lock and unlock button for the trunk make sure it is unlocked then try it with the remote.

this sounds like a broken wire inside the corrugated tubing for the boot/trunk loom located by the right hand hinge on the lid

I have a e60 525i that I just brought and it has the same problem, the owners manual suggests driving with the key in the ignition for a long time as the batteries are rechargable and may have gone flat, i have tried this and it doesnt seem to work

i also read something about getting water in the aerial about the rear window and that wrecks the receiever. My car isn;t under warranty so not looking forwarding to taking it to bmw