I have a 2002 Mercedes Benz C240. My car won't start every now and then and have to charge up the battery. Thought the battery might be bad so bought a brand new battery and still happens and gets a message on dashboard regarding ESP default. What is wrong with this car.

I think you should check if the alternator is charging the battery.with engine runningcheck battery voltageshould be 9.6 to 12 volts,now turn on headlights ,air conditioner & all accessories,check voltage at battery again,voltage should be around 14 volts indicating the alternator is charging the battery,.check all wiring in charging circuit before condeming the alternator also alternators fusible link

Hi there.
You must have a drain some where.
Make sure the brake light are NOT staying on. if they are, it will drain the battery. this is due to faulty brake light switch.
I think the ESP light is related to the brake light switch.