Power to second screen. Hello, should a second screen attached with the proper cables run off the

Question

Hello, should a second screen attached with the proper cables run off the built in battery supply from the main unit? at the moment the second screen will only work if the mains power supply is plugged in to it directly or the in car cigar adaptors are, it won't work just on the internal battery power. Is this a fault and should I take it back or is this normal as have always had it plugged into some power supply. Thank you for any help with this.