elecrical problem i think?

here`s what i`m experiencing i have a 1999 vauxhall combo. it feels like the battery is flat but when i charge it i can leave the van days and it will turn first turn of the key. but if i then drive it 2 work 8 miles and turn off and immediately try to restart no go! feels as though no power in battery. when i go back in 30 mins it starts? but then the trip home at night with lights on, i cannot restart even with a wait of 30 mins just wont turn at all. please help