Charged battery will be dead in about 8:hours after the cables are put on and tightened

Two things come to mind first is the battery holding its charge after it is fully charged, if so remove the + cable from the battery and connect a amp meter between the battery post and the cable , this should almost zero amps if every thing is switched off, there will be a small draw from the clock or alarm system, look for a light in the boot or glove box or cab light, any of the above will run the battery down over time

You will need a multimeter with DC amps setting (usually 10A). You disconnect the negative battery cable and place one lead from your meter on the battery terminal(neg) and the other lead to the negative cable. You should be showing some amperage reading(0.65A i.e.). Make sure Everything is off in car and door closed too, so you don't get an inaccurate reading. Pull fuses one at a time until you find the circuit that's draining your battery( meter reading should drop to zeroA or close to it). If that doesn't work, try disconnecting your alternator(connector and BAT, terminal) and see if the numbers on the meter drop. If so, your alt. is at fault. Always double check your readings so you don't replace parts that are good.

Hi , you need to have the battery checked for condition, it may not be retaining charge because one or more of the cells is in bad condition. Assuming you have a 12v battery, it should be producing rather more than that while the engine is running and the charging circuit is performaing it's task of keeping it charged. Ideally this value will be around the 14 v mark. If the value is below the 12v the battery requires then the battery will not charge but slowly become depleted. Somebody should check the battery output with the engine running , with a multimeter, to establish whether this is the case.
If the output is below 12v have the alternator checked, check the drive belt is appropriately tensioned so that the alternator can be properly driven. It may be that the alternator itself is faultyand needs servicing or replacement.
It might be that there is a fault with one of the components in the electrical system that is the battery to drain, this is not very unusual and can sometimes be traced to components such as the BCM, (body control module,) an electrician can establish this for you.
It should be a fairly straightforward task to track the cause of this problem in the case the battery/belt/alternator are at fault.