Just changed the battery to my 19999 a4 1.8

Depends on the device capabilities and the type of cells or battery it uses - the common type of jump starter has a sealed lead acid battery and recently high performance types have been introduced that are lighter and smaller and more expensive due to being based on lithium cells.

It is important the lead acid type is never allowed to become discharged as if left discharged for even a short time it will be damaged and in a few weeks could be completely ruined. A jump starter should be recharged after every use and routinely charged every month or two.

A top up charge as above would usually last anything from half an hour to several hours depending on the battery capacity and state of charge. A jump starter designed to start a big diesel will take longer than one designed to start small to medium petrol engines.

If the unit is completely discharged and in the unlikely event it is still healthy, a complete recharge could take days...

Change the petrol check ht leads replace sparks plugs

change the oil and filter first if you can turn the motor by hand to make sure its not siezed. pull the plugs and put a little diesel in them put plugs back in . make sure your battery is good and crankit it a little to the oil moving through out the engine

The clicking may be a starter motor problem, if it's been stood for a while it might need starting to get everything back to normal, eg the cars memory......

Any car that stood for so long need to have all the belts change before use as the belts crack and loose flexibility, the cam belt will most probably go first and will damage the valves. Most probably jumped a tooth on the cam gear and timing will be out and will not start. Check all the timing marks if still in position. Petrol can also be the problem as the filters clog with the dirt in the tank after a long period not running.

it sounds like the alternator needs to be replaced

sounds like you have a lazy starter, had this problem a while back.

Can you hear it clicking at gas door when you hit switch? Try having someone hit switch while you push in a little on it. Once open, lubricate little release part inside.

change battery ,if it starts with jump leads battery flat or altenator not charging

Well his first mistake was buying a Duramax Isuzu Diesel... But with that said, Diesel has a tendency to grow bacteria in the gas tank. Flush the gas tank and change the fluids in the engine and you should be fine. You may notice that some of the gaskets on the engine may leak now that they have dried up. they will either reseal themselfs in time or with thightening or you will have to replace the gasket. I don't think that will be the case after only a few years though.