New Battery last six months then gradually wont start in the end wont start at all without jumping it.

Try unhooking the battery it does a sort of reset. And check fuses all fuses and relays

Did the new battery work when replaced? If so then it is the charging system! Look at the alternator first!

Interesting,this sounds just like no output from the alternator. If the alternator is good then it has to be the cabling or connections to the battery. If you have a multimeter take a reading on the cabling then read what is actually reaching the battery posts.

This behaviour is typical of worn starter motor main contacts, at the bottom of the starter solenoid.

Could be a wet or bad belt, belt tensioner or one of the other pulleys.

the battery may be shorted out. do you have a meter to check a batteries condition? if not take the battery into the parts store and they can tell you if it has a dead short in it which means the plates inside the battery have sulphated together and wont allow current to pass through them. lead acid batteries are real bad about this if they go completly dead even if they are newer batteries.

The most likely one is the battery went flat causing brown out of the ECU unit or the boy racer add on .I get no end of problems caused by add on so called car alarms from the long term airport parking lads and 9 out of ten times even after removing the add on alarm its still will not start and invariably end up with either a dealer fitted new ECU or one from a scrappy with the key chip and the transponder .

I had same prob with Grand Cherokee - no rhyme nor reason as to when, where or why it wouldn't start. After two new starters the shop finally found a starter wire/cable that was corroded on the inside. Mech' could move it back and forth and it would / would not start. Changed cable six months ago - no problems since. Hope it helps.

You have a new battery and it cranks. Turning over is cranking. You are saying it won't start after sitting for six months? What is an old mechanic? Was the cable installed at home during the one year layoff? It sounds as though you failed to hold the old mechanic responsible for installing the correct cable or adjusting the one you have ,so it will start. You have a safety switch issue by the sound of it. Make sure you have the right cable and then find out how to install and if applicable,adjust properly

i had the sam prob but mine was the battery.....make sure all your conections a are solid in the battery...maybe check some fuses too?