Just a suggestion but I recommend getting a 12 Volt Impact Wrench off Ebay.
They cost about $50 and I have used mine for everything from wheel nuts to rebuilding diffs.
Only thing is the wheel nut sockets they come with are rubbish but a few extra bucks at a Tool shop or Coventry's for a 21mm Impact Socket and it's game on.
Link is for illustration only... I don't know this seller so cannot recommend them one way or the other
http://www.ebay.com.au/itm/NEW-12v-E...item4ad0947d1d