Batt DIY...cake in nice weather in your garage or driveway; maybe not so cake in YYZ land in March, dunno. Took me 30 mins the first time, lollygagging...
If you are doing a batt swap, I highly rec'd bringing new batt home and doing a slow, low amp maintenance charge on it overnight or longer. Few new batts in the store, are in top shape, and many have been subject to a high amp 'hot shot' charge before you buy it.
There are a few DIYs on the net, and on this site...here is my version, and just above the first pic is a link to more pics and info.
GL, mD
http://www.xoutpost.com/bmw-sav-foru...apped-out.html