Buy a $25-$35 OEM iPhone 5 Battery from Ebay and replace it yourself. It will void your warranty (If you even still have one) and you better make sure it is OEM.
As for the install complexity level... My grandma could do it.
2008 335xi E90 Alpine White - Sold - 8/19/10
> 2011 335xi E90 Jet Black - Sold - 6/27/12 > 2012 335IS E92 Crimson Red - Current