Just to give you an idea of how fast battery costs have declined, here is a chart (via futurist Ramez Naam) showing how the amount of energy that can be stored in lithium-ion batteries per $100 rose from 1991 to 2005.More at Bloomberg View.
That data is from a 2009 study by Duke University. Nor did storage efficiency stop after 2005; according to Naam, the cost of electric-car batteries declined by 40 percent from 2010 to 2013. The Tesla gigafactories are projected to drive costs down at an even faster rate.
These declines, unlike the recent 50 percent drop in oil prices, are not temporary. They are driven by increasing demand, which spurs technological progress -- not by reduced demand, which lowers the oil price.