I have an Anker Astro Pro2 20,000 mAh multi voltage external battery that I have connected via the DC output to a MagSafe 2 lead. The MagSafe 2 Charger that came with my MacBook is rated 20V. The Anker has 2 settings (16V and 19V) that charge my rMBP 15". It appears 19V charges the laptop quicker. Is it as simple as that, 19V charges quicker so use that or is the 16V more efficient in some way? Am I overthinking things here?