My dad uses a Verizon Galaxy Nexus, the same one he's had for two years. Whenever I call him his first response is invariably, "talk fast, my battery is dying!" It's become his catchphrase. He's counting the days till he's eligible for an upgrade to a DROID MAXX. Until then, he would probably be well-served by the Anker Astro E4, an external battery pack with a massive 13000mAh capacity. It's available on Amazon for a reasonable $44, but apply the coupon code ANKERE4B (black) or ANKERE4W (white) to get it for $38.99 plus free shipping.
The Anker Astro E4 holds enough energy to charge most smartphones at least six times over. It's got dual USB ports and a maximum output of 5 volts, so you can charge a phone and tablet simultaneously. At 5x3x1 inches, it won't take up much more space than your current phone in a bag or pocket, and it includes an LED flashlight just for the heck of it. It's available in both black and white finishes, and Amazon reviews are almost universally positive.
This promotional deal is good for today (Friday, September 6th) only, but the regular price is pretty good to start with, so no worries if you're tied up. To get the cheaper price and free standard shipping, apply the coupon code above on the Amazon checkout page right before paying. (Make sure you've got the right code and color combination!) Those of you who have Amazon Prime can get free two-day shipping even without the coupon.