My 520 was used so much the battery life became annoyingly short. I bought a 520 plus partly because I wanted more memory for maps and partly fr teh battery. At the time I couldn't find instructions fo replacing the 520 battery.

That said, when I rode doubles I usually used a battery extender. I had a "Minty Boost" I soldered myself. With lithium batteries I was good forever. Later I got a small usb brick I velcroed to the stem. The Garmin will use it while running. That may be your best and cheapest option for long rides. Good luck.