So I bought battery brick from Amazon.ca it was a 20,000 mAh, it's a 5v battery pack. I also just bought a Milwaukee 9 Ah 18v battery for my tools and beside it I saw an adapter that let me click onto the top of the battery the ability to use it to charge my phone. I could not tell what one was better when it comes to capacity. Because one is an 18V battery and the other is 5V. How can I calculate what is a better battery to use to charge my battery if my phone requires 5V 3A to charge. If I only had room for 1 battery on a trip what one should I bring?
Update: My phone battery is 4000 mAh so lets assume that every time i charge it I want to fill it up.. I really don't care what I am charging though but what battery listed above would last longer... therefore charge my phone more if you assume 5V 3A when charging it!