I have two RAVPower brand power banks for sale. I love this brand and think these are excellent products - they simply don't recharge as fast as I would like, and I've ended up settling on another product by the same brand that fits my needs better. These are on the larger side, but will provide plenty of power while you're out on the trail. If you carry a smartphone and/or GPS and use rechargeable batteries in your headlamps and/or whatever else, these power banks will ensure you can keep everything charged when out for a while.
Both are in brand new condition, just tested out for a couple days, shipped in the original packaging with original USB cables and padded stuffsack. As such I'm asking for $10 off the Amazon prices:
https://www.amazon.com/gp/product/B012NIQG5E - 26,800mAh power bank, massive capacity, recharges at 5V 2.1A. Asking $40
https://www.amazon.com/gp/product/B01G1XH46M - 22,000mAh power bank, also pretty big capacity, recharges at 5V 2.4A, so faster than the above. Asking $30.
For full disclosure, I ended up preferring their 20,100mAh power bank (https://www.amazon.com/gp/product/B019IFIJW8) for my needs, because it supports recharging at 5V 3.0A with an appropriate charger, and in combination with it's slightly smaller capacity, recharges faster than either of the above that I'm selling. If you're fine with leaving the power bank charging overnight after fully draining it, the above products are great!