I've had a couple of these power banks from Aukey for two years now and they've been great at keeping my devices charged when camping but usually are left in the car glove box for emergencies
Aukey 20000 mAh power bank
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Specifications:
capacity of 20,000 mAh
5V 3.4A dual USB output
lightweight (388g / 13.7oz)
They were £19.99 when I bought them and are similarly priced on amazon today and there are many sizes and brands of power bank to suit your needs or budget but they are a great addition to any B.O.B. or vehicle
Take care
Roo