Gone up to £7.99 now
Go for a reputable brand and read reviews on Amazon. I've owned Xiaomi, Anker, Ravpower and a Samsung battery packs. None of them damaged my devices, although the Samsung died after a few months.
Used loads of different portable chargers - they can’t do any damage, the worst thing they can do is not charge
Question: Many of us need & use powerbanks for our tablets, phones etc Those devices are expensive to replace - if a charger causes damage. How do we judge how safe/reliable any particular charger is? What specific things do we need to look for / take into account? It would be a shame to loose a £100-£1000 device over an unknown design?