Battery power station (not working) (Biggleswade)

3 years old, worked well initially but now doesn't seem to be charging properly. When I plug it in to charge it goes to 100% fast and stops charging, but then drops back as soon as it's unplugged. I had an identical one with the same problem and was told it was the charge controller.
I don't know if it will be repairable or reusable, but wanted to try to give it away before putting it out for recycling! The specifications can be found here: https://www.amazon.co.uk/dp/B0B4RWSZ8H
Sorry, it won't include the charger as I've kept that to use with a different power bank. The input looks to be a standard 12 volt round plug.
Please only take this if you can work with it safely as there's a risk the batteries may still have a charge and give someone a zap if they dismantle it incorrectly!

Fair Chance Policy: I’ll wait before choosing a recipient to give more people a chance to reply.