So i just received the battery i was talking about here
it kind of works, but it has a very strange behaviour: basically when i connect it to a load (just a board, nothing intensive), it just doesn't give energy and the board remains off. If then i connect the other connector to the wall plug, the battery "wakes up" and power the board. Then i can disconnect the battery from the wall and it keeps running for hours.
What can it be ? If i then disconnect the battery from the board and reconnect to it, it is again "asleep" and i need to connect to the wall plug.
Never heard of this behaviour from a battery