I agree with the opinion of "railman", i.e. the Power for all Alliance, 18 V, green line.
As the hoover is not exactly a lightweight, larger and bigger batteries do not necessarily make it any easier to handle!
2 or 3Ah are more than enough. The dust container only holds 3 litres anyway!

They are compatible from an electrical point of view. However, the thicker batteries may not fit into the wall bracket. They must then be charged externally. See also https://www.powerforall-alliance.com/de...