First round of testing completed.

4Ah robi battery lasts for 3 phone charges.
5Ah robi battery lasts for 4 phone charges.

I guess not surprising.

With a 4Ah (18v) battery and a phone battery of 5Ah (3.8v). Number of charges = (4x18)/(5x3.8) = 3.7 (minus losses)

Might need to get a bigger robi battery for festival use ....